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UBC Theses and Dissertations

Quantitative performance characterization of image quality and radiation dose for dental CBCT machine (CS9300) Abouei, Elham


Purpose: To characterize the performance of cone beam CT (CBCT) used in dentistry, investigating quantitatively the image quality and radiation dose during dental CBCT over different settings for partial rotation of the x-ray tube. Methods: Image quality and dose measurements were done on a variable field of view (FOV) dental CBCT (Carestream 9300). X-ray parameters for clinical settings were adjustable for 2-10 mA, 60-90 kVp, and two optional voxel size values, with fixed time for each protocol and FOV. The phantoms were positioned in the FOV to imitate clinical positioning. Image quality was assessed by scanning a cylindrical poly-methyl methacrylate (PMMA) image quality phantom (SEDENTEXCT IQ), and the images were analyzed using ImageJ to calculate image quality parameters such as noise, uniformity, contrast to noise ratio (CNR), and spatial resolution. A protocol proposed by SEDENTEXCT, dose index 1 (DI1), was applied to dose measurements obtained using a thimble ionization chamber and cylindrical PMMA dose index phantom (SEDENTEXCT DI). Dose distributions were obtained using Gafchromic film. Results: The image noise was 6-12.5% which, when normalized to the difference of mean voxel value of PMMA and air, was comparable between different FOVs. Uniformity was 93.5-99.7% across the images. CNR was 0.5-4.2, 0.2-4.6, 3.7-11.7, 4.3-17.8, and 6.3-14.3 for LDPE, POM, PTFE, air, and aluminum, respectively. The measured FWHM and spatial resolution were larger than the voxel size. FWHM were 0.49-0.65 mm; spatial resolution was 194.81-467.68. Dose distributions were symmetric about the rotation angle’s bisector. For large and medium FOVs at 4 mA, 80-90 kVp, and 180-250 μm, DI1 values were in the range of 1.26-3.23 mGy. DI1 values were between 1.01-1.93 mGy for small FOV (5x5 cm²) at 4-5 mA,75-84 kVp, and 200 μm. Conclusion: Noise and spatial resolution decreased and the CNR increased by increasing kVp; the geometric distortion, AAV, FWHM were very similar or the same when increasing the kVp. When FOV size increased, image noise increased and CNR decreased. FWHM and spatial resolution have no correlation with the voxel size. DI1 values were increased by increasing tube current (mA), tube voltage (kVp), and/or FOV.

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