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Phantom dosimetry of i-CAT CBCT and i-CAT panoramic radiographs in pediatric patients Choi, Ella 2014

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PHANTOM DOSIMETRY OF i-CAT CBCT AND i-CAT PANORAMIC RADIOGRAPHS IN PEDIATRIC PATIENTS by Ella Choi  D.D.S., The University of Alberta, 2010  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  MASTER OF SCIENCE  in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES  (Craniofacial Science)    THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)   June 2014  © Ella Choi, 2014      ii Abstract  Objectives: To obtain head dimensions from patients who received dental CBCT at BC Children’s Hospital (BCCH), to apply this information to design and construct small child and adolescent head phantoms, and to measure and compare the absorbed radiation doses from CBCT and panoramic radiographs using small child, adolescent and adult head phantoms. Materials and Methods: Patients who received dental CBCT at BCCH were surveyed. Head dimensions from each subject’s image were measured to develop adolescent and small child head phantoms. The most commonly used dental CBCT imaging protocols were examined. Absorbed doses were measured for small child, adolescent and adult head phantoms with i-CAT CBCT and Planmeca panoramic radiograph machines.  Results: In the patient survey, 32 patients met the inclusion criteria. The most common indications for CBCT referral were for orthodontic treatment, followed by craniofacial abnormality and cleft lip and palate. A small child phantom was developed to represent the child patients with craniofacial abnormality and an adolescent phantom was developed to represent healthy orthodontic patients. Absorbed radiation doses varied depending on machine, imaging protocol, size of phantom and location of the ion chamber in the phantoms. For CBCT images, the highest radiation was measured in the small child phantom while the lowest radiation was measured in the adult phantom. For panoramic radiographs, the i-CAT CBCT panoramic option was compared to the Planmeca panoramic radiograph machine. For both machines, the small child phantom measured the highest while the adult phantom measured the lowest radiation. For     iii the adolescent phantom, lower values were measured with the Planmeca machine while lower values were measured with i-CAT CBCT panoramic option for the small child phantom. Conclusion: Two groups of pediatric patients were referred for dental CBCT at BCCH: young patients with craniofacial abnormality and healthy adolescent patients for orthodontic assessment. A consistent trend was observed for both CBCT and panoramic radiographs: the highest dose was measured in the smallest phantom while the lowest dose was measured in the largest phantom. Radiation in pediatric population is more detrimental than in adult population and it is important to child size the dose and protocol.        iv Preface  Identification and design of the research program, performance were performed by the author and research supervisor, Dr. Nancy L. Ford. Performance of the research and analysis of the research data were conducted by the author. Regular committee meetings were held where the research supervisor, pediatric dentist and oral maxillofacial radiologist were in attendance. Ethics was approved by UBC Research Ethics Board. (H11-03449). The online ethics training module Tri-Council Policy Statement: Ethical Conduct for Research Involving Humans Course on Research Ethics (TCPS 2: CORE) was issued and completed on 14 December, 2011     v Table of Contents  Abstract .......................................................................................................................................... ii	  Preface ........................................................................................................................................... iv	  Table of Contents ...........................................................................................................................v	  List of Tables ................................................................................................................................ ix	  List of Figures .............................................................................................................................. xii	  List of Symbols ........................................................................................................................... xiv	  List of Abbreviations ................................................................................................................... xv	  Acknowledgements .................................................................................................................... xvi	  Dedication .................................................................................................................................. xvii	  Chapter  1: Introduction ...............................................................................................................1	  1.1	   Introduction of Dental CBCT ............................................................................................ 1	  1.2	   How Dental CBCT Works ................................................................................................. 1	  1.3	   Advantages of CBCT ......................................................................................................... 2	  1.3.1	   Accessibility ................................................................................................................ 2	  1.3.2	   Rapid Scan Time ......................................................................................................... 3	  1.3.3	   Field of View .............................................................................................................. 3	  1.3.4	   Image Accuracy .......................................................................................................... 4	  1.3.5	   Multiplanar Image ....................................................................................................... 5	  1.3.6	   Post-Processing ........................................................................................................... 5	  1.4	   Disadvantages of CBCT .................................................................................................... 6	  1.4.1	   Higher Radiation Dose ................................................................................................ 6	      vi 1.4.2	   Field of View (FOV) ................................................................................................... 7	  1.4.3	   Artifacts ....................................................................................................................... 8	  1.4.4	   Limited Soft Tissue Imaging ...................................................................................... 9	  1.4.5	   Hounsfield Units ....................................................................................................... 10	  1.5	   Increasing Use of CBCT in Dentistry .............................................................................. 10	  1.6	   Different CBCT Machines and Imaging Protocols .......................................................... 13	  1.7	   Radiation Risk .................................................................................................................. 14	  1.7.1	   Deterministic and Stochastic Effects ........................................................................ 15	  1.7.2	   Measuring Radiation ................................................................................................. 17	  1.7.3	   Limitations of Effective Dose ................................................................................... 17	  1.7.4	   Dose Index ................................................................................................................ 19	  1.7.5	   Radiation Risk in the General Population ................................................................. 20	  1.8	   Radiation Risk in Pediatric Population ............................................................................ 21	  1.9	   Radiation Risk in Medically Compromised ..................................................................... 23	  1.10	   Dose Reduction Strategies ............................................................................................. 24	  1.11	   Aims of Study ................................................................................................................ 27	  Chapter  2: Materials and Methods ...........................................................................................28	  2.1	   Part 1. Patient Survey ....................................................................................................... 28	  2.2	   Part 2. Phantom Dosimetry Study .................................................................................... 32	  2.2.1	   Materials ................................................................................................................... 32	  2.2.2	   Methods..................................................................................................................... 36	  2.2.2.1	   Phantom Dosimetry Study for CBCT ................................................................ 37	      vii 2.2.2.2	   Phantom Dosimetry Study for Panoramic Radiograph ...................................... 38	  2.3	   Statistical Analysis and Uncertainty ................................................................................ 39	  Chapter  3: Results .......................................................................................................................40	  3.1	   Part 1. Patient Survey ....................................................................................................... 40	  3.1.1	   Sample Size, Gender and Age .................................................................................. 40	  3.1.2	   Indication for CBCT ................................................................................................. 40	  3.1.3	   Previous Radiation Exposure .................................................................................... 42	  3.1.4	   Common CBCT Imaging Protocols for i-CAT ......................................................... 43	  3.1.5	   Dose Index ................................................................................................................ 43	  3.1.6	   Head CBCT Measurements ...................................................................................... 44	  3.2	   Part 2. Phantom Dosimetry Study .................................................................................... 49	  3.2.1	   Conversion of Units .................................................................................................. 49	  3.2.2	   CBCT Radiation........................................................................................................ 50	  3.2.3	   Panoramic Radiograph Radiation ............................................................................. 53	  Chapter  4: Discussion .................................................................................................................58	  4.1	   Part 1. Patient Survey ....................................................................................................... 58	  4.1.1	   Sample size, Gender and Age ................................................................................... 58	  4.1.2	   Indication for CBCT ................................................................................................. 58	  4.1.3	   Previous Radiation Exposure .................................................................................... 60	  4.1.4	   Common CBCT Imaging Protocols for i-CAT ......................................................... 61	  4.1.5	   Head CBCT Measurements ...................................................................................... 61	  4.2	   Part 2. Phantom Dosimetry Study .................................................................................... 62	      viii 4.2.1	   Phantom Head Size ................................................................................................... 62	  4.2.2	   CBCT Radiation........................................................................................................ 63	  4.2.3	   Panoramic Radiograph Radiation ............................................................................. 65	  4.3	   Sources of Error ............................................................................................................... 67	  Chapter  5: Conclusions and Recommendations ......................................................................69	  5.1	   Patient Survey .................................................................................................................. 69	  5.2	   Phantom Dosimetry Study ............................................................................................... 69	  5.3	   Strengths .......................................................................................................................... 71	  5.4	   Limitations ....................................................................................................................... 71	  5.4.1	   Patient Survey ........................................................................................................... 71	  5.4.2	   Phantom Dosimetry Study ........................................................................................ 72	  5.5	   Recommendations and Clinical Relevance ...................................................................... 73	  5.6	   Future Directions ............................................................................................................. 74	  Bibliography .................................................................................................................................76	  Appendix A: Phantom CBCT Dosimetry Data .........................................................................82	  Appendix B: Phantom Panoramic Radiograph Dosimetry Data with i-CAT CBCT NG .....85	  Appendix C: Phantom Panoramic Radiograph Dosimetry Data with Planmeca Panoramic Machine .........................................................................................................................................87	       ix List of Tables  Table 1. Comparison of Effective Doses from Different Dental CBCT Units ............................. 16	  Table 2. Tissue-weighting Factors for Calculation of Effective Dose – ICRP 1990 and 2007 Recommendations  for Tissue in the FOV During CBCT exposure: Dental Related Select Tissues....................................................................................................................................................... 18	  Table 3. Anatomic Analysis on a CBCT Study: Skeletal Landmarks .......................................... 29	  Table 4. Anatomic Analysis on a CBCT Study: Distances Between Skeletal Landmarks in Sagittal View ................................................................................................................................. 30	  Table 5. Anatomic Analysis on a CBCT Study: Distances Between Skeletal Landmarks in Coronal View ................................................................................................................................ 31	  Table 6. Energy Absorption per Unit Mass Provided by the National Institute of Standards and Technology ................................................................................................................................... 34	  Table 7. Description of Phantoms ................................................................................................. 36	  Table 8. Common CBCT Imaging Protocols. ............................................................................... 37	  Table 9. Panoramic Exposure Values Employed at BC Children’s Hospital ............................... 38	  Table 10. Panoramic Exposure Values Employed at University of British Columbia Dental Clinic ............................................................................................................................................. 39	  Table 11. Dose-Area-Product (DAP) for Different CBCT and Panoramic Radiograph Settings on i-CAT ............................................................................................................................................ 43	  Table 12. I-CAT CBCT Radiation Measurement with Head Phantoms: Protocol1 ..................... 50	  Table 13. I-CAT CBCT Radiation Measurement with Head Phantoms: Protocol 2 .................... 51	      x Table 14. Comparison of Adult Imaging Protocol for i-CAT and Planmeca Panoramic Radiograph .................................................................................................................................... 53	  Table 15. Comparison of Adolescent Imaging Protocol for i-CAT and Planmeca Panoramic Radiograph .................................................................................................................................... 53	  Table 16. Comparison of Small Child Imaging Protocol for i-CAT and Planmeca Panoramic Radiograph .................................................................................................................................... 53	  Table 17. I-CAT and Planmeca Panoramic Radiograph Radiation Measurement Using Adult Imaging Protocol. .......................................................................................................................... 54	  Table 18. I-CAT and Planmeca Panoramic Radiograph Radiation Measurement Using Adolescent Imaging Protocol. ....................................................................................................... 55	  Table 19. I-CAT and Planmeca Panoramic Radiograph Radiation Measurement Using Small Child Imaging Protocol ................................................................................................................. 56 Table A.1 Adult Phantom Dosimetry Data, i-CAT CBCT NG………………………………….82 Table A.2 Adolescent Phantom Dosimetry Data, i-CAT CBCT NG………………….………...83 Table A.3 Small Child Phantom Dosimetry Data, i-CAT CBCT NG………………………….. 84 Table B.1 Adult Phantom Dosimetry Data, i-CAT CBCT NG, Large Protocol …………...….. 85 Table B.2 Adolescent Phantom Dosimetry Data, i-CAT CBCT NG, Large Protocol………….. 85 Table B.3 Adolescent Phantom, i-CAT CBCT NG, Small Protocol…………………………...  86 Table B.4 Small Child Phantom, i-CAT CBCT NG, Small Protocol…………………………..  86 Table C.1 Adult Phantom Dosimetry Data, Planmeca Panoramic Machine, Adult Protocol……87 Table C.2 Adolescent Phantom Dosimetry Data, Planmeca Panoramic Machine, Adolescent Protocol. ……………………….…………………….…………………….…………………….87     xi Table C.3 Adolescent Phantom Dosimetry Data, Planmeca Panoramic Machine, 7-12 year old protocol. ……………………….…………………….…………………….……………………88 Table C.4 Small Child Phantom Dosimetry Data, Planmeca Panoramic Machine, Under 6 Year of Age Protocol. ……………………….…………………….…………………….……………88       xii List of Figures  Figure 1. Anatomic Analysis in Sagittal View…………………………………………………. 30 Figure 2. Anatomic Analysis in Coronal View…………………………………………………. 31 Figure 3. Phantom Study Set-up and Panoramic Radiograph Set-up ........................................... 33	  Figure 4. Head Phantom Size Comparison from the Top View ................................................... 35	  Figure 5. Top View of a Phantom Head…………………………………………………………37	  Figure 6. Age Distribution of Thirty-two Children who Met the Inclusion Criteria .................... 40	  Figure 7. Indication for Dental CBCT in Thirty-two Patients at BC Children’s Hospital………41 Figure 8. Relative Scale of Two Most Common CBCT Imaging FOV Settings at BC Children’s Hospital ......................................................................................................................................... 43	  Figure 9. Distance Between Upper Incisor (U1) and Posterior Point of First Cervical Vertebrae (Posterior C1) Plotted Against Age with Linear Regression  ....................................................... 44	  Figure 10. Distance Between Upper Incisor (U1) and Anterior Point of First Cervical Vertebrae (Anterior C1) Plotted Against Age with Linear Regression ......................................................... 45	  Figure 11. Distance Between Maxillary Anterior Nasal Septum (ANS) and Posterior Nasal Septum (PNS) Plotted Against Age with Linear Regression  ....................................................... 45	  Figure 12. The Largest Skeletal Anteroposterior Distance Plotted Against Age with Linear Regression  .................................................................................................................................... 46	  Figure 13. The Largest Anteroposterior Soft Tissue Distance Including Nose Plotted Against Age with Linear Regression  ......................................................................................................... 46	  Figure 14. The Largest Anteroposterior Soft Tissue Distance Excluding Nose Plotted Against Age with Linear Regression  ......................................................................................................... 46	      xiii Figure 15. Distance Between Left and Right Condyles Plotted Against Age with Linear Regression  .................................................................................................................................... 47	  Figure 16. Distance Between Left and Right Gonions Plotted Against Age with Linear Regression  .................................................................................................................................... 47	  Figure 17. The Widest Left and Right Zygomatic Width Plotted Against Age with Linear Regression  .................................................................................................................................... 47	  Figure 18. Maxillary Width Plotted Against Age with Linear Regression  .................................. 47	  Figure 19. The Widest Skeletal Horizontal Width Plotted Against Age with Linear Regression  48	  Figure 20. The Widest Soft Tissue Horizontal Widths With Ears Plotted Against Age with Linear Regression  .................................................................................................................................... 48	  Figure 21. The Widest Soft Tissue Horizontal Widths Without Ears Plotted Against Age with Linear Regression  ........................................................................................................................ 48	  Figure 22. Distance Between Gonion and Condyle Plotted Against Age with Linear Regression ....................................................................................................................................................... 49	  Figure 23. Average Absorbed Radiation Dose (mGy) in Adult (left), Adolescent (middle) and Small Child (right) Phantoms: Protocol 1 ..................................................................................... 52	  Figure 24. Average Absorbed Radiation Dose (mGy) in Adult (left), Adolescent (middle) and Small Child (right) Phantoms: Protocol 2. .................................................................................... 52	   Figure 25. Distance from X-ray Source and Radiation……………………………………….....64 Figure 26. Size of Phantom and Radiation. ………………………………………......................65      xiv List of Symbols  Σ: Summation     xv List of Abbreviations  ALARA: As Low As Reasonably Achievable BC: British Columbia BCCA: British Columbia Cancer Agency BCCH: British Columbia Children’s Hospital BMI: Body Mass Index CBCT: Cone Beam Computed Tomography CT: Computed Tomography FOV: Field of View ICRP: International Commission on Radiological Protection i-CAT NG: i-CAT Next Generation kVp: peak kilovoltage LNT: Linear No-Threshold  mA: milliampere MDCT: Multi-Detector Computed Tomography mGy: milliGray mSv: milliSievert PMMA: Polymethyl Methacrylate  RCDSO: Royal College of Dental Surgeons of Ontario  TAD: Temporary Anchorage Device TLD: Thermo Luminescent Dosimeter μSv: microSievert     xvi Acknowledgements  I offer my enduring gratitude to my research supervisor, Dr. Nancy Ford, whose penetrating questions taught me to think critically and offered sharp insight. From working with Dr. Ford, I not only learned about radiation, but also newly gained immense respect for all scholars and researchers in science.  I thank my committee members Dr. David MacDonald and Dr. Rosamund Harrison for their guidance and suggestions.  I thank Dr. Pierre Deman for words of encouragement inside and outside the laboratory.  I am thankful for UBC Faculty of Dentistry Research Equipment Grant and UBC Faculty of Dentistry graduate student research funding, which provided equipment essential for my research.          xvii Dedication  I would like to dedicate this thesis to my mother, Younghee Choi. She has always put higher education as a priority and her values are instilled in mine. Without her, I would not have attempted nine years of post-secondary education. I am forever indebted to her for her unconditional love and endless support.  My sister, Sabina Choi, has been a constant source of inspiration and love. I am grateful for her positive energy, calm and encouragement.  My fiancé, Jonathan Hoyles, has been enthusiastic throughout this journey. He is my rock during the difficult times. I am eternally grateful for his love.       1 Chapter  1: Introduction  1.1 Introduction of Dental CBCT Developing a faster and safer imaging modality has been an endless quest in dentistry. The introduction of two-dimensional dental radiography in the 1890s was responsible for major progress in dental radiology (3).  While adequate in most dental situations, two-dimensional images have a number of limitations. Distortion and superimposition have been a constant obstacle to better diagnosis and treatment. CBCT is the latest imaging modality that may resolve some of these limitations. Accurate multiplanar images allow practitioners to better visualize the working field. In the practice of dentistry, the cost of the machine and higher radiation doses are often barriers to CT usage. For many years, only patients seen in hospital settings had access to CT. This situation changed in the late 1990s when dental CBCT devices became widely available commercially (4).  With the development of new detector technology, data processing power and computer programs, dental CBCTs became capable of producing multiplanar images of high resolution at a lower cost (3). Today, many dental practitioners either own or have a ready access to CBCT imaging. With the advent of user-friendly software, lower cost, smaller size and attempts to lower the radiation dose, the popularity of CBCT in dentistry is on the rise (5).   1.2 How Dental CBCT Works CBCT is an image acquisition technique where a multiplanar image is reconstructed from two-dimensional projection views around an object. In a dental CBCT machine, an x-ray source and     2 detectors are mounted on opposite sides of a rotating gantry. From the x-ray source, a divergent cone-shaped beam of ionizing radiation is emitted, which is detected by an area detector on the opposite side. The CBCT gives three-dimensional and multiplanar images from single rotation in contrast to Multi-Detector CT (MDCT), where the patient is translated through the x-ray beam during multiple rotations. During an x-ray exposure from CBCT, the x-ray source and detector rotate around the patient in a synchronized fashion. The rotation can be a full turn or a partial turn depending on the desired FOV and device settings.   1.3 Advantages of CBCT  1.3.1 Accessibility The two greatest reasons for dental CBCT’s popularity are the reduction in size and cost compared to medical MDCT. Dental CBCT has a smaller footprint than a MDCT and a patient can sit up or stand up rather than lie down. Due to the upright positioning, a more accurate representation of jaw positioning is obtained. A dental CBCT is not as heavy as medical CT and has a smaller footprint. Therefore, a dentist does not need to have the floor reinforced prior to dental CBCT installation. In addition, dental CBCT does not need the high-voltage electricity required by a medical CT. These advantages allow dentists to install the dental CBCT machine in their offices, thus offering a more convenient in-office imaging experience for the patient. Better accessibility and affordability have led to increased purchase and use of dental CBCT. Even     3 though the radiation from CBCT is less than MDCT, radiation protection strategy is still required for CBCT.   1.3.2 Rapid Scan Time Compared to MDCT where multiple gantry rotations are required for a single scan, a CBCT requires one gantry rotation for a scan. This advantage reduces the chance of subject movement, reduces radiation exposure because of the shortened scan time and may have encouraged the increased use of CBCT on children. Because young children often have difficulty staying still, they may need general anesthesia or sedation for medical imaging. With rapid scan time, younger children can tolerate sitting still for the approximately 10 to 20 seconds needed to complete the scan.   1.3.3 Field of View  In radiology, field of view (FOV) is the volume of the image. Many dental CBCT machines offer different fields of view to match the area of interest. The FOV options include single tooth volume (small), maxilla or mandible volume (medium) and craniofacial volume (large). The different options are available for customization for each patient and area of interest. The ability to change the FOV is a crucial tool to avoid unnecessary radiation exposure to areas that are not of interest. Every dental CBCT machine has different extent of adjustability. Most dental CBCT do not offer small, medium and large volumes. It is important to avoid irradiating areas that are not of interest, especially if the patient is young and the cells are susceptible to radiation damage. Therefore, the dentist who is interested in purchasing a dental CBCT needs to ensure that the     4 FOV options are adequate for their practice.  1.3.4 Image Accuracy CBCT studies boast high diagnostic quality images for bone imaging compared to two-dimensional conventional dental radiographs (6). Dental CBCT images are a reliable and accurate imaging modality for linear measurements that are relevant to dental applications (7, 8). During the scan, projections are obtained at predetermined projection intervals. For instance, 180 projections can be obtained from a 180° arc with a projection interval of 1°. The number of images comprising the finished image depends on the frame rate, completeness of the arc and speed of the rotation (3). The number of projections in each scan can be either fixed or variable depending on the machine. If the selected volume element (voxel) size is small, a higher number of projections are required for the three-dimensional image. The spatial resolution measured by voxel size can be changed to the desired degree of image quality and radiation. A higher number of projections enables greater spatial and contrast resolution, increased signal-to-noise ratio and reduced artifacts (3). However, the higher number of projections also means a longer scan time and higher radiation dose.  Dental CBCT allows measurements in multiple planes and is perceived to be useful for many applications such as orthodontics, implantology, oral pathology, and oral surgery. It is assumed that presumed accuracy increases the success rate of the treatment. Whether the higher accuracy from CBCT imaging significantly increases the treatment success rate has not yet been demonstrated in the literature.      5 1.3.5 Multiplanar Image The ability to generate multiplanar images is what distinguishes dental CBCT from the other dental imaging modalities. This advantage allows the clinician to visualize the areas of interest that are not physically visible and allows accurate measurements. By enhancing visualization of the craniofacial complexes, dental CBCT helps in identifying the normal and abnormal structures. However, dental CBCT is not necessarily indicated for all situations requiring precise measurements. For example, in orthodontic treatment planning, the difference in diagnostic quality between multiplanar images from CBCT and a two-dimensional cephalogram is clinically irrelevant and insignificant, thus not justifying the higher radiation from CBCT (9). There is inadequate research to demonstrate that three-dimensional or multiplanar images lead to a higher success rate in dental procedures.   1.3.6 Post-Processing The most clinically useful aspect of CBCT is the highly sophisticated yet user-friendly, easy-to-handle software that allows multiplanar images to be viewed in axial, sagittal and coronal planes. The clinician can measure lengths, distances, angles and volumes of relevant anatomical structures in multiplanar views. For instance, for an impacted maxillary canine, the exact position, the length of the canine, the angulation of the impaction, and the distance to the adjacent teeth can be determined prior to the tooth exposure surgery. Another common application of post-processing is post-treatment assessment. For example, after a cleft palate bone graft, a CBCT study can be used to determine the quality, width, length and thickness of the     6 bone graft (10). Many dental clinicians consider the pre- and post-treatment information from CBCT studies to be a valuable part of dental care.  1.4 Disadvantages of CBCT While the advantages are many, dental CBCT use should be the exception, not the norm in daily dental practice because of the higher radiation. Despite the higher image quality, dental CBCT should not replace conventional diagnostic intraoral radiographs (11). For many dental assessments, dental CBCT images contain much more information and radiation than necessary. Surprisingly, some studies consider dental CBCT as a low radiation and low cost imaging modality alternative (10, 12). One should interpret these studies with caution and be careful not to compare dental CBCT to medical CT. Dental CBCT may be used instead of MDCT in cases of suspected pathology or trauma; this practice would decrease the amount of radiation to the patient. However, most dental CBCT use is in addition to two-dimensional x-rays, not instead of medical CT. This trend increases the total radiation to patient from dental sources.  1.4.1 Higher Radiation Dose The most advertised merit of dental CBCT is that it emits reduced radiation dose, which is true in comparison to other CT modalities in medicine. The conventional MDCT requires multiple rotations for a single scan, whereas a CBCT scan only requires one rotation, thus decreasing the radiation. The conventional head CT radiation dose is reported to be approximately 314 to 2426 µSv (13, 14) This is significantly higher than maxilla and mandible dental CBCT dose of 36.9 to     7 1073 µSv (13, 15) This dose varies widely depending on the machine model, type, FOV, resolution, frame rate, sensor sensitivity, scan time, kVp and mA selected.  If dental CBCT use were limited to replacing medical CTs for craniofacial visualization, the total radiation dose to the patient would indeed be reduced (16). Radiation doses from full FOV dental CBCT scans have been reported to range from 2-23% of doses of MDCT exams (17). However, dental CBCT often does not replace existing imaging modality but is supplemental to conventional radiographs such as bitewing, periapical, panoramic and cephalometric radiographs. The radiation doses from full FOV dental CBCT scans have been measured to be 4 to 42 times the dose from a panoramic radiograph (17). Therefore, the clinicians ordering dental CBCT should be mindful that frequent use of supplemental CBCT increases the total radiation dose to patients.   1.4.2 Field of View (FOV) The FOV and scanned volume are dependent on the size of the detectors. In dental CBCT machines, every machine has a different detector size that is fixed and cannot be changed. Some machines have options for changing the FOV. In practice, the dental CBCT machines can be classified by their FOV size. Large scans visualize areas of craniofacial anatomy such as base of skull, brain, maxilla and mandible. Medium scans visualize maxilla and/or mandible. Small over focused scans are used for small areas of interest in a dentoalveolar lesion. Radiation exposure to the base of skull and brain is unnecessary in most dental cases. In a small child or an adult with a small head, the area outside the tissues of interest can easily be exposed with fixed diameter and large FOV. By reducing the FOV height, you can prevent exposing structures that are not of     8 interest. On the other hand, the Kodak 9000 CBCT has a small FOV and produces a cylinder shaped three-dimensional image of 50 mm in diameter and 37 mm height. In machines that have small fixed FOV, the machine can be adequate for visualizing a small area of interest. If a larger area needs to be visualized, the operator may have to take multiple scans and stitch the images together. However, stitching may lead to increased dose because overlapping scans of area of interest are required for optimal stitching. Positioning a patient is challenging with a small FOV. If an operator is not skilled enough to obtain the required image or if the patient moves during the scan, a re-scan may be necessary. A dental CBCT machine with customizable FOV would allow optimization of image quality and total radiation to patient if used by a knowledgeable dentist. Therefore, the operator should be familiar with dental CBCT and have appropriate training in order to prevent unnecessary re-takes.  1.4.3 Artifacts An artifact is an artificial image that is not normally present but caused by external manipulation. Artifacts can prevent correct interpretation of CBCT images, resulting in a possible re-take. Four types of artifacts are x-ray beam hardening, patient-related, scanner-related and cone beam related.  The first artifact is x-ray beam hardening which is due to the increase in average energy level of the beam when the low-energy protons are filtered out. The beam hardening artifact results in distortion of metallic structures, and streaks and dark bands can appear between two dense objects (6). Despite the reduced image quality, Esmaeili et al. still recommended the use of CBCT for patients with extensive restorations, multiple prostheses or implants (6). Some ways to     9 reduce beam hardening are reduction of FOV to avoid scanning metal restorations or implants, collimation, modification of patient positioning or scanning the dental arches separately.  The second artifact is patient-related. Movement of the patient during the exposure causes patient-related artifacts in the image. Using a head restraint and shortening the scan time can reduce this impact of patient movement.  The third artifact is scanner-related. Circular-shaped radiopacities appear due to scanner detection imperfections or poor calibration (3). This problem will persist and show at every scan. Lastly, there are three cone beam related artifacts. The first is partial volume averaging, which occurs when the size of object is smaller than voxel size. The resultant image shows a “step” appearance(18). By selecting the smallest voxel, the more accurate image of the object can be obtained and the partial volume averaging effect can be reduced. Secondly, under sampling where a reduced number of data samples constitute a scan leading to sharp edges and noisy images. Lastly, there is cone-beam effect, which is most prevalent in the periphery portions of the scan where the information is reduced due to the divergence of the x-ray beam.  Image artifacts can make CBCT images unsuitable for dental assessment and diagnosis, especially if the patient’s dentition is heavily restored. A dentist should take suggested precautions to prevent artifacts from appearing on images. If artifacts are present, they should be identified and a decision should be made about necessity of re-takes.  1.4.4 Limited Soft Tissue Imaging Dental CBCT scans usually reveal only limited internal soft tissue information and low-contrast resolution due to poorer bit-depth. Therefore, locally invasive lesions such as ameloblastomas     10 require the use of contrast medium for visualization (19). Unfortunately, contrast media cannot be used in CBCT scans because contrast medium is delivered intravenously while the patient is in a supine position. Most dental CBCTs require the patient to sit upright, which of course prevents the delivery of the medium. In practice, no dentist would deliver an IV contrast as an in-office procedure because of possibility of syncope and allergic reactions. Due to the limited ability to image soft tissue and the impracticality of IV delivery, other imaging modalities such as medical CT, MRI or ultrasound should be considered as alternatives.  1.4.5 Hounsfield Units Hounsfield units (HU), also called CT numbers, are a quantitative scale for radiodensity. Air is represented by a minimum of -1000 HU, water is 0 HU and dense bone or metal can be up to 3000 HU (19). In current dental practice, HU are not used as part of bone density analysis. Some investigations suggest that HU should be determined for dental CBCT studies (20, 21). On the other hand, others argue against the use of HU in dental CBCT due to the variability of CBCT machines and the lack of calibration (22, 23). Without universal calibration, the same bony area can have different gray scale depending on the reconstruction algorithm (4).  1.5 Increasing Use of CBCT in Dentistry The use of CBCT in dentistry has recently increased because machines have become commercially available for private practice. CBCT has created a revolution in dentistry because the imaging is not only used for diagnosis but also guides surgical procedures. It is a new trend that affects all disciplines of dentistry including oral and maxillofacial surgery, endodontics, oral     11 pathology, implantology and orthodontics as part of diagnosis, surgical procedure and post-treatment assessment.  Conclusions of a 2009 systematic review demonstrated that the most common uses of dental CBCT are for maxillofacial surgery (41%), dentoalveolar pathology (29%), orthodontics (16%) and implantology (13%) (4). Other applications included endodontics, periodontics and forensic dentistry. The most common reasons for CBCT were for assessment of impacted canines, third molars, and mesiodens. Other clinical indications included supernumerary teeth, assessment of dentoalveolar trauma, root resorption and foreign body (4). In maxillofacial surgery, CBCT scan was most commonly performed for temporomandibular joint assessment and for arthrography to measure the thickness of the glenoid fossa, despite the fact that MRI is the preferred diagnostic imaging tool. Other frequent indications were the investigation of odontogenic cysts and tumors, trauma, cleft pathology, orthognathic surgery, intra-operative imaging, navigation, oral cancer, osteomyelitis, bisphosphonate related osteonecrosis of jaw and obstructive sleep apnea (4).  For cleft pathology, CBCT allows evaluation of alveolar bone grafts and assessment of nasal deformity that two-dimensional imaging cannot provide.  For orthodontic purposes, mini-implant, cephalometry and tooth positions were the most common indications for CBCT scans (4). According to a survey of American and Canadian postgraduate orthodontic programs, 73.3% of the programs use dental CBCT on a regular basis (5). However, in a systematic review that examined the role of CBCT in orthodontics, the authors concluded that CBCT does not offer better diagnostic potential, treatment planning or treatment outcome compared to conventional imaging modalities (24).  For placement of a     12 temporary anchorage device (TAD), the thickness of palate or mandible is measured from the scans to determine safe sites for placement. Information regarding tooth position and inclination is important prior to mini-implant placement surgery. However, in a systematic review, CBCT use for TAD did not result in better outcomes (4). The only area that CBCT added value was for airway diagnostics such as sleep apnea.  Finally in implantology, implant planning and surgical guidance template assessment were the main reason for CBCT use (4). Certainly, dental CBCT has many use criteria for multiple applications. Affordability and compact size have increased accessibility to dental CBCT. Unfortunately, well controlled studies and guidelines were not in place before the rapid uptake of dental CBCT by private dental practitioners (25). What is most concerning is that no specific patient selection criteria have been established, and in some clinics improper criteria have been developed. This situation likely results in increased and unnecessary radiation to the population. It is very important that the clinicians respect the “As Low As Reasonably Achievable” (ALARA) principle when using dental CBCT to justify the additional radiation from dental CBCT compared to conventional radiographs. As an extension to ALARA principle, the Alliance for Radiation Safety in Pediatric Imaging also known as “Image Gently” was founded in 2008 to advocate two imaging principles: to use professional judgment in selecting imaging and to ensure ALARA principle is employed for children without losing diagnostic quality (26). At the present time, no well-constructed studies have demonstrated that the use of dental CBCT has increased the quality of patient care. A systematic review that examined the efficacy of     13 CBCT concluded that the benefit of CBCT to dental care is too minimal to justify the amount of radiation (24). Another problem is that dental CBCT machines are mainly purchased and utilized by specialist dentists rather than radiologists (27). This situation may lead to medico-legal issues, as dental clinicians often lack the comprehensive knowledge and experience to interpret the scan (28). For CBCT machines with a large FOV, regions outside of the orthognathic areas such as base of skull and neck are irradiated. In these cases, a medical radiologist should be analyzing the CBCT studies.  Some dental CBCT machines also have a panoramic feature. This feature is attractive for clinicians since only one machine is needed for both CBCT and panoramic radiographs. The panoramic image reconstructed from CBCT is considered to be superior to a conventional panoramic radiograph because the CBCT-generated image does not display superimposed secondary images and is considered to be geometrically more accurate (19). Specifically, reconstructed panoramic images taken by a dental CBCT lacks superimposition of vertebrae or secondary image from mandible angle from the opposite side. There have been limited studies regarding radiation dose of panoramic radiographs taken using a CBCT machine compared to the panoramic radiographs taken using a conventional panoramic machine.   1.6 Different CBCT Machines and Imaging Protocols Great variations exist among CBCT machines that are commercially available for the maxillofacial region. Brand names include NewTom, PSR 9000, 3D Accuitomo, Arcadis Orbic, i-CAT, CB Mercuray, Ortho-CT, Galileos, Powermobil, Siremobil, DentoCat, CB Throne,     14 PreXion, Kodak and Carestream. There is an ongoing influx of newer models in the market, and it is difficult to evaluate, compare and calibrate all dental CBCT devices. Any variation in dental CBCT imaging protocol can result in differences in reported dose. In many dental CBCT studies, doses are reported as effective dose despite many shortcomings of this measurement. Effective radiation doses in microsieverts from different CBCT units are summarized in Table 1. The amount of radiation varies greatly depending on the machine, models, imaging protocols and location of dosimetry in the phantom. Even for the same machines and same imaging protocols, different studies report different values for the effective doses. The imaging protocols that are reported in the literature include values for tube voltage (kV), mA, gray scale depth, exposure control, exposure time (seconds), scan time (seconds), radiation source, exposure time per image, rotation angle (degree), projections per rotation, detector type, detector size, FOV, scanned volume dimensions, scanned volume height, scanned volume diameter, pixel size, patient positioning, proprietary software, total filtration, front panel attenuation and software used. The greatest discrepancies were shown in exposure time, scan time, FOV, detector size, scanned volume and pixel size (17). It is a fine balance between imaging quality and radiation because the amount of radiation increases as image quality increases. The optimization point is different for every machine and imaging protocol. Being familiar with optimization of imaging quality and radiation dose is an important step towards reducing radiation.  1.7 Radiation Risk Radiation is the emission of energy as electromagnetic waves. The radiation under discussion in this study is ionizing radiation which produces charged ions, and can have harmful effects on     15 human health.  Ionizing radiation can come naturally from radioactive materials and cosmic sources or artificially from x-ray tubes and particle accelerators (29). The carcinogenic effects of high radiation exposure are well studied, whereas the extent of the carcinogenic effect of low dose diagnostic radiation is not yet well known (30).   1.7.1 Deterministic and Stochastic Effects The two effects of ionizing radiation are deterministic and stochastic. The deterministic effects occur above a threshold dose. These effects include radiation burns, radiation syndrome, cataracts and radiation-induced thyroiditis (31). For the stochastic effect, the probability of occurrence is dependent on absorbed dose. However, the severity of stochastic effect is independent of absorbed dose. The stochastic effect can result in damages to DNA, cellular death, mutagenesis, teratogenesis, genetic transformation, cognitive decline and heart disease (31). In a study that evaluated 49 healthy children after CBCT for orthodontic assessment and conventional radiographs, cytotoxicity was verified (32). Both deterministic and stochastic effects are serious and neither is worse than the other. Deterministic effects are not likely from dental radiation exposure. The probability of a child seeing stochastic effects from a single CBCT scan is very low. However, the probability will increase with repeated CBCTs. In our study, we assume the linear no-threshold (LNT) model of radiation-induced carcinogenesis. This theory states that there is no threshold below which radiation risk is zero. The International Commission on Radiological Protection (ICRP) is an independent and international organization that provides recommendations and guidelines on radiation protection.      16 In the latest recommendations on radiological protection, ICRP 103 uses the LNT model to estimate the radiation risk (2).    Table 1. Comparison of Effective Doses from Different Dental CBCT Units  Maxillofacial region (Large FOV) Dentoalveolar region (Medium FOV) Localized region (Small FOV) CBCT units Effective Dose (µSv) CBCT units Effective Dose (µSv) CBCT units Effective Dose (µSv) NewTom 3G 682 573     NewTom VGi 1941 NewTom VGi 2651   i-CAT NG 831 742 773 i-CAT Standard 691 34 (lower jaw) 3 77 (upper jaw) 3   Illuma Standard 982     Illuma Ultra 4982     Galileos Comfort 841 Galileos default 702   Kodak 9500 1631 Kodak 9500 921 Kodak 9000 3D (upper jaw) 191     Kodak 9000 3D (lower jaw) 401     ProMax 3D 4882   3D Accuitomo  541 3D Accuitomo (upper jaw) 431 29 (front) 3 44 (premolar and canine) 3       CB Mercuray Standard 5692 CB Mercuray 5602 CB Mercuray2 4072 CB Mercuray Maximum 10732     1. Pauwels R, Beinsberger J, Collaert B, Theodorakou C, Rogers J, Walker A, et al. Effective dose range for dental cone beam computed tomography scanners. Eur J Radiol. 2012 Feb;81(2):267-71 2. Ludlow JB, Ivanovic M. Comparative dosimetry of dental CBCT devices and 64-slice CT for oral and maxillofacial radiology. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2008 Jul;106(1):106-14. 3. Loubele M, Bogaerts R, Van Dijck E, Pauwels R, Vanheusden S, Suetens P, et al. Comparison between effective radiation dose of CBCT and MSCT scanners for dentomaxillofacial applications. Eur J Radiol. 2009 Sep;71(3):461-8.     17 1.7.2 Measuring Radiation Absorbed dose is the energy of ionizing radiation that is absorbed per unit mass. It is measured with commercially available dosimetry equipment with high reproducibility. It is denoted by DT, and its units are Gy and Rad. 1 Gy means 1 joule of energy is absorbed per kg of matter. Rad stands for radiation absorbed dose. 1 rad is equal to 0.01 Gy.  Equivalent dose is calculated by the following formula HT=Σ WRŸDT, where WR is radiation weighting factor. It is 1 in the case of X-ray radiation. Therefore, for x-rays, 1 Sv is equal to 1 Gy. In this formula, the unit for HT is Gy.  With a phantom model, the absorbed dose is calculated by using a radiation measurement probe inserted into specific locations in a phantom. The above formula is used to calculate the absorbed dose for each imaging protocol and location.  The problem with studies that examine the radiation dose is that often they use a commercially available adult phantom. This “one size fits all” adult phantom does not give clinicians insight into the radiation dose to the pediatric patient. Estimating pediatric dose based on the adult phantom has been shown to underestimate the pediatric radiation doses (33). In our study, custom adolescent and small child phantoms were developed to prevent the underestimation of pediatric radiation doses.   1.7.3 Limitations of Effective Dose  In dentistry, there is advocacy for the use of effective dose in dosimetry of CBCT. The effective dose measures the relative contribution of the different organs and tissues to the overall risk of radiation detriment and estimates the probability of cancer induction and genetic effects of     18 ionizing radiation (4). It is not a measure of radiation, but is a measure of adverse health risk from radiation. Another limitation of effective dose in dosimetry is that the measurements do not differentiate between gender and age. Therefore, it is not an accurate way to determine the radiation dose. In addition, the effective dose measurements are not easily obtainable for a practitioner and are not applicable to pediatric populations. Measuring the effective dose has several limitations. Traditionally, the thermoluminescent dosimeter (TLD) and RANDO phantoms are used to measure the organ and total effective doses. Using the TLD technique is a labour-intensive procedure without standards regarding the number and location of measuring points (34). Some studies show that effective doses measured with TLDs have low reproducibility, especially for those TLDs placed on the theoretical skin surface. The low reproducibility can be attributed to the fact that it is difficult to align the locations of organs of interest and TLD rods (35). The effective dose is calculated by the following formula E=Σ WTŸHT, where WT  is t weighting factor and HT is the equivalent dose (Table 2). Table 2. Tissue-Weighting Factors for Calculation of Effective Dose – ICRP 1990 and 2007 Recommendations  for Tissue in the FOV During CBCT exposure: Dental Related Select Tissues (1, 2) Tissue 1990 WT 2007 WT Bone Marrow 0.12 0.12 Esophagus 0.05 0.04 Gonads 0.20 0.08 Thyroid 0.05 0.04 Bone Surface 0.01 0.01 Brain Remainder 0.01 Salivary Glands - 0.01 Skin 0.01 0.01      19   There are two sets of ICRP tissue weighting factors, one published in 1990 and another in 2007, resulting in two different values for tissue weighting factor. It is imperative to indicate which version is used in a study. The biggest limit of effective dose is that the tissue weighting factors do not reflect individual patient risk. The measurements in the phantom do not correspond to patients, because the effective atomic number and mass density of tissues and organs vary greatly (35). The radiation absorption differs depending on the composition of tissue at the site of radiation exposure. For example, some patients may have more fat content, while others have higher muscle content. Tissue weighting factors are mean values representing averages across both sex and age, which underestimates the radiation to more vulnerable populations such as females and children. It is well known that females are more susceptible to radiation by up to a two-fold increase (36). The risk of damage to the reproductive system and risk of cataracts are significantly higher in females than in males (31).  Younger children are more susceptible to radiation, but the effective dose does not consider this issue of age (36). Because effective dose has low reproducibility and is inappropriate for use in pediatric population, the absorbed dose will be employed in this study.  1.7.4 Dose Index In 2011, SedentexCT recommended Dose-Area-Product (DAP) as dental CBCT dose audit as it is a practical quantitative estimation of total radiation delivered during exposure. It is calculated by absorbed dose in air (mGy) multiplied by area (cm2) of x-ray beam. The unit for DAP is mGy*cm2. The i-CAT NG machine provides DAP values for each imaging protocol. DAP was     20 examined but not used in our study because it estimates radiation delivered by a machine but does not estimate the amount of radiation absorbed by the patient.  1.7.5 Radiation Risk in the General Population Radiation risk has always been a concern for the general public. However, in recent years dental radiation has not been a grave concern due to the perceived low dose of radiation per person compared to background radiation and medical exposure.  The total amount of radiation from dental examinations comprises only 0.06% of total annual radiation dose (37). However, these 5-year old statistics are not reflective of the new trend in dentistry where an increasing number of radiation exposures may be part of elective assessment and treatment.  In dentistry, the amount and effect of radiation from dental procedures have not been very clear to either the public or professionals. A popular trend in private dental practices is conversion from analog to digital radiography. Many dental x-ray machine manufacturers claim the radiation per each radiation exposure is decreased with digital radiography, hence decreasing the total amount radiation emitted to patients. The change from D speed film to digital radiography will decrease radiation and is beneficial to patients when appropriate settings are used.  What has added to the amount of radiation received by dental patients is the introduction of dental CBCT. Even though CBCT is advertised as a low dose three-dimensional imaging modality, dental CBCT study is often ordered in addition to conventional two-dimensional radiographs. The advantages of CBCT are well known, but the potential harmful effects are just starting to be publicized. Different machines have different settings for the scan and different operators use different imaging protocols. Often the imaging protocols in-use are not evidence     21 based. Rather, they are based on personal experience or manufacturer’s recommendation without credible scientific evidence. There have been some attempts to optimize the image quality using the lowest radiation possible. Unfortunately, many machines do not have an option to modify settings such as voxel size, kVp or mA, thus decreasing the operator’s ability to customize radiation dose for each patient. The balance between radiation dose and image quality should be emphasized since they are closely interrelated. As image quality improves, the radiation dose to the patient also increases. ICRP in 2001 defined low dose radiation to below 0.2 mGy per imaging study. Dental CBCT is low dose compared to MDCT, but it is important to employ ALARA principle at all times. Compared to panoramic radiographs or bitewings, dental CBCT is certainly a high dose machine. Guidelines for this sophisticated imaging modality are being developed across Canada such as Standard of Practice published by Royal College of Dental Surgeons of Ontario (RCDSO) in 2011(38). However, the radiation doses from different CBCT machines are not calibrated and imaging protocols for different patient sizes and ages are not yet available. These guidelines are for the general population and do not necessarily reflect the need for a different limit for the pediatric population.    1.8 Radiation Risk in Pediatric Population The relatively high radiation dose from dental CBCT in the pediatric population has given rise to growing concern of a possible increase in future cancer risk. Cheng et al. determined that the additional secondary cancer risk from a CBCT scan may be as high as 2.8% (39). In current dental practices, low-dose radiation exposures are of interest. In 2006, the BEIR VII report stated     22 that children are more susceptible to carcinogenesis after low-dose radiation (29). It is important to study the absorbed radiation dose in pediatric patients for the following reasons.  1) The dose to patient is a function of radiation from the x-ray source and the size of patient (33).  In a larger patient, the superficial tissues attenuate the radiation beam, decreasing radiation to the centre. With the smaller head size in pediatric patients, there is less attenuation of radiation and more radiation reaches the centre (36).  2) Radiation has a cumulative effect over a lifetime. When pediatric patients are exposed to radiation from any source, the stochastic effects remain for a lifetime. The concept of acceptable radiation limit is misleading because there is no safe amount of radiation allowable per year. There is no lower radiation limit at which detrimental effects clinically take place.  3) Children have a longer life span after radiation. Thus, there is a longer time period for the stochastic effects to result in detrimental medical conditions such as a genetic mutation or cancer (40).  4) Children are more sensitive to radiation (41).  The size and position of tissues of children are different from adults. The tissues and cells are rapidly undergoing growth, differentiation, replication and division, making them more prone to the damage of radiation (25). Thyroid, breasts and gonads are especially sensitive to radiation in children. The cancer risk per sievert is highest at a younger age and decreases with age (42).      23 Awareness of radiation risk to the pediatric population is surprisingly low. Many machines are not equipped with functions to customize settings for pediatric patients. New studies are investigating the positive relationship between dental radiation and risk of meningioma (43). Such studies are challenging to conduct because it is difficult to distinguish the effects of dental radiation from the effects of other radiation sources, including background radiation. In medicine, there is a plethora of studies on excessive medical radiation. Recent publications report that radiation exposure from MDCT scans in childhood can increase the risk of cancers such as leukemia and brain tumors. In a retrospective cohort study, 180,000 young people were followed for 17 years in the United Kingdom. It was concluded that cumulative doses of 50 mGy can triple the risk of leukemia and 60 mGy can triple the risk of brain cancer (44). A recent Australian study investigated cancer incidence rate by following 680,000 people for 20 years and recorded CT exposure in childhood or adolescence. The incidence of cancer was 24% higher in exposed compared to the unexposed group (45).   1.9 Radiation Risk in Medically Compromised Pediatric patients with medical comorbidities often have extensive history of radiation exposure either for diagnostic or treatment purposes. Since the higher the radiation dose and the younger the age at exposure increase the risk of cancer, the dental practitioner has to be mindful of pediatric patients who may have a history of extra radiation exposure. In dentistry, the patient with previous exposures to the head and neck is of particular concern. High risk groups include patients with 1) history of MDCT imaging of brain, neck, upper spine or facial area due to neoplasms; 2) craniofacial abnormalities because these children may have a series of imaging     24 studies to assess the extent of abnormality, to treatment plan for the reparative surgery, and to monitor healing; 3) history of head and neck trauma, where a MDCT or CBCT was required to assess the bone fractures; and 4) exposure to high dose and multiple fractions of radiation therapy for cancer. When managing these vulnerable groups of medically compromised pediatric patients, the dental professional should actively reduce the radiation dose and attempt to avoid unnecessary radiation.   1.10 Dose Reduction Strategies Anytime a patient is exposed to radiation the ALARA principle should be respected (46). When using dental CBCT, different modifications can be made to adhere to the ALARA principle.  1. The dental professional should be familiar with dental CBCT devices. It is important to optimize the imaging protocol to balance radiation and image quality for different age groups and gender (47). The radiation dose is optimized when there is least amount of radiation emitted to provide diagnostic image quality (48). Many CBCT machines, but not all, allow modification of kVp, mA, voxel size and FOV, which in turn can optimize the radiation emitted. Always use the smallest FOV to see the area of interest and reduce the exposure time, kVp and mA if possible. Knowing the CBCT machine includes doing appropriate research before purchasing the dental CBCT. An informed consumer should request information from manufacturers regarding the estimated radiation dose for each imaging protocol, ability to modify the imaging protocols to optimize for children, and any research articles that are associated with the specific model of CBCT machine.      25 2. The dental professional should be familiar with guidelines and help develop guidelines and imaging protocols for dental CBCT use in pediatric patients. There are new guidelines such as RCDSO 2011 and the SedentexCT project that discusses radiation protection that is specific for CBCT for oral and maxillofacial radiology (38, 49). There is a movement in medical radiology to increase awareness of radiation safety in pediatric populations, which can be seen in campaigns such as Image Gently (40). 3. Quality standards and quality assurance protocols, as well as regular calibration of the dental CBCT machine, should be in place for use in dental practices. Proper training of staff can promote radiation safety. 4. Appropriate justification for referral for dental CBCT is essential. There are many different ways to obtain the information necessary for treatment without exposing pediatric patients to higher radiation from dental CBCT. In current dental practice, there is no consensus on patient criteria for dental CBCT, and referral is solely based on the dental practitioner’s clinical judgment.  5. Correct positioning of the patient can reduce the radiation and prevent re-takes. In many clinical situations, laser light beam markers are used to position patients. Maintenance programs and regular calibrations are required to ensure the laser light beams are actually aligned with the radiation sensor. The use of immobilization devices such as a bite guard, chin rest and head positioner can be especially useful in the pediatric population to reduce movement and moving out of optimal position.  6. Using the preview function of the dental CBCT helps the operator to verify the positioning of the machine. Children who are unfamiliar with the CBCT machine can     26 make sudden movements due to surprise or fear of the machine. The “dry run mode” without x-rays can help the child understand and become familiarized with the procedure prior to radiation exposure, thus preventing sudden movements during the actual radiation exposure. 7. Shielding can significantly reduce the radiation to the patient. The use of protective cover such as appropriately child sized lead apron can significantly decrease radiation to radiosensitive and rapidly dividing cells in children. In a study that examined the effect of leaded glasses, thyroid shield and collimation on dental CBCT radiation in an adult female phantom, collimation alone reduced the radiation dose to the brain by up to 91%, the leaded glasses reduced radiation to the lens of the eye by up to 74%, and the thyroid shield reduced radiation to the thyroid up to 42% (50). These numbers are supported by earlier studies of the thyroid collar that reduced the dose up to 40% (15, 17). The reduction of absorbed dose to the eye can reduce the risk of radiation-induced cataract development (51).  8. Manufacturers can play a significant role in dose reduction as well. A study examined effective dose before and after the manufacturer’s installation of additional filtration and kVp adjustments in the Kodak 9500 CBCT units. The changes resulted in significant dose reduction of an average of 43% at all adult settings (52). The challenges in dose reduction strategies in the pediatric population come from the lack of information, research, consensus and guidelines. In dentistry, the struggle is compounded, because the industry is largely privatized and keeping track of the CBCT imaging protocols     27 employed and their usage is very difficult. Dental professionals need to be mindful that what they do now lasts for the children’s lifetime.   1.11 Aims of Study  Part 1. Patient Survey at BC Children’s Hospital The aim of the patient survey was to use head dimensions measured from a sample of patients who received dental CBCT at BC Children’s Hospital to develop small child and adolescent phantoms.  Part 2. Phantom Dosimetry Study for CBCT The aim of phantom dosimetry study was to compare the absorbed radiation dose in small child, adolescent and adult head phantoms using i-CAT NG dental CBCT, using the most common imaging protocols at BC Children’s Hospital.  Part 3. Phantom Dosimetry Study for Panoramic Radiographs The aim of phantom dosimetry study was to determine the difference in absorbed radiation dose in small child, adolescent and adult head phantoms from dental CBCT generated panoramic radiograph and conventional panoramic radiograph.     28 Chapter  2: Materials and Methods  2.1 Part 1. Patient Survey A retrospective patient survey was conducted at BC Children’s Hospital with ethics approval (H11-03449). Inclusion criteria were patients under the age of 18 years at time of exposure who received dental CBCT scans in the Department of Dentistry at BC Children’s Hospital from August 2010 to December 2011. There were no exclusion criteria. The CBCT device at BC Children’s Hospital is i-CAT NG CBCT.  Specifically, the following data was obtained to better understand the characteristics of children who received dental CBCT scans. The information was obtained from the software i-CAT Vision.  • Gender and age of the patient at the time of exposure  • Indication for CBCT  • Previous history of dental and MDCT scans for this diagnosis (if available)  • Technique and imaging protocol employed  o Values of kVp and mA o FOV dimension o Voxel size o Scan time o Exposure time • Dose area product     29 Using the software i-CAT Vision, CBCT images were analyzed for each patient to determine the distances between selected landmarks (Table 3). We selected easily identifiable landmarks that are likely to be seen in most patients. Each set of measurements was plotted on a graph with length as x-axis and age as y-axis. Graphs were plotted to determine whether there was a correlation between age and the landmark distances. The following anatomical landmarks and corresponding definitions were used to measure select distances for anatomic analysis on CBCT studies (Tables 3, 4, 5).  Table 3. Anatomic Analysis on a CBCT Study: Skeletal Landmarks (53) Landmark Abbreviation Definition Upper Incisor  UI The lowest incisal point of upper central incisor First Cervical Vertebrae Anterior C1 The most anterior point of atlas, the first cervical vertebrae  Posterior C1 The most posterior point of atlas, the first cervical vertebrae Gonion Go The most inferior and posterior point at the angle formed by the ramus and body of the mandible Condyle Co The most superior point of the condyle head Anterior Nasal Spine ANS The spinous process of the maxilla forming the most anterior projection of the floor of the nasal cavity Posterior Nasal Spine PNS The spinous process formed by the most posterior projection of the junction of the palatine bones in the midlines of the floor of the nasal cavity Zygomatic Zyg The most lateral point of the zygomatic arch      30  Table 4. Anatomic Analysis on a CBCT Study: Distances between Skeletal Landmarks in Sagittal View Sagittal Plane  1 UI- anterior C1  2 UI- posterior C1  3 U1 to Go-Co If C1 was not captured in CBCT, a perpendicular line was drawn from U1 to Go-Co 4 Go-Co  5 ANS-PNS  6 Skeletal AP The longest anterior-posterior skeletal distance parallel to the border of CBCT study 7 Soft Tissue AP The longest anterior-posterior soft tissue distance without nose parallel to the border of CBCT study 8 Soft Tissue AP Nose The longest anterior-posterior soft tissue distance including nose parallel to the border of CBCT study   Figure 1. Anatomic Analysis in Sagittal View.      31             Table 5. Anatomic Analysis on a CBCT Study: Distances between Skeletal Landmarks in Coronal View Coronal Plane   1 Co (L-R) The distance between the left and right condyles 2 Go (L-R) The distance between the left and right gonions 3 Zygomatic Width The distance between the left and right zygomas 4 Maxillary Skeletal Width The distance between the left and right maxillares 5 Skeletal Width The widest skeletal width parallel to the occlusal plane connecting 36 and 46 buccal cusp tips 6 Soft Tissue Width The widest soft tissue distance not including ears at the level of the occlusal plane connecting 36 and 46 buccal cusp tips 7 Soft Tissue Width Ears The widest soft tissue distance including ears parallel to the occlusal plane connecting 36 and 46 buccal cusp tips  Figure 2. Anatomic Analysis in Coronal View      32 Confidentiality procedure: CBCT studies were investigated at BC Children’s Hospital using the software i-CAT Vision. From the software, the patients’ names and medical record numbers were obtained. The patient’s dental and medical charts were accessed if they were patients of BC Children’s Hospital. De-identified demographic information and measurements from CBCT images were entered into an Excel Spreadsheet. Subjects were identified by numbers assigned in the order of CBCT studies that were taken.  2.2 Part 2. Phantom Dosimetry Study The phantom study was an experimental study to measure absorbed radiation doses in adult, adolescent and small child phantoms.   2.2.1 Materials  In this study, the doses absorbed by the phantoms were measured on an i-CAT NG CBCT machine. The panoramic radiography unit was PROMAX manufactured by Planmeca, Helsinki, Finland. The radiation dosimetry equipment was RADCAL® Accu-Dose® Model 2186 Version 7.03 with a thimble ion chamber 10X6-0.6-CT and Radcal Model 9660A Ion Chamber Digitizer.      33     Figure 3. Phantom Study Set-up. (1) i-CAT CBCT and Panoramic Radiograph Set-up. (2) Close-up of the Set-up. (3) Planmeca PROMAX Panoramic Radiograph Set-up. (4) RADCAL Accu-Dose. (5) Thimble Ion Chamber  (1)    (2)    (3)    (4)      (5)      34 The adult phantom is a commercially available SedentexCT DI by Leeds Test Objects, Ltd.  The adolescent phantom and pediatric phantom were developed to allow better estimation of the radiation dose in the pediatric population. They were custom made by British Columbia Cancer Agency (BCCA) Genome Sciences Centre based on measurements obtained in the patient survey. All phantoms were made of polymethyl methacrylate (PMMA), which is a transparent thermoplastic material. PMMA shows similar energy absorption to muscle and water content of the maxillofacial region (Table 6).   Table 6. Energy Absorption per Unit Mass Provided by the National Institute of Standards and Technology (54)  Medium g/cm2 Bone 0.0980 Muscle 0.0285 PMMA* 0.0237 Water 0.0282 Air 0.0264 *PMMA: Polymethyl Methacrylate   We used the terms “small child” and “adolescent” phantom to differentiate between different sizes of pediatric patients. Our aim was to ensure that the small child phantom included the smallest dimensions of children who received dental CBCT. In our patient survey, the youngest child was 5 years of age. Since we only had one subject of this age, we could not calculate an average of head dimensions. CT Pediatric Head Dose Phantom Model 76-424-4156 is available commercially from the company Fluke Biomedical as part of a nested CT phantom that is used for clinical CT to measure body and head doses. This pediatric head phantom had a diameter of     35 100 mm, so we fabricated our own phantom with the same dimensions. Using the dimensions of the commercially available phantom will ensure that other studies can easily replicate our study design and validate the results. The diameter of 100 mm is a conservative value compared to the measurements from five to seven year olds in our study. Four children who were five to seven years of age had horizontal width range of 107 to 139 mm, and anteroposterior distance range of 143 to 151 mm.  In our study, the smallest value for the horizontal width was 104 mm for a healthy 9 year old. Having a phantom with a diameter of 100 mm would ensure that absorbed dose for a very small child is accounted for.  The size of the adolescent phantom was obtained using data from the patient survey. The most common indication for dental CBCT was orthodontic issues such as transposed or impacted canines. Since most children without skeletal abnormalities initiate orthodontic intervention between ages 11-14, the average head dimensions of children of these ages was determined (55-57). The calculated value from 12 patients was 134 mm in the coronal plane and 149 mm in sagittal plane. Since our adult phantom had a diameter of 150 mm, we fabricated an adolescent phantom with a diameter of 135 mm. The number was rounded up to the closest 5 mm for ease of fabrication. The figure and table below show the actual measurements and relative size of adult, adolescent and small child phantoms (Figure 4 and Table 7).  Figure 4. Head Phantom Size Comparison from the Top View. (1) Adult: Diameter of 150 mm, (2) Adolescent: Diameter of 135 mm, (3) Small Child: Diameter of 100mm              (1)               (2)                (3)      36   2.2.2 Methods Each phantom was centred in the FOV with the ion chamber positioned at the midline of the image height using laser alignment lights. The probe hole close to the perimeter was rotated by 90 degrees between scans to measure the radiation in theoretical patient right, left, front and back. Each phantom has one probe hole in the centre.  The median point between the centre and surface were also measured in theoretical patient right, left, front and back. In addition, surface radiation was measured to estimate the skin dose in theoretical patient right, left, front and back (Figure 5). Measurements were made three times in each position of the phantom, and an average was used for comparison and statistical analysis. This protocol is in accordance with Health Protection Agency (HPA)’s 2010 recommendation for testing radiation output repeatability averaging a minimum of three measurements (58).        Table 7. Description of Phantoms Phantom Source Material Diameter (mm) Height (mm) Probe Holes Adult SedentexCT DI, Leeds Test Objects, Ltd. PMMA* 150 160 5 Adolescent BCCA** Genome Sciences Centre PMMA 135  150 5 Small Child BCCA Genome Sciences Centre PMMA 100  150 5 *PMMA: Polymethyl Methacrylate  ** BCCA: British Columbia Cancer Agency      37  2.2.2.1 Phantom Dosimetry Study for CBCT Two CBCT imaging protocols were chosen, as they were the most common protocols in practice at BC Children’s Hospital. The most commonly used protocols for the i-CAT are presented in Table 8. All phantoms were imaged with both protocols.   Table 8. Common CBCT Imaging Protocols. The most common protocols for the variable values  Common Imaging Protocol #1 Common Imaging Protocol #2 kVp 120 120 mA 5 5 FOV 60 mm 130 mm Resolution (Voxel Size) 0.4 mm 0.4 mm Exposure Time  4 seconds 4 seconds   Figure 5. Top View of a Phantom Head. The radiation was measured in 13 different locations where the probes were inserted. The radiation was measured three times in each spot and its average was obtained.                  38 2.2.2.2 Phantom Dosimetry Study for Panoramic Radiograph Two panoramic radiography devices were used to measure and compare the absorbed radiation for adult, adolescent and small child head phantoms. The first machine was the i-CAT where the panoramic function was used to generate an image. The two available settings for the panoramic radiography using i-CAT are presented below (Table 9). For the adult phantom, “Large” protocol was used with i-CAT. For the adolescent phantom, both “Large” and “Small” protocols were used with i-CAT. For the small child phantom, “small” protocol was used.  Table 9. Panoramic Exposure Values Employed at BC Children’s Hospital Settings Acquisition Time (s) kVp mA Small 18.3 84 5 Large 20 94 5   The second machine was Planmeca Panoramic Machine. The acquisition protocol was selected based on the University of British Columbia Dentistry guidelines for panoramic radiographs as shown below (Table 10). The protocol is selected by patient age and size. The laser alignment lights were used to line up with the centre of the phantoms. Since the phantom cannot be placed on to the chin rest or close to the bite guide, it was positioned as close as possible to the chin rest.  For the small child phantom, the “Small child up to 6 y.o” protocol was used with the exposure time of 14 seconds, kVp of 60 and mA of 4. For the adolescent phantom, the protocol “Child 7 to 12 y.o” and “Adolescent” were used. For the adult phantom, the “small adult” protocol was used to ensure we account for all sizes of pediatric patients were accounted for (Table 10).      39   2.3 Statistical Analysis and Uncertainty Linear regression (R2) was calculated for head measurements in the patient survey. R2 values are included in Figures 9 to 22. For phantom dosimetry study, standard deviation of the three values in each imaging protocol was calculated to determine the variation from the average. The standard deviations are included in the tables with the ± symbol in front of the value.  Table 10. Panoramic Exposure Values Employed at University of British Columbia Dental Clinic Patient Type Time (s) kVp mA Small child up to 6 y.o 14 60 4 Child 7 to 12 y.o 14 62 5 Adolescent 16 64 7 Small Adult 16 66 9      40 Chapter  3: Results  3.1 Part 1. Patient Survey 3.1.1 Sample Size, Gender and Age Thirty two children, 16 boys and 16 girls, met the inclusion criteria for the study from August 2010 to December 2011. No patient was excluded from the study. The age range was 5 to 17 years of age. The age distribution is plotted in Figure 6.   Figure 6. Age Distribution of Thirty-two Children who Met the Inclusion Criteria   3.1.2 Indication for CBCT The indication for CBCT ranged from orthodontic assessment, craniofacial abnormality, cleft lip and palate, pre- and post-surgical assessment and oral pathology (Figure 7).   0	  1	  2	  3	  4	  5	  5	   6	   7	   8	   9	   10	   11	   12	   13	   14	   15	   16	   17	  Number of Patients Age (year) Age Distribution     41  The term orthodontic patients refer to those who were referred for CBCT by an orthodontist. Specific indications in this category included mesiodens (1 patient), congenitally missing teeth (1 patient), impacted canines (8 patients), transposed canines (1 patient) and unknown (2 patients). The most common reason for referral was impacted canines (n=8). Craniofacial abnormalities included Goldenhar syndrome (4 patients), Apert syndrome (1 patient), hemifacial microsomia (1 patient), nevoid basal cell carcinoma syndrome (1 patient) and lacrimo auriculo dento digital (LADD) syndrome (1 patient). Many of these patients with craniofacial abnormality had cleft lip and/or palate. However, these children were not included in the referral for cleft lip and/or palate unless the CBCT was specifically for the cleft. Similarly, if they were to undergo surgery to correct features associated with a syndrome, they were only included in the craniofacial abnormality group. If the patients with craniofacial abnormalities were having surgery for other reasons, they were included in the indication for surgical assessment. There were six patients referred for assessment of existing or Figure 7. Indication for Dental CBCT in Thirty-two Patients at BC Children’s Hospital       42 repaired cleft lip and/or palate. Oral pathology referrals included assessment of isolated entities such as head and neck lymphatic malformation (1 patient) and radiolucent lesion (1 patient). If the patient had oral pathology but required oral surgery, the patient was categorized into oral surgery referrals, which included conditions such as chronic recurrent multifocal osteomyelitis, Le Fort surgery and mandible advancement surgery.  In summary, there were two main groups of patients who received dental CBCT at BC Children’s Hospital over 17 month period (2010 – 2011). Nineteen patients were receiving dental CBCT for medical and dental reasons as an alternative to head MDCT. These patients tended to be young and pre-adolescents. Thirteen patients were presumably healthy teenagers who were receiving dental CBCT as part of orthodontic planning and assessment. Since these 13 healthy patients were referred from orthodontists in the community, they did not have a record at the BC Children’s Hospital.   3.1.3 Previous Radiation Exposure  Thirteen patients were from private orthodontic practice; therefore we could not determine their previous radiation exposure from MDCT or dental CBCT. Of the remaining 19 patients, 6 had recorded previous CT radiation exposure for previous dental surgery for impacted teeth, cleft palate or retrognathism. It was not charted whether the CT radiation exposure was from a dental CBCT or a MDCT. One patient had a history of radiotherapy of pituitary gland.        43 3.1.4 Common CBCT Imaging Protocols for i-CAT The i-CAT machine had fixed values for kVp at 120 and mA at 5. The two most commonly used imaging protocols in this study were: 1) 60 mm FOV, 0.4 voxel size and exposure time of 4 seconds and 2) 130 mm FOV, 0.4 voxel size and exposure time of 4 seconds (Table 11 and Figure 8).   Figure 8. Relative Scale of Two Most Common CBCT Imaging FOV Settings at BC Children’s Hospital   3.1.5 Dose Index The dose index is provided by the i-CAT machine manufacturers after every exposure in Dose-Area-Product (DAP) unit as follows (Table 11).  Table 11. Dose-Area-Product (DAP) for Different CBCT and Panoramic Radiograph Settings on i-CAT Setting kVp mA mAs Diameter (mm) Height (mm) Voxel (mm) Exposure time (second) Acquisition Time (second) DAP (mGy*cm2) CBCT 120 5 18.54 160 60 0.4 4 8.9 302.9 CBCT 120 5 18.54 160 130 0.4 4 8.9 623.9 Pan Large 94 5 100.04     20 146.4 Pan Small 84 5 91.7     18.3 91      44  3.1.6 Head CBCT Measurements The measurements for selected distances in sagittal and coronal planes were plotted against age to determine whether there was a relationship between the magnitude of distance and age. Measuring head dimension of patients at BC Children’s Hospital was important to ensure we accounted for different shape and sizes of patients when we fabricated small child and adolescent phantoms. In Figures 9-22, it is easy to see dimensions and range of each data set. Linear regression was calculated for each set of measurements to quantify any relationships. The distance between upper incisor and posterior point of first cervical vertebrae were measured to determine whether there was a relationship between the anteroposterior growth and age. This distance was measured in 30 out of 32 patients. In two patients, the posterior point of first cervical vertebrae was not in view in the CBCT study. Figure 9 shows that there was no significant relationship between the age and distance between upper incisor and first cervical vertebrae.      Figure 9. Distance Between Upper Incisor (U1) and Posterior Point of First Cervical Vertebrae (Posterior C1) Plotted Against Age with Linear Regression      45  The distance between upper incisor and anterior point of first cervical vertebrae were measured to determine whether there was a relationship between the anteroposterior growth and age. This distance was measured in all 32 patients. Figure 10 shows that there was no obvious relationship between the age and length between upper incisor and first cervical vertebrae.   The distance between maxillary anterior and posterior nasal septum was measured to determine whether there was a relationship between the anteroposterior growth and age. This distance was measured in 26 out of 32 patients. Figure 11 shows that there was no obvious relationship between the age and distance between anterior and posterior nasal septum.   Figure 10. Distance Between Upper Incisor (U1) and Anterior Point of First Cervical Vertebrae (Anterior C1) Plotted Against Age with Linear Regression    Figure 11. Distance Between Maxillary Anterior Nasal Septum (ANS) and Posterior Nasal Septum (PNS) Plotted Against Age with Linear Regression      46  The largest skeletal anteroposterior distance shown on CBCT study was measured to determine whether there was a relationship between the anteroposterior size and age. In two patients either anterior or posterior end of skeletal structure was not in view for the CBCT study. This distance was measured in 30 out of 32 patients. Figure 12 shows that there was no obvious relationship between the age and anteroposterior size.     The largest anteroposterior soft tissue distances including nose (figure 13) and excluding nose (figure 14) were measured to determine whether there was a relationship between the anteroposterior soft tissue growth and age. The distance including nose was measured in 26 out of 32 patients whereas the distance excluding nose was measured in all 32 patients. Figures 13 Figure 14. The Largest Anteroposterior Soft Tissue Distance Excluding Nose Plotted Against Age with Linear Regression  Figure 13. The Largest Anteroposterior Soft Tissue Distance Including Nose Plotted Against Age with Linear Regression Figure 12. The Largest Skeletal Anteroposterior Distance Plotted Against Age with Linear Regression       47 and 14 show that there was no obvious relationship between the age and the largest anteroposterior soft tissue distance excluding or including nose.   The distances between left and right condyles and gonions were measured to determine whether there was a relationship between the coronal hard tissue growth and age. This distance between condyles was measured in 22 out of 32 patients and the distance between gonions was measured in 28 out of 32 patients. Figures 15 and 16 show that there were no obvious relationship between the age and the distance between left and right condyles or gonions. Figure 15. Distance Between Left and Right Condyles Plotted Against Age with Linear Regression  Figure 16. Distance Between Left and Right Gonions Plotted Against Age with Linear Regression  Figure 17. The Widest Left and Right Zygomatic Width Plotted Against Age with Linear Regression  Figure 18. Maxillary Width Plotted Against Age with Linear Regression       48 The widest left and right zygomatic width and maxillary width were measured to determine whether there was a relationship between the coronal growth and age. The zygomatic width distance was measured in 20 out of 32 patients and the maxillary width was measured in 30 out of 32 patients. Figures 17 and 18 show that there was no obvious relationship between the age and the widest left and right zygomatic width or maxillary width.   The widest skeletal horizontal width was measured to determine whether there was a relationship between the coronal skeletal growth and age. This distance was measured in 31 out of 32 patients. In one patient, the most lateral point on either side was not in view of CBCT study. Figure 19 shows that there was no obvious relationship between the age and the widest skeletal horizontal width.          Figure 19. The Widest Skeletal Horizontal Width Plotted Against Age with Linear Regression   Figure 20. The Widest Soft Tissue Horizontal Widths with Ears Plotted Against Age with Linear Regression   Figure 21. The Widest Soft Tissue Horizontal Widths without Ears Plotted Against Age with Linear Regression       49 The widest soft tissue horizontal width with and without ears were measured and plotted against age to determine whether there was a relationship between the coronal soft tissue growth and age. The distance with ears was measured in 29 out of 32 patients and the distance without ears was measured in 31 out of 32 patients. Figures 20 and 21 show that there were no obvious relationship between the age and the widest soft tissue horizontal widths with or without ears.   Lastly, the distance between gonion to condyle was measured to determine whether there was a relationship between the vertical growth and age. This distance was measured in 22 out of 32 patients. Figure 22 shows that there was no obvious relationship between the age and distance between gonion and condyle.   3.2 Part 2. Phantom Dosimetry Study  3.2.1 Conversion of Units The radiation exposure was measured in mR three times in each location. The average of three measurements was calculated. The average was converted to mGy in air using the formula (59):   Average Radiation Dose (mR) x 0.00876 = Average Radiation dose in Air (mGy) Figure 22. Distance Between Gonion and Condyle Plotted Against Age with Linear Regression       50 Then, the average radiation dose in Air (mGy) was converted to the average radiation dose in PMMA (mGy) using the formula (59):    3.2.2 CBCT Radiation The most radiation was measured in the small child phantom, the second highest radiation was measured in the adolescent phantom and lastly, the lowest radiation was measured in the adult phantom. For all three phantoms, the highest absorbed radiation was measured at the periphery  Average Radiation Dose in Air (mGy) x 0.87 = Average Radiation Dose in PMMA (mGy) Table 12. i-CAT CBCT Radiation Measurement with Head Phantoms: Protocol 1. FOV 60 mm.160 mm diameter, 120 kVp, 5 mA, 0.4 voxel, 4s exposure time. The center, middle and periphery measurements are in PMMA and the surface measurements are in air. Average Value is in Bold. Standard Deviation is Denoted by ±. Phantom Location Average Absorbed Dose (mGy) in PMMA* Adult Center  1.091 ± 0.004 Middle 1.165 ± 0.002 Periphery 1.314 ± 0.003 Surface 1.158 ± 0.002 Adolescent Center 1.530 ± 0.002 Periphery 1.648 ± 0.002 Surface 1.371 ± 0.003 Child Center 1.854 ± 0.009 Periphery 1.880 ± 0.003 Surface 1.712 ± 0.004 *PMMA: Polymethyl Methacrylate       51 of the phantom in PMMA, the second highest radiation was measured in the centre and the lowest on the surface in the air. Table 12 and 13 show the average values with standard deviation at each imaging protocol and phantoms.   Table 13. i-CAT CBCT Radiation Measurement with Head Phantoms: Protocol 2. FOV 130 mm, 160 mm diameter, 120 kVp, 5 mA, 0.4 voxel, 4s exposure time. The center, middle and periphery measurements are in PMMA and the surface measurements are in air. Average Value is in Bold. Standard Deviation is Denoted by ±. Phantom Location Average Absorbed Dose (mGy) in PMMA* Adult Center 1.448 ± 0.003 Middle 1.389 ± 0.002 Periphery 1.520 ± 0.002 Surface 1.283 ± 0.001 Adolescent Center 1.948 ± 0.005 Periphery 1.932 ± 0.003 Surface 1.511 ± 0.005 Child Center 2.196 ± 0.003 Periphery 2.077 ± 0.011 Surface 1.906 ± 0.003 *PMMA: Polymethyl Methacrylate   The simplified illustrations of average absorbed radiation in adult, adolescent and small child phantoms at each location are shown at two most common imaging protocols: FOV of 60 mm     52 and 130 mm. The numbers were rounded up to the nearest tenths and middle location values of adult phantoms were not included in the diagram for ease of comparison (Figures 23 and 24).    Figure 23. Average Absorbed Radiation Dose (mGy) in Adult (left), Adolescent (middle) and Small Child (right) Phantoms: Protocol 1. The imaging protocol is set at FOV 60 mm, 160 mm diameter, 120 kVp, 5 mA, 0.4 voxel size, 4 second exposure time.                          Figure 24. Average Absorbed Radiation Dose (mGy) in Adult (left), Adolescent (middle) and Small Child (right) Phantoms: Protocol 2. The imaging protocol is set at FOV 130 mm, 160 mm diameter, 120 kVp, 5 mA, 0.4 voxel size, 4 second exposure time.         53 3.2.3 Panoramic Radiograph Radiation There was a wide variability between radiation dose measurements from i-CAT NG CBCT and Planmeca panoramic radiographs. The imaging protocol settings for i-CAT and Planmeca are compared in Tables 14-16.   Table 14. Comparison of Adult Imaging Protocol for i-CAT and Planmeca Panoramic Radiograph   i-CAT Planmeca Protocol Large Adult kVp 94 66 mA 5 9 Acquisition time 20 s 16 s  Table 15. Comparison of Adolescent Imaging Protocol for i-CAT and Planmeca Panoramic Radiograph   i-CAT Planmeca Protocol Large Small Adolescent 7-12 year old  kVp 94 84 64 62 mA 5 5 7 5 Acquisition time 20 s 18.3 s 16 s 14 s  Table 16. Comparison of Small Child Imaging Protocol for i-CAT and Planmeca Panoramic Radiograph   i-CAT Planmeca Protocol Small Under 6 Year of Age kVp 84 60 mA 5 4 Acquisition time 18.3 s 14 s  Because both machines operate under different conditions and imaging protocols (i-CAT: small and large vs. Planmeca: Under 6 Year of Age, Adolescent 7-12 year old, Adult), comparing absorbed doses from i-CAT to Planmeca is difficult even when using the same phantom head for     54 each machine. The averaged results with standard deviation can be found in Tables 17-19. The focus should be on comparing trends, not values.   For i-CAT CBCT generated panoramic radiograph of the adult phantom, the highest radiation dose was measured at the side of the phantom at the periphery. This measurement was obtained by averaging the left and right side dose measurements. In contrast, for Planmeca generated panoramic radiograph of the adult phantom, the highest radiation dose was measured in the front periphery. The rear periphery dose was higher than the side periphery dose. The measurements at the periphery were consistently higher than surface for both i-CAT and Planmeca. Table 17. i-CAT and Planmeca Panoramic Radiograph Radiation Measurement Using Adult Imaging Protocol. Average Absorbed Dose Value is in Bold. Standard Deviation is Denoted by ±.  Adult Phantom i-CAT Planmeca Protocol  Large Adult Centre 0.359 ± 0.007 0.249 ± 0.005 Periphery Side 0.505 ± 0.004 0.303 ± 0.004  Front 0.178 ± 0.003 0.425 ± 0.006  Back 0.291 ± 0.006 0.336 ± 0.002 Surface Side 0.383 ± 0.005 0.382 ± 0.006  Front 0.074 ± 0.004 0.181 ± 0.004  Back 0.214 ± 0.002 0.336 ± 0.004 *Average Absorbed Dose (mGy) in PMMA     55 For i-CAT CBCT generated panoramic radiograph of the adolescent phantom, both large and small imaging protocols were used to measure the radiation dose. For both protocols, the highest radiation dose was seen in the centre, and the lowest on the surface. For Planmeca generated panoramic radiograph of the adolescent phantom, both adolescent and 7-12 year old protocols were used. The highest dose was measured on hypothetical patient’s right and left surfaces. For i-CAT, it is obvious that the large setting instead of small protocol for an adolescent increased the absorbed dose measured. The same trend is seen for Planmeca, where choosing an adolescent protocol instead of 7-12 year old protocol increased the absorbed dose measured.   Table 18. i-CAT and Planmeca Panoramic Radiograph Radiation Measurement Using Adolescent Imaging Protocol. Average Absorbed Dose Value is in Bold. Standard Deviation is Denoted by ±. Adolescent Phantom i-CAT Planmeca  Protocol Large Small Adolescent 7-12 year old Centre 0.750 ± 0.005 0.455 ± 0.004 0.402 ± 0.004 0.163 ± 0.002 Periphery Side 0.481 ± 0.005 0.348 ± 0.003 0.431 ± 0.004 0.181 ± 0.004  Front 0.499 ± 0.004 0.298 ± 0 0.335 ± 0.002 0.184 ± 0.007  Back 0.315 ± 0.005 0.205 ± 0.003 0.319 ± 0.002 0.190 ± 0.001 Surface Side 0.377 ± 0.003 0.267 ± 0.002 0.540 ± 0.004 0.283 ± 0.002  Front 0.144 ± 0.008 0.079 ± 0.001 0.301 ± 0.001 0.060 ± 0.012  Back 0.231 ± 0.002 0.150 ± 0.009 0.096 ± 0.005 0.163 ± 0.006 *Average Absorbed Dose (mGy) in PMMA      56 For i-CAT CBCT generated panoramic radiograph of the small child phantom, the lowest dose was measured on the surface. The highest doses were measured at the periphery and the second highest in the centre. For Planmeca generated panoramic radiograph of the small child phantom, the lowest dose was measured in the centre. The highest measurements were at the periphery, and the second highest radiation doses were measured on the surface.   Table 19. i-CAT and Planmeca Panoramic Radiograph Radiation Measurement Using Small Child Imaging Protocol. Average Absorbed Dose Value is in Bold. Standard Deviation is Denoted by ±. Small Child Phantom i-CAT Planmeca Protocol Small Under 6 year of age Centre 0.326 ± 0.005 0.319 ± 0.002 Periphery Side 0.336 ± 0.001 0.475 ± 0.005  Front 0.541 ± 0.002 0.657 ± 0.006  Back 0.223 ± 0.007 0.530 ± 0.002 Surface Side 0.303 ± 0.005 0.577 ± 0.003  Front 0.254 ± 0.003 0.229 ± 0.005  Back 0.178 ± 0.003 0.525 ± 0.004 *Average Absorbed Dose (mGy) in PMMA     57  The CBCT dosimetry for multiplanar images showed that the highest radiation dose was measured in periphery, the second highest at centre and the lowest on the surface. This trend does not hold for panoramic radiographs. However, the importance of child-sizing the dose using different protocols for child and adult is obvious from both CBCT and panoramic dosimetry studies.      58 Chapter  4: Discussion  4.1 Part 1. Patient Survey 4.1.1 Sample size, Gender and Age The i-CAT CBCT was installed in the BC Children’s Hospital Dental Clinic in August 2010 for imaging of children with craniofacial abnormalities as a substitute for MDCT. The infrequent use of the dental CBCT during this time period is evident by the small sample size. This may be due to that fact that the i-CAT CBCT was just installed in August 2010. Considering there were 13 patients referred from private orthodontic practice, the actual number of hospital patients receiving the CBCT was only 19 in the seventeen-month study period. There was an equal distribution of genders in the study sample.   4.1.2 Indication for CBCT The increasing dental CBCT use is a concern when there are insufficient randomized controlled trials and systematic reviews supporting the use of this imaging modality. In an ideal setting, all levels of evidence should be considered before clinically adopting a new diagnostic imaging modality (60). Alarmingly, some centres in USA have included CBCT into routine dental examinations (61).  The most common dental CBCT indication at BC Children’s Hospital was orthodontics referral from the community. More specifically, impacted canine was the most common indication. The parallax method has been the conventional way to determine the position of the impacted canine (61). In the general population, the prevalence of impacted maxillary canine is about 3% (62),     59 which is a relatively high prevalence rate, but the impacted tooth alone does not justify exposing 3% of the adolescent population to CBCT. According to Liu’s study on localization of impacted maxillary canines, CBCT study does not reduce treatment time or support a more accurate exposure of impacted teeth (62). From a systematic review of CBCT use in orthodontics, van Vlijmen reported that there was not enough evidence for use of CBCT for implant placement, cephalometrics, orthognathic surgery, root angulation assessment, tooth impaction and cleft lip and/or palate (24). The only area that showed significant benefit from dental CBCT use was for the evaluation of the airway. In contrast, other investigations examining use of CBCT in orthodontics recommend five situations in which CBCT is indicated: severe facial asymmetry or facial disharmony, sleep medicine, impacted maxillary canines, or minidental implants, where parallax technique does not work (63). The conflicting evidence in the literature, recommendations and guidelines are confusing for practicing dentists to assess risk and benefit for each case.  In patients with cleft lip and palate, palate closure surgery is performed about 9 to 12 months of age and bone graft is performed around 8 to 9 years of age at BC Children’s Hospital. For these procedures, obtaining accurate and detailed information in the osseous cleft to examine the bone defect and tooth germs is valuable. However, limiting radiation in younger patients is also very important, especially at very young age (64).  A systematic review reported a lack of randomized controlled trials to support the use of CBCT to improve oral surgery treatment outcomes (65). The systematic review found that CBCT does not perform better than panoramic radiographs in preventing postoperative complications (65).     60 The benefit of CBCT in oral surgery assessment and treatment outcome has not been supported in the literature.  The advantage of CBCT in diagnosis of oral pathological conditions has not yet been determined. In an article examining the advantage of CBCT in differentiating granuloma from radicular cyst, the CBCT proved not to be a reliable diagnostic method (66). The authors concluded that conventional radiography, surgical biopsy and histopathological evaluation are still the gold standard and CBCT should only be used when necessary (66).   Incidental findings are more likely with CBCT than with panoramic or periapical radiographs (67). The prevalence of incidental findings in medical radiology is surprisingly high at 4 to 31% (68, 69). In a study investigating dental CBCT images in orthodontic patients, 66% of 329 CBCT scans had incidental findings (70). Of these scans, 168 needed follow-up and 2 had clinical significance. The incidence of oral cysts, impacted teeth, and supernumerary teeth are higher with CBCT. However, most of the incidental findings do not affect dental treatment planning.  4.1.3 Previous Radiation Exposure  Six patients out of 32 patients had previous CT radiation exposure for impacted teeth, cleft palate and retrognathism. This finding suggests that patients with craniofacial abnormality may be exposed to more radiation than their healthy counterparts. Craniofacial abnormality often predisposes patients to cleft lip and/or palate and impacted teeth due to the arch length loss (71). Having a cleft palate alone predisposes the patient to more supernumerary teeth and increased need for a bone graft, which may require multiple radiation exposures for assessment and treatment (72, 73). Pediatric dentists, oral surgeons and orthodontists need to be mindful that     61 their patients with craniofacial abnormalities and cleft palate may have a history of multiple radiation exposures. Pediatric patients with history of high dose radiation should be examined with care and all attempts should be taken to reduce further radiation exposure to them.   4.1.4 Common CBCT Imaging Protocols for i-CAT The common CBCT imaging protocols for multiplanar three-dimensional images and panoramic option used at BC Children’s Hospital may not be representative of what is commonly used in private practice. Many different dental CBCT models and vendors are available. Some studies examining the commonly used CBCT imaging protocols show that the protocols and radiation vary vastly depending on the devices and the model (17, 74). The imaging protocols used in this study are only applicable to i-CAT NG and Planmeca.  4.1.5 Head CBCT Measurements The age of patient and the size of head in sagittal and coronal planes showed no correlations based on linear regression (R2). This lack of correlation was likely for several reasons: difference in ethnic backgrounds, presence of craniofacial abnormality in many patients and small sample size. However, other investigators found no significant racial, national or geographic differences in head circumferences. For healthy children, the average head circumference at yearly intervals up to age 18 is well studied (75). Nellhaus noted that arrested head growth marks potential developmental delay whereas the accelerated growth signifies more correctable conditions such as hydrocephalus, subdural hematomas or effusions (75). In our study, head measurements were     62 used as the basis for the dimensions of the small child and adolescent phantoms. The aim was to include the range of children from the very small child to very large child sizes.   4.2 Part 2. Phantom Dosimetry Study Positioning of the phantoms was different from customary patient positioning. The conventional tools such as head support, head strap, gate, chin support and chin cup and chair were not used because of the shape of the phantom set-up. The only common positioning tool that both phantom and patient shared was the laser alignment light. These lights have horizontal and vertical components. The horizontal light is to be positioned at the occlusal plane between the lips. The vertical light is to be positioned 1.5 inches in front of condyle in humans. In our study, the alignments were centered horizontally and vertically on the phantom for the ease of position replication for the study.  4.2.1 Phantom Head Size The size of the child phantom head was based on average pediatric head sizes in the literature, commercially available pediatric head phantoms, and the smallest measurements from our BC Children’s Hospital patient survey. The size of the adolescent phantom was determined by average facial width as measured on 10 subjects from BCCH aged 11 to 14 years. This age group is most commonly referred for orthodontic assessment and thus will subsequently receive CBCT. Our approach is in accordance with two studies that described the mean age at the beginning of orthodontic treatment was 12.5 years and 12.2 years of age respectively (55, 76).       63 4.2.2 CBCT Radiation In our study, the highest radiation was measured at the periphery, the second highest at centre and the lowest on the surface of the phantom. This finding is in partial contrast to studies that found the maximum dose at the centre and reduced doses at the periphery (77, 78). The studies of Sawyer (77) and Amer (78) respectively attribute their findings to the reduced scatter radiation near the edge of the FOV and the divergent beam in the CBCT.  The adult phantom’s surface attenuates more radiation due to its size (33). Therefore, more radiation reaches the centre of the phantom as the size decreases. When the same imaging protocols were used for all three phantoms, the highest radiation doses were measured in the small child phantom. Knowing children’s sensitivity to radiation and risk involved, one should weigh the risk and benefit carefully before ordering CBCT imaging for children. Only when two-dimensional imaging is impossible or impractical, should a CBCT image be considered.  In a previous study that compared adult and child radiation equivalent doses from two different dental CBCT, different amounts of radiation were measured depending on the CBCT devices, location of centre, size and FOV (25). When the same CBCT imaging protocol was used for both children and adults, higher radiation was measured in the head and neck of children compared to adults (29, 51). Our study showed the same trend when the adolescent phantom was exposed to x-ray using large and small protocols with i-CAT Panoramic Radiograph Option.  The “large” protocol resulted in higher absorbed dose than the “small” protocol. Therefore, it is important to use a pediatric imaging protocol for small children and adolescents.  The variation measured in each location in and on the phantoms is related to the distance from the x-ray source, medium through which the x-ray travels and scatter radiation.      64 In CBCT, the x-ray beam is emitted in a cone shape. The x-ray beams are more concentrated closer to the radiation source. As the distance increases from the x-ray source, the x-ray beam is spread over a larger area and less radiation is measured (Figure 25).  Figure 25. Distance from X-ray Source and Radiation. From x-ray source 1, the lowest radiation would be measured from point A, the second lowest from point B and the highest from point C. From x-ray source 2, the lowest radiation would be measured from point C, the second lowest from point B and the highest from point A.    PMMA material attenuates x-ray energy as it passes through. The more distance the x-ray beam travels through PMMA, the more the x-ray beam is attenuated and less x-rays reach the centre. The smaller the diameter of PMMA phantom, the less the x-ray beam is attenuated and more x-rays reach each location inside the phantom. This is evident in our study because all radiation measurements were higher in small child phantom compared to measurements made in adolescent and adult phantoms (Figure 26).       65  Scattered radiation also contributes to the radiation dose. On the surface, there is only minimal back scatter radiation. Inside the phantom, there is more scatter radiation. This explains why in our study the measured radiation dose on the surface of the phantom was lower than that measured in the periphery or centre of the phantom.  4.2.3 Panoramic Radiograph Radiation Panoramic radiographs are one of the most common dental radiographic films. Since the dental arch is not in a simple horseshoe arch shape, multiple centres of rotations are often used when taking a panoramic radiograph. Ideally, maxilla and mandible should be positioned in the focal trough and be clearly seen on the resulting image. The Planmeca machine has a rotating centre of the radiation beam that shifts on a pre-programmed trajectory behind the subject’s head during exposure. In an article that studied the absorbed dose from conventional panoramic radiography, Figure 26. Size of Phantom and Radiation. The white circle represents an adult phantom and the grey circle represents a small child phantom. This diagram shows relative absorption through thick (adult) or thin (small child) PMMA slab before striking ion chamber in the centre. At centre of adult phantom, more x-ray is attenuated compared to the centre of small child phantom. Therefore, less radiation would be measured at centre of adult phantom compared to the centre of small child phantom.     66 the authors observed the highest radiation dose was measured close to the rotation axes of the x-ray beam (79). This trend was not seen in our study with the Planmeca machine.  In our study, the overall trend that we saw with the CBCT phantom study was also observed with the panoramic phantom study. The highest radiation was measured with the small child phantom, the second highest radiation was measured with the adolescent phantom and the lowest radiation was measured with the adult phantom in all locations in and on the phantoms. The variability was seen in the distribution of radiation depending on the machine, imaging protocol and size of phantom. The imaging protocols for i-CAT and Planmeca were matched for each phantom but their kVp, mA and exposure times were very different. For the adult phantom, absorbed doses from i-CAT and Planmeca were similar. For both machines, the highest dose was measured at the periphery of phantom and the second highest dose on the surface of phantom. For the adolescent phantom, the absorbed dose from Planmeca measured lower than i-CAT. For both machines, the highest dose was measured at the centre, the second at the periphery and the lowest on the surface of the phantom. For the small child phantom, the absorbed dose from i-CAT was lower than Planmeca. For both machines, the lowest dose was measured at the periphery, the second at the centre and the lowest on the surface of the phantom. The variability in radiation dose depending on different machines and imaging protocols demonstrates no machine to be superior. The Planmeca absorbed dose was lower for the adolescent phantom while i-CAT was lower for the small child phantom.  For i-CAT, when the same adolescent phantom was used with different imaging protocols, the absorbed dose was higher with the “large” protocol compared to “small” protocol. Similarly, for Planmeca, when the same adolescent phantom was used with different imaging protocols, the     67 absorbed dose was higher with the adolescent protocol compared to the 7-12 year old protocol. This difference illustrated the importance of using the appropriate protocols for pediatric populations. Protocols and resulting radiation doses from different machines were not directly comparable because their kVp, mA and exposure times were vastly different. Surprisingly the radiation dose measured using i-CAT CBCT was not necessarily higher than that measured with conventional panoramic radiograph machine.  Radiation dose is related to the distance from the x-ray source and the path of the beam through the patient. The variability in radiation dose measurement in different locations can be attributed to difference in proximity to the radiation source due to the difference in size of each phantom. The radiation is higher when you are closer to the x-ray source. While the surface of adult phantom was closer to the x-ray source, the center of the adult phantom would be further from it compared to adolescent or small child phantoms. This explains how the surface dose was higher than the center in adult phantom, while the center dose was higher than the surface dose for adolescent and small child phantoms. Another reason for the difference in radiation dose may be the differences in path of the x-ray beam between i-CAT panoramic mode and Planmeca. Because of the lack of studies regarding panoramic radiographs taken with an i-CAT, a direct comparison with other studies is not possible at this point. In addition, no study to date has measured the radiation dose in different locations in and on PMMA phantoms.      4.3 Sources of Error  Sources of errors can compromise the accuracy and precision of an experimental study.      68 Systemic errors that affect measurement accuracy include faulty calibration of radiation devices, inadequately maintained radiation measurement instruments, and incorrect reading by the investigator. In the measurement of the anatomical distances on the CBCT images, incorrect labelling of anatomical landmarks and incorrect measurements of distances may have occurred. Because of the gray-scale of CBCT images, the distinction from one tissue to the other can be unclear on CBCT studies. Some random errors may have occurred during measuring of anatomical distances or when reading the absorbed radiation dose. Also, the dosimeter reading tends to fluctuate during the reading, which may result in random error. In order to minimize errors, the experiments were repeated three times and the average and standard deviations were calculated. All measurements for each phantom were taken in one sitting in order to eliminate variability in set-up.  It is difficult to compare the measurements from this study to measurements from other studies for two main reasons. Many articles investigating dental CBCT dosimetry use effective dose despite the fact that the absorbed dose is a better metric. Effective dose is a measure of radiation risk, not of radiation dose. It is inadequate for use in pediatric patients because the effective dose does not take demographic factors such as gender, age and BMI into consideration. In addition, previous studies show that effective doses are clinically very complex to obtain and have low reproducibility (34). Secondly, no other study examined radiation dose in adolescents or small children phantoms.       69 Chapter  5: Conclusions and Recommendations  5.1 Patient Survey Over a 17-month period from August 2010 to December 2011, 32 children of a variety of ages received CBCT at BC Children’s Hospital. There were two main groups of patients. The first group had a craniofacial abnormality and tended to be young children, 5 to 9 years of age. The second group was healthy adolescent patients aged 11 to 14 years who were referred from the community for dental CBCT for orthodontic assessment.    Important imaging protocol information was obtained to be used in the phantom study. The two most common imaging protocols both shared 120 kVp, 5 mA, 0.4 voxel size and exposure time of 4 seconds. They only differed in the size of the FOV: one was 160 mm diameter by 60 mm high and the other was 160 mm diameter by 130 mm high. Since there was only one dentist who was operating the i-CAT at BC Children’s Hospital, the two most common imaging protocols may not be representative of what is commonly used with other i-CAT machines. The information on head dimensions in sagittal and coronal planes was important to ensure that the phantoms to be developed covered the range of patients at BC Children’s Hospital Dental Clinic.  5.2 Phantom Dosimetry Study The most important conclusion from the phantom study was the overall trend. While radiation doses from our study are only specific to i-CAT NG CBCT and imaging protocols used, the overall trend of smaller head sizes measuring higher radiation is likely seen with other CBCT machines and imaging protocols. When the imaging protocols were held constant, the highest     70 absorbed dose was measured in all locations in the small child phantom and the lowest absorbed dose was measured in the adult phantom. The highest radiation was measured at the periphery, the second highest at centre and the lowest on the surface of small child, adolescent and adult phantoms. In the panoramic radiograph study, each phantom absorbed different amounts of radiation from the two different radiation devices. The trend from the CBCT study was consistently seen in panoramic study in the highest radiation in all locations in the small child phantom, the second highest in the adolescent phantom and the lowest in the adult phantom. For all three phantoms, it was difficult to determine which machine resulted in lower absorbed radiation dose. This result demonstrates that different machines produce images in different ways and that one machine does not fit all patients. It is also inferred that panoramic option of i-CAT CBCT can be an alternative to conventional panoramic radiograph machine. As clearly demonstrated in the panoramic dosimetry study with the adolescent phantom, using higher radiation protocol such as “large” for i-CAT and “adolescent” for Planmeca, results in higher radiation dose in patients. The different protocols on panoramic radiograph machine include lower radiation protocol such as “small” for i-CAT and “7-12 year old” for Planmeca that seem better for the pediatric population.  Since our study shows that small pediatric patients receive more radiation during CBCT, appropriate pediatric imaging protocols should be developed specifically for different dental CBCTs.       71 5.3 Strengths The strength of this study is that it is the first study to develop phantoms for a dosimetry study based on pediatric patient demographics. No other study has examined the radiation doses in the adolescent and small child using head phantoms. To our knowledge, it is also the only study that examines radiation doses from a panoramic radiograph using a dental CBCT machine.   5.4 Limitations  5.4.1 Patient Survey Dental CBCT is a relatively new imaging modality and has only been in use since August 2010 at BC Children’s Hospital. With so few published studies, it is impossible to compare the BC Children’s Hospital Dental Clinic characteristics with patients who receive CBCT in other hospitals. Since the study was based in a tertiary care hospital, the findings are not representative of private practice. In this study, all CBCTs were taken by one community orthodontist. The use of CBCT in a general dentistry practice may differ from a specialty dentistry setting in a hospital. Due to the late implementation of record protocol at BC Children’s Hospital, not all relevant information was recorded in the CBCT record for each subject. The information on the medical history and previous radiation exposure was often incomplete. Since all orthodontic patients were not hospital patients but rather a private orthodontist’s patients, their records were inaccessible for this study.       72 5.4.2 Phantom Dosimetry Study A phantom that better represents a pediatric patient can be developed to more accurately measure the absorbed radiation. The current phantoms are cylindrical in shape and are not anatomically correct. In the average healthy human, the head is in an ovoid shape and the length of the head is longer than the width. Therefore, the radiation absorption and scatter pattern in phantoms would not be accurately representative of humans. The phantom heads in this study are made of uniform density PMMA. A phantom made of PMMA mimics the density of muscle, water and air. The energy absorption per unit mass of these tissues is very similar to that of PMMA (Table 6). However, the energy absorption per unit mass of bone is higher than PMMA. This difference may change the radiation scatter pattern in the phantom. Furthermore, the position, size, thickness and density of tissues and organs are different in children compared to adults. In many studies, TLD and RANDO phantoms are often used. The RANDO phantoms come in two models, male and female. However, they do not offer a child head phantom for measuring absorbed or effective dose. In addition, RANDO phantoms have different materials that mimic tissues such as soft tissue, lungs, skeletons and breasts. However, a more anatomically correct composition cannot guarantee that RANDO can measure radiation doses more accurately compared to PMMA phantoms. This is because the density and energy absorption per unit mass of these tissues would be different for each individual depending on BMI, age and gender.  The reported dose of radiation in the literature can vary depending on scanner models, imaging protocols, measurement, and calculation techniques (49, 80). In this study, the positioning of phantoms was different from positioning of human patients. The only alignment device employed was the laser beam. Due to the physical shape of the phantom, other alignment devices     73 such as a bite guard chin rest and head positioner could not be used. It is unknown if the laser light beams are actually aligned with the radiation sensor in the dental CBCT device. In humans, the horizontal light is to be positioned at the occlusal plane between the lips, and the vertical light is to be positioned 1.5 inches in front of condyle in humans. In contrast, the alignments were centered horizontally and vertically for the head phantoms. These factors may have resulted in a different radiation centre, and thus inaccurate absorbed dose measurements. In order to reduce error, similar positioning was ensured for all images.  5.5 Recommendations and Clinical Relevance When deciding to purchase a CBCT, a dental professional should know what to look for. Choose a machine that allows the clinician to customize and change kVp, mA, voxel size and FOV for each patient. It is imperative to ensure that large FOVs are not used as standard imaging technique. Not only do large fields of view increase radiation dose, but they can also put the dentist in a legal dilemma if there was a failure to comply with legal regulations (81). By law, dentists are required to identify and report findings. It is important to check whether the dental CBCT device comes with a pediatric setting, as using this setting can reduce the absorbed dose significantly (25). The clinician should request information from manufacturers regarding pediatric settings and imaging protocols and expected radiation dose.  When ordering a CBCT study for a patient, the medical history and previous radiation exposure of the patient should be carefully considered. Among dentists and physicians there is a tendency to underestimate the radiation to pediatric patients (82). Younger and female children are more susceptible to radiation, and unnecessary CBCT radiation should be avoided (29). CBCT should     74 never be a part of standard exam due to the high radiation dose. It should only be ordered when two-dimensional imaging is inadequate, or when the benefit of CBCT outweighs the risk of stochastic effect of radiation.  When taking a dental CBCT, select the appropriate imaging protocol for the patient’s gender, age and size. In addition, do a “dry-run” to ensure correct positioning of patient. This is especially important in pediatric patient since they can move unexpectedly or easily become scared.   5.6 Future Directions A phantom study with small child and adolescent head phantoms should be conducted for different CBCT machines and imaging protocols. This is because every CBCT device needs calibration and radiation measurement, and one imaging protocol does not work with all machines (25). For different machines and imaging protocols, trends will be the same; the smallest head phantom will measure the highest radiation whereas the largest head phantom will measure the lowest radiation when the identical imaging protocols were used. However, the actual dose values will be different, so each machine and imaging protocol needs to be measured separately.  Furthermore, the optimization of radiation dose should be investigated for child and adolescent patients. The image quality assessment using different CBCT devices, imaging protocols and phantoms is needed to ensure the diagnostic quality of images at the lowest radiation dose possible, especially for younger children. The further study in optimization in pediatric population would help clinicians adhere to the ALARA principles.  Another dimension to this optimization in pediatric population is to study the differences between genders since it is known     75 that the radiation risk is higher for adult females (29). In addition, the investigation into different body mass index (BMI) and its effect on measured radiation should be studied to compare the effect of radiation in low and high BMI patients. It is known that patients with lower BMI measure higher radiation doses due to smaller size (83). Patients with high BMI confer higher image noise in their images, validating adaptation of imaging protocol to BMI (84). Greater amount of x-ray radiation is required to penetrate thicker tissue, which increases radiation dose to the patient required for a diagnostic image. This is especially relevant because childhood obesity is on the rise (85). Dental CBCT is a valuable tool and its popularity will only increase. As with any other technology, it is important to train dentists and provide them with guidelines for reference. Sedentex and European Academy of Dental and Maxillofacial Radiology guidelines are a good place to start (86). Recent effort in British Columbia includes guidelines that were prepared by BC Centre for Disease Control (87). This document included referral criteria and radiation safety suggestions, but it lacked an emphasis on sensitivity of the pediatric population to ionizing radiation. 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Basic principles for use of dental cone beam computed tomography: consensus guidelines of the European Academy of Dental and Maxillofacial Radiology. Dentomaxillofac Radiol. 2009 May;38(4):187-95. 87. Control BCfD. Guidelines on Radiation Protection & Quality Assurance Applicable to Dental Cone Beam Computed Tomography (CBCT). Vancouver, British Columbia, Canada2012. 88. ImageGently. Image Gently: The Alliance for Radiation Safety in Pediatric Imaging.  2014 [updated April 2, 2014; cited 2014 April 21]; Available from: http://www.imagegently.org.       82 Appendix A: Phantom CBCT Dosimetry Data Table A.1 Adult Phantom Dosimetry Data, i-CAT CBCT NG. (kVp 120, mA 5, mAs 18.54, diameter 160 mm, voxel size 0.4, exposure time 4 seconds, acquisition time 8.9 seconds)  FOV DAP (mGy*cm2) Exposure 1 (mR) Exposure 2(mR) Exposure 3(mR) Protocol 1      Right side of the patient 60 302.9 173.2 172.6 172.6 Back hole at the edge of the field 60 302.9 168.7 169.2 169.7 Left side of the patient 60 302.9 173.9 174 174.2 Front hole of the patient 60 302.9 173.4 173 174 Middle on the right  60 302.9 154.3 153.9 154 Middle hole in the back 60 302.9 151.9 151.8 152.3 Middle of patient left 60 302.9 152.3 151.8 152.2 Middle of the patient front 60 302.9 153.5 153.2 153.2 Central hole without holder (middle) 60 302.9 142.6 143.7 143       Surface      Patient right 60 302.9 142.4 143.4 143.1 Patient back 60 302.9 153.3 153.5 153.4 Patient left 60 302.9 163.1 163.1 163.5 Patient front 60 302.9 148.3 148.3 148       Protocol 2      Right side of the patient 130 623.9 201.3 200.9 201.2 Back hole at the edge of the field 130 623.9 200 199.5 199.7 Left side of the patient 130 623.9 202.1 201.6 201.5 Front hole of the patient 130 623.9 195.3 195.4 195.6 Middle on the right  130 623.9 183.8 183.4 183.6 Middle hole in the back 130 623.9 183.6 183.7 183.4 Middle of patient left 130 623.9 181.3 181.7 181.3 Middle of the patient front 130 623.9 180.2 180.6 180.2 Central hole without holder (middle) 130 302.9 189.9 189.7 190.4       Patient right 130 623.9 159.3 159.6 159.1 Patient back 130 623.9 169 169 168.8 Patient left 130 623.9 180.4 180.4 180 Patient front 130 623.9 165 165 165.1        83 Table A.2 Adolescent Phantom Dosimetry Data, i-CAT CBCT NG. (kVp 120, mA 5, mAs 18.54, diameter 160 mm, voxel size 0.4, exposure time 4 seconds, acquisition time 8.9 seconds)  FOV (mm) DAP (mGy*cm2) Exposure 1 (mR) exposure 2 (mR) exposure 3 (mR) Protocol 1      Right side of the patient 60 302.9 222.9 222.9 223.3 Back hole at the edge of the field 60 302.9 212.2 212.3 212.3 Left side of the patient 60 302.9 209.1 209 208.5 Front hole of the patient 60 302.9 220.3 221 221.4 Central hole without holder (middle) 60 302.9 201.1 200.5 200.9       Surface      Patient right 60 302.9 186.2 185.1 186.1 Patient back 60 302.9 177.7 177 177.4 Patient left 60 302.9 175.6 175.6 175.3 Patient front 60 302.9 181.5 180.5 180.8       Protocol 2      Right side of the patient 130 623.9 260.6 261 259.9 Back hole at the edge of the field 130 623.9 251 250.9 251.8 Left side of the patient 130 623.9 246.3 245.8 245.9 Front hole of the patient 130 623.9 256.2 257 256.2 Central hole without holder (middle) 130 302.9 254.9 255.8 256.3       Patient right 130 623.9 198.6 200.3 199.8 Patient back 130 623.9 201.2 198.7 199.2 Patient left 130 623.9 192.9 193.1 192.7 Patient front 130 623.9 200.3 201 200.8            84 Table A.3 Small Child Phantom Dosimetry Data, i-CAT CBCT NG. (kVp 120, mA 5, mAs 18.54, diameter 160 mm, voxel size 0.4, exposure time 4 seconds, acquisition time 8.9 seconds)  FOV (mm) DAP (mGy*cm2) Exposure 1 (mR) exposure 2 (mR) exposure 3 (mR) Protocol 1      Right side of the patient 60 302.9 243.8 241.9 244.2 Back hole at the edge of the field 60 302.9 253.5 253.7 252.6 Left side of the patient 60 302.9 248.8 249 248.7 Front hole of the patient 60 302.9 241 241 240 Middle on the right  60 302.9 244.2 244.1 244.3 Middle hole in the back 60 302.9 249.4 248.9 249.4 Middle of patient left 60 302.9 247.7 247.2 247.6 Middle of the patient front 60 302.9 245 244.3 244.5 Central hole without holder (middle) 60 302.9 242.3 242.7 242.1       Surface      Patient right 60 302.9 228.5 228.4 228.5 Patient back 60 302.9 228.7 227.4 227.1 Patient left 60 302.9 221.4 221.5 220.7 Patient front 60 302.9 221.6 221.4 220.2       Protocol 2      Right side of the patient 130 623.9 288.6 288.1 287.8 Back hole at the edge of the field 130 623.9 278.3 279.1 280.5 Left side of the patient 130 623.9 274.4 273.5 275.3 Front hole of the patient 130 623.9 262.5 267.6 266.4 Middle on the right  130 623.9 271 270.8 270.7 Middle hole in the back 130 623.9 291.2 288.7 288.9 Middle of patient left 130 623.9 284.7 288.6 287.7 Middle of the patient front 130 623.9 284.2 283.9 284.2 Central hole without holder (middle) 130 302.9 284.6 284 285       Patient right 130 623.9 254 252.2 253 Patient back 130 623.9 253.2 252.7 252.8 Patient left 130 623.9 246.8 246.7 246.6 Patient front 130 623.9 247.7 247.5 247.4       85 Appendix B: Phantom Panoramic Radiograph Dosimetry Data with i-CAT CBCT NG  Table B.1 Adult Phantom Dosimetry Data, i-CAT CBCT NG, Large Protocol (kVp 94, mA 5, DAP 146.4 mGy*cm2, acquisition time 20 seconds) Setting Position Exposure 1 (mR) Exposure 2 (mR) Exposure 3 (mR) Large      Centre 47.9 46.1 47.3  Right 64.7 64.6 65.2  Left 67.4 67 68.4  Back 38.5 37.3 38.7  Front 23.1 23.8 23.5       Surface     Right 42.5 44.8 43.7  Left 56.5 57 56.9  Back  27.8 28.3 28.1  Front 9.7 9.7 9.8  Table B.2 Adolescent Phantom Dosimetry Data, i-CAT CBCT NG, Large Protocol (kVp 94, mA 5, DAP 146.4 mGy*cm2, acquisition time 20 seconds) Setting Position Exposure 1 (mR) Exposure 2 (mR) Exposure 3 (mR) Large:   Centre 98.4 97.7 99  Right 69.4 68.9 68  Left 58 57.6 56.8  Back 40.6 41.5 42  Front 66 65 65.3       Surface     Right 51.2 52.3 52.3  Left 47.1 46.7 46.9  Back  30.7 30.2 30.1  Front 18.2 20.1 18.6       86 Table B.3 Adolescent Phantom Dosimetry Data, i-CAT CBCT NG, Small Protocol (kVp 84, mA 5, DAP 91.0 mGy*cm2, acquisition time 18.3 seconds) Setting Position Exposure 1 (mR) Exposure 2 (mR) Exposure 3 (mR) Small   Centre 59.4 60.4 59.4  Right 43.8 43.5 43.1  Left 47.6 48.4 47.2  Back 26.5 26.9 27.4  Front 39 39 39       Surface     Right 39.8 39.8 39.7  Left 29.9 30.7 30.5  Back  20.7 18.4 20  Front 10.5 10.2 10.5  Table B.4 Small Child Phantom Dosimetry Data, i-CAT CBCT NG, Small Protocol (kVp 84, mA 5, DAP 91.0 mGy*cm2, acquisition time 18.3 seconds) Setting Position Exposure 1 (mR) Exposure 2 (mR) Exposure 3 (mR) Small  Centre 42.2 42.7 43.4  Right 45.2 45.1 45.1  Left 43.1 42.8 43  Back 30.3 28.9 28.7  Front 71.3 71 70.8       Surface     Right 36.8 36.4 35  Left 43.1 44 43.6  Back  23.8 22.9 23.2  Front 33.6 32.9 33.3        87 Appendix C: Phantom Panoramic Radiograph Dosimetry Data with Planmeca Panoramic Machine  Table C.1 Adult Phantom Dosimetry Data, Planmeca Panoramic Machine, Small Adult Protocol. (kVp 66, mA 9, acquisition time 16 seconds) Setting Position Exposure 1 (mR) Exposure 2 (mR) Exposure 3 (mR) Large      Centre 32.3 33.4 32.2  Right 45.1 44.1 44  Left 35.5 35 34.6  Back 48.6 48.2 48.7  Front 60.4 58.9 60.1       Surface     Right 49.8 49.9 50.5  Left 49.9 50.1 52.2  Back  43.5 44.4 44.5  Front 24.3 24 23.1  Table C.2 Adolescent Phantom Dosimetry Data, Planmeca Panoramic Machine, Adolescent Protocol. (kVp 64, mA 7, acquisition time 16 seconds) Setting Position Exposure 1 (mR) Exposure 2 (mR) Exposure 3 (mR) Large      Centre 52.2 52.9 53.1  Right 71.9 71.1 71  Left 42.1 42.1 41.2  Back 44.9 44.6 45  Front 40.7 40.3 40.4       Surface     Right 78.6 79.4 78.8  Left 63.5 62.6 62  Back  39.6 39.6 39.3  Front 11.9 13.2 12.8      88 Table C.3 Adolescent Phantom Dosimetry Data, Planmeca Panoramic Machine, 7-12 year old protocol. (kVp 62, mA 5, acquisition time 14 seconds) Setting Position Exposure 1 (mR) Exposure 2 (mR) Exposure 3 (mR) Large      Centre 21.1 21.3 21.6  Right 23.6 24.7 22.9  Left 23.5 24 23.7  Back 25.8 25.5 25.6  Front 23.2 22.4 24.2       Surface     Right 46.8 47.4 47.2  Left 26.9 26.9 27.3  Back  20.6 21.8 22.1  Front 6.8 7.2 9.6  Table C.4 Small Child Phantom Dosimetry Data, Planmeca Panoramic Machine, Under 6 Year of Age Protocol. (kVp 60, mA 4, acquisition time 14 seconds) Setting Position Exposure 1 (mR) Exposure 2 (mR) Exposure 3 (mR) Large      Centre 41.7 41.7 42.1  Right 64.2 63.1 63.1  Left 122.9 123.2 124.3  Back 25.2 24.6 24.7  Front 30.1 30.8 31.6       Surface     Right 27.4 28.1 26.9  Left 33.8 34.2 33.7  Back  19.7 20.2 20.8  Front 8.8 7.5 8.4    

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