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Diagnostics 14 (7): 775 (2024)
Abu-Omar, Ahmad
Murray, Nicolas
Ali, Ismail T.
Khosa, Faisal
Barrett, Sarah
Sheikh, Adnan
Nicolaou, Savvas
Tamburrini, Stefania
Iacobellis, Francesca
Sica, Giacomo
Granata, Vincenza
Saba, Luca
Masala, Salvatore
Scaglione, Mariano
Advancing medical technology revolutionizes our ability to diagnose various disease processes. Conventional Single-Energy Computed Tomography (SECT) has multiple inherent limitations for providing definite diagnoses in certain clinical contexts. Dual-Energy Computed Tomography (DECT) has been in use since 2006 and has constantly evolved providing various applications to assist radiologists in reaching certain diagnoses SECT is rather unable to identify. DECT may also complement the role of SECT by supporting radiologists to confidently make diagnoses in certain clinically challenging scenarios. In this review article, we briefly describe the principles of X-ray attenuation. We detail principles for DECT and describe multiple systems associated with this technology. We describe various DECT techniques and algorithms including virtual monoenergetic imaging (VMI), virtual non-contrast (VNC) imaging, Iodine quantification techniques including Iodine overlay map (IOM), and two- and three-material decomposition algorithms that can be utilized to demonstrate a multitude of pathologies. Lastly, we provide our readers commentary on examples pertaining to the practical implementation of DECT’s diverse techniques in the Gastrointestinal, Genitourinary, Biliary, Musculoskeletal, and Neuroradiology systems.
Citation: Abu-Omar, A.; Murray, N.;Ali, I.T.; Khosa, F.; Barrett, S.; Sheikh,A.; Nicolaou, S.; Tamburrini, S.;Iacobellis, F.; Sica, G.; et al. Utility ofDual-Energy Computed Tomographyin Clinical Conundra. Diagnostics 2024,14, 775. https://doi.org/10.3390/diagnostics14070775Academic Editor: Ibrahim YelReceived: 29 January 2024Revised: 25 March 2024Accepted: 27 March 2024Published: 7 April 2024Copyright: © 2024 by the authors.Licensee MDPI, Basel, Switzerland.This article is an open access articledistributed under the terms andconditions of the Creative CommonsAttribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).diagnosticsReviewUtility of Dual-Energy Computed Tomography inClinical ConundraAhmad Abu-Omar 1,* , Nicolas Murray 1 , Ismail T. Ali 1, Faisal Khosa 1 , Sarah Barrett 1, Adnan Sheikh 1,Savvas Nicolaou 1, Stefania Tamburrini 2, Francesca Iacobellis 3 , Giacomo Sica 4 , Vincenza Granata 5,Luca Saba 6 , Salvatore Masala 7 and Mariano Scaglione 7,8,91 Department of Emergency Radiology, University of British Columbia, Vancouver General Hospital,Vancouver, BC V5Z 1M9, Canada; ismail.ali@vch.ca (I.T.A.)2 Department of Radiology, Ospedale del Mare-ASL NA1 Centro, Via Enrico Russo 11, 80147 Naples, Italy3 Department of General and Emergency Radiology, A. Cardarelli Hospital, Via A. Cardarelli 9,80131 Naples, Italy; iacobellisf@gmail.com4 Department of Radiology, Monaldi Hospital, Azienda Ospedaliera dei Colli, 80131 Naples, Italy;gsica@sirm.org5 Division of Radiology, Istituto Nazionale Tumori IRCCS Fondazione Pascale—IRCCS Di Napoli,80131 Naples, Italy6 Medical Oncology Department, AOU Cagliari, Policlinico Di Monserrato (CA), 09042 Monserrato, Italy7 Department of Medicine, Surgery and Pharmacy, University of Sassari, Viale S. Pietro, 07100 Sassari, Italy;samasala@uniss.it (S.M.)8 Department of Radiology, Pineta Grande Hospital, 81030 Castel Volturno, Italy9 Department of Radiology, James Cook University Hospital, Marton Road, Middlesbrough TS4 3BW, UK* Correspondence: aomar@doctors.org.ukAbstract: Advancing medical technology revolutionizes our ability to diagnose various disease pro-cesses. Conventional Single-Energy Computed Tomography (SECT) has multiple inherent limitationsfor providing definite diagnoses in certain clinical contexts. Dual-Energy Computed Tomography(DECT) has been in use since 2006 and has constantly evolved providing various applications to assistradiologists in reaching certain diagnoses SECT is rather unable to identify. DECT may also comple-ment the role of SECT by supporting radiologists to confidently make diagnoses in certain clinicallychallenging scenarios. In this review article, we briefly describe the principles of X-ray attenuation.We detail principles for DECT and describe multiple systems associated with this technology. Wedescribe various DECT techniques and algorithms including virtual monoenergetic imaging (VMI),virtual non-contrast (VNC) imaging, Iodine quantification techniques including Iodine overlay map(IOM), and two- and three-material decomposition algorithms that can be utilized to demonstrate amultitude of pathologies. Lastly, we provide our readers commentary on examples pertaining to thepractical implementation of DECT’s diverse techniques in the Gastrointestinal, Genitourinary, Biliary,Musculoskeletal, and Neuroradiology systems.Keywords: dual energy scanned projection radiography; dual energy CT application; dual energy CTsystem; gastrointestinal; genitourinary; musculoskeletal; neuroradiology1. IntroductionSingle-Energy CT (SECT) has long been validated as the standard imaging modalityin acute clinical settings [1]. Most SECT scans are acquired at 120 kVp, producing a poly-energetic X-ray beam with an average energy of 75 keV and an energy peak of 120 kVp [2].SECT has become a first-line tool in emergency departments due to its accessibility andhigh diagnostic accuracy [3]. However, in certain situations, the diagnostic limitationsof SECT are evident due to multiple inherent factors including the phase of the studyand the inability to separate different materials that possess similar attenuation on SECT.Dual-Energy CT (DECT) has been used in clinical practice since 2006 [4]. It has emerged asDiagnostics 2024, 14, 775. https://doi.org/10.3390/diagnostics14070775 https://www.mdpi.com/journal/diagnosticsDiagnostics 2024, 14, 775 2 of 21a promising tool with multiple clinical applications already demonstrated. DECT can shedlight on some of the diagnostic challenges encountered when using conventional SECT,aiding radiologists and physicians to problem-solve clinical conundrums and facilitatepatient management.2. Principles of AttenuationCT images represent the attenuation of X-ray photons by different body constituents,with attenuation expressed in Hounsfield Units (HU) [5]. Attenuation refers to the weak-ening of X-ray photons as they pass through a material and the attenuation of a specificmaterial is related to its density, effective atomic number (Z), and the energy level of the in-coming X-ray beam [5]. The linear attenuation coefficient (µ) is a material-specific constantquantifying the fraction of attenuated incident X-ray photons per unit thickness of thatmaterial. It is directly proportional to (Z) and increases with increasing physical density ofthe absorbing material. This linear coefficient can be normalized as the mass attenuationcoefficient per unit density of a material to produce a value that is constant for a givenelement independent of the density of the material.Two dominant physics principles account for X-ray attenuation; the Photoelectric effectand Compton effect. The Photoelectric effect is one of the principal forms of interactionof X-ray photons with matter. The incident photon interacts with and ejects an inner shellelectron of an atom. The probability of the effect is at its maximum when the energy ofthe incident photon is equal to or just greater than the binding energy of the electron in itsshell (known as the k-edge value). The removed electron is called a photoelectron and theincident photon is completely absorbed. The photoelectric effect is directly proportionalto (Z) of a material and inversely proportional to the incident photon’s energy, i.e., theprobability of the photoelectric effect increases with higher (Z) and lower energy levels.The most abundant elements in human tissue have k-edges too low to be detectableranging from 0.1 keV to 0.53 keV [6]. Although calcium has a relatively low k-edge value(4 keV), its high value relative to soft tissue causes markedly greater attenuation of low-energy photons producing inherent image contrast. Iodine has a k-edge value of 33.2 keV,within the range of energies used in diagnostic imaging, making it ideal for use in contrastmedia due to a combination of mass attenuation coefficient and the photoelectric effect.SECT is limited by its sole reliance on CT HU for quantitation [7]. This explainswhy, depending on their relative densities, two materials of differing composition such ascalcium and iodine may yield the same measurable HU when subjected to a single X-raybeam but produce different HU values when exposed to different energy levels at DECT,independent of their density value. Because iodine and calcium have high (Z), they aremore susceptible to the photoelectric effect. Contrarily, soft tissue demonstrates a weakphotoelectric effect and less variation of attenuation values with different energy levels [8].The Compton effect is the main cause of scattered radiation. It occurs due to theinteraction of the incident photon with free unattached electrons or loosely bound outershell electrons. Unlike the photoelectric effect, the Compton effect is relatively independentof the photon’s energy level.3. Principles of DECTDECT assesses attenuation by materials when submitted to high (140–150 kVp) andlow energy (80–100 kVp) photon beams and acquires two datasets for the imaged anatom-ical range. Post-processing of dual-energy data can be conducted either before (in theprojection-space domain) or after (in the image-space domain) the reconstruction of high-or low-energy images, depending on the scanner [9,10]. Postprocessing manipulation isutilized to reconstruct three different types of images: mixed images, material-specificimages, and virtual monoenergetic images [11].Images derived from a combination of the high and low-energy datasets are createdand are called mixed images. Using varying percentages of the high and low kVp datasets,Diagnostics 2024, 14, 775 3 of 21the resultant images simulate SECT acquired at 120 kVp [12] and are sent to the PictureArchiving and Communication System (PACS) for interpretation.DECT material-specific images are created after evaluating the interaction of all bodyconstituents with high and low energy levels. These images are generated through dif-ferential attenuation analysis to obtain a dual-energy index. Postprocessing software isused to calculate the attenuation properties of each voxel at low and high energy and amathematical algorithm is used to determine the proportion of dominant materials withinthe voxel based on a three-material decomposition technique to evaluate the representationof iodine, calcium, or fat in each voxel [13]. Once the proportions are known, different setsof images are generated after subtracting each constituent.Virtual non-contrast (VNC) images, sometimes referred to as virtual unenhanced(VUE) images depending on the vendor, are constructed after the voxels containing iodineare excluded from the image. VNC images help in avoiding the need for extra timeand radiation dose associated with acquiring a separate true non-contrast scan. Theseadvantages are particularly evident in multiphasic studies such as CT angiography, CTurography, or multiphasic liver CT, where the necessity for a true non-contrast scan canbe omitted. This can result in reductions in radiation dose ranging from 19% to 60%,depending on the specific protocol used [14–19].Likewise, iodine-only images and virtual non-calcium (VNCa) images can be gener-ated. It is also possible to superimpose a color-coded iodine overlay map (IOM) to quantifythe amount of iodine in a specific region of interest.DECT allows the extrapolation of images for various single-energy levels. The resul-tant virtual monoenergetic image (VMI) simulates the image that would be obtained ifthe scanner were to emit a true monoenergetic beam of X-rays. Using low-energy imagesimproves the contrast-to-noise ratio by increasing the attenuation of materials with highatomic numbers such as iodine at the expense of decreased spatial resolution and increasednoise. This facilitates the detection of iodine-containing structures such as contrast-opacifiedvessels and hypervascular tumors rendering them more conspicuous [20]. It also enablesa decrease in the volume of intravenous iodinated contrast administered which can beuseful in patients with reduced renal function [21]. For instance, without compromisingdiagnostic accuracy, reductions in iodinated contrast dosage of up to 50% can be attained byutilizing 40- and 50-keV monoenergetic images in abdominal aorta CT angiography [22–25].Similarly, reductions of 50% in iodinated contrast dosage for CT urography and 40% forcoronary CT angiography are also feasible [26,27].Conversely, high-energy images improve signal-to-noise ratio and ameliorate the effectof streak artifact but with decreased attenuation of body constituents at higher energies,thereby limiting soft tissue contrast resolution.DECT postprocessing application also utilizes Rho-Z Maps (Z effective) to reconstructimages based on the material decomposition of substances. The Rho Z decomposition tech-nique involves separation of materials based on their effective number (Rho) and electrondensity (z) [28]. Through material composition analysis, this application can identify andcharacterize different materials depending on their relative atomic number. This can beuseful in assessing enhancement of an imaged radiodense structure or confirming absenceof iodine within the structure [20].4. DECT Systems4.1. Dual-SourceDual-source scanner with dual detector arrays utilizes two X-ray sources and twodata-acquisition systems mounted on the gantry in an orthogonal fashion [29]. Each X-raysource is equipped with its own high-voltage generator allowing independent control ofthe X-ray tube potential and current. High-energy dataset is obtained at 120 or 140 kVpand low-energy dataset acquired at 80 or 100 kVp [30]. Using two separate X-ray sourcesallows beam filtration and current modulation in each tube resulting in optimization ofimage quality [30]. However, the dual-source images suffer from limited temporal andDiagnostics 2024, 14, 775 4 of 21spatial registration because the high and low-energy scans are acquired at slightly differenttimes. Furthermore, the available field of view is limited to 33cm, somewhat hinderingevaluation in patients with large body habitus [30].4.2. Single-Source with Fast Kilovoltage SwitchingThis prototype employs a single source scanner with a single detector layer. It iscontingent on a single X-ray source with fast switching between two kilovoltage settings(80 and 140 kVp) at intervals of 0.5 ms during a single gantry rotation to generate highand low-energy X-ray acquisitions [30]. Unlike the dual-source scanner, the tube currentremains constant for both energy levels and cannot be altered simultaneously. The exposuretime ratio is varied between the 80 and 140 kVp acquisitions (typically 65% of exposure timeat 80 kVp and 35% at 140 kVp) in order to optimize the contrast-to-noise ratio. The detectorsystem has a fast-sampling capability to acquire the alternating high and low-energy data.The advantages of the fast kilovoltage switching scanner include good temporal and spatialregistration [30]. Furthermore, the available field of view is increased to 50 cm for imageanalysis. Drawbacks include limited spectral separation between the high and low-energydatasets [30]. Additionally, the noise-to-signal ratio is higher on images obtained at lowerenergy levels as the tube current cannot be modulated at the same time the peak voltageis altered [30].4.3. Single-Source with Dual-Layer DetectorThis scanner detector system is modified with two scintillation layers arrayed oneon top of each other to receive separate high and low-energy data from a single X-raysource [30]. The top detector layer captures low-energy data whilst the bottom layercaptures high-energy data. This prototype offers excellent temporal and spatial registrationbut has limited energy separation with substantial spectral overlap.4.4. Single-Source with Twin-Beam FiltrationThis system utilizes a single energy source with a split filter placed in the collimator.The filter is composed of gold and tin arranged in close proximity along the longitudinalaxis to facilitate separation of the high and low-energy X-ray spectra. The respective halvesof the detector capture these distinct energy spectra [31,32]. By employing tin and gold, thefilter segregates the low and high-energy components of the polychromatic X-ray beam,consequently achieving spectral separation [33,34]. This system is cost-effective and doesnot require significant hardware modification. Installing a split filter into the tube collimatorof an established CT scanner is undertaken to enable dual-energy imaging [31–33]. Thisenhancement allows for image acquisition across the entire field of view, while automatictube current modulation ensures optimal radiation dosage for the patient [31,33].Nonetheless, the spectral separation achieved through a split filter is relatively re-stricted when contrasted with the capability of a dual-source dual-energy scanner [32,34].Moreover, there exists the possibility of cross-scattering, wherein one side of the beam mayinfluence the adjacent side of the detector. Additionally, a higher power output from theX-ray tube becomes imperative due to the beam prefiltration, posing a limitation particu-larly in imaging obese patients. Furthermore, temporal discrepancies between low- andhigh-energy data are observed [31].4.5. Single-Source with Sequential (Dual-Spin) AcquisitionSequential acquisition, proposed as one of the initial methodologies for obtainingdual-energy images without specialized hardware, involves scanning the entire volumesequentially at low and high kilovoltage settings [34]. The system functions in either anaxial or helical mode. In the axial mode, two gantry rotations, each at a different energylevel, are acquired at each table position before the table moves in the z-axis dimension [7].In the helical mode, the entire scan volume undergoes scanning with a helical acquisition atone energy level, immediately followed by a helical acquisition of the same scan volume atDiagnostics 2024, 14, 775 5 of 21another energy level. As two acquisitions are performed, each acquisition is executed at halfthe dose typically used in a conventional 120 kVp scan to mitigate the risk of heightenedradiation exposure [8].This approach, while effective, introduces a considerable delay between acquiring thetwo datasets. However, this delay can be mitigated to some degree by alternating the tubepotential between each successive gantry rotation [7,31,32].No significant hardware alterations are required with this system. Any CT scanner canperform sequential scanning at two different voltages, allowing the datasets to be combinedfor spectral analysis [31].By imaging at two different potentials at the same location during each gantry rotation,the views align precisely, facilitating projection space alignment and material decomposi-tion [33]. Moreover, automatic exposure control can be applied, allowing for tube currentmodulation at different potentials to achieve optimal noise levels [33].Drawbacks include spectral distortion due to patient movement between acquisitions.Additionally, implementing this method in cardiac studies and angiographic acquisitionscan be challenging due to significant temporal skew, necessitating high temporal resolu-tion [31]. Consequently, its application is primarily limited to relatively static organs andnon-contrast studies [34,35]. Table 1 summarizes the strengths and weaknesses of variousDECT technologiesTable 1. Strengths of Weaknesses of various DECT systems.Technology Advantages DisadvantagesDual source Optimized image quality Limited temporal and spatial resolutionSmall FOV *Rapid kVp switching Good temporal and spatial registrationLimited spectral separationHigh noise-to-signal ratio of low-energy imagesRequires specialized hardwareDual layer detectors Excellent temporal and spatial resolution High spectral overlapRequires specialized hardwareTwin-beam filtrationCost-effectiveFull spectral FOV * available forimage acquisitionTemporal discrepancy between high- andlow-energy dataCross-scatterIntrinsic lower energies due to filtrationOverlapping spectra in center and edge of the beamDual spinCost-effective (can be performed onany scanner)Optimized tube current modulationFull spectral FOV *No cross scatterSpectral distortion secondary to motion artefact* FOV—Field of view.5. DECT Applications5.1. Examples in the Gastrointestinal System5.1.1. BowelImaging of the unprepared bowel by conventional SECT may have limitations whichcan be resolved by using DECT. The principle of DECT for bowel pathology relies primarilyon identifying whether mural enhancement is present or absent after administration ofintravenous contrast [36]. It is important to avoid administering enteric iodinated contrastwhen DECT postprocessing techniques are used as enteric contrast would compromiseassessment of mural enhancement [11].The subtle differences in mural attenuation in inflammatory conditions of the bowelcan be accentuated by employing iodine quantification postprocessing techniques. Lee et al.revealed that mucosal hyperemia related to active Crohn’s disease was most conspicuousDiagnostics 2024, 14, 775 6 of 21on VMI at 40 keV, as compared to conventional SECT obtained at 120 kVp, increasing thesensitivity and negative predictive value for diagnosing active Crohn’s disease [37].A suspected large bowel neoplasm can appear more conspicuous on IOM due toiodine uptake which can increase the confidence in raising the probability of an underlyingmalignant process by the reporting radiologist [11].Bowel ischemia is ascribed to arterial thromboembolic phenomena or venous occlusionas well as hypoperfusion secondary to low-flow states. Segmental hypoenhanced orunenhanced bowel wall strongly suggests ischemia. These changes can be subtle onconventional CT. The application of low-keV VMI increases the difference in attenuationbetween ischemic and non-ischemic segments of bowel [11]. Similarly, IOM increasesthe conspicuity of non-enhancing bowel helping radiologists to confidently attribute thefinding to ischemia (Figure 1). In the latter stages of ischemia, submucosal hemorrhage mayensue resulting in mural hyperattenuation, which can be mistaken for mural enhancementon conventional SECT. VNC images and IOM can be utilized to demonstrate intramuralhemorrhage and decreased iodine mural uptake respectively supporting the diagnosis ofbowel ischemia [11].Diagnostics 2024, 14, x FOR PEER REVIEW 7 of 21 Figure 1. From left to right—Contrast enhanced conventional 120 kVp and iodine overlay map (IOM) coronal images of the abdomen. Closed-loop small bowel obstruction secondary to a right inguinal hernia (*). The small bowel loop demonstrates no mural enhancement; more conspicuous on the IOM (*) which is highly suggestive of an incarcerated hernia. Gastrointestinal bleeding is caused by many conditions including bleeding disorders, anticoagulant therapy, and neoplasms [38]. VNC images can confirm the hyperattenuat-ing nature of intramural or intraluminal blood whilst IOM helps to exclude the presence of an underlying enhancing lesion. Furthermore, IOM can increase the conspicuity of in-traluminal extravasation of contrast in keeping with active bleeding and enable the differ-entiation of these areas of active extravasation from hyperattenuating digestive material or intestinal foreign bodies. Equally, low-keV VMI can identify active hemorrhage by pro-moting the conspicuity of iodine. Another advantage of contrast-enhanced DECT in the setting of gastrointestinal bleeding and bowel ischemia is its ability to create VMI derived from contrast-enhanced phases, allowing up to 30% reduction in radiation dose when compared to the triphasic studies usually performed as standard in suspected gastroin-testinal bleeding or bowel ischemia [39]. 5.1.2. Appendix In acute appendicitis, IOM can help increase the confidence of diagnosis by detecting areas of mural hyperenhancement owing to hyperemia; a feature that can be subtle on conventional CT in the early stages of inflammation [11]. Likewise, IOM can be utilized to depict complications of appendicitis including ischemia, gangrene, or perforation demon-strated by discontinuity of enhancement of the bowel wall. This finding can be subtle or even undetectable on conventional SECT. Figure 1. From left to right—Contrast enhanced conventional 120 kVp and iodine overlay map (IOM)coronal images of the abdomen. Closed-loop small bowel obstruction secondary to a right inguinalhernia (*). The small bowel loop demonstrates o mural enhancement; more conspicuous on the IOM(*) which is highly suggestive of an incarcerated hernia.Gastrointestinal bleeding is caused by many conditions including bleeding disorders,anticoagulant therapy, and neoplasms [38]. VNC images can confirm the hyperattenuatingnature of intramural or intraluminal blood whilst IOM helps to exclude the presence of anunderlying enhancing lesion. Furthermore, IOM can increase the conspicuity of intralumi-nal extravasation of contrast in keeping with active bleeding and enable the differentiationof these areas of act ve extravasatio from hyperattenuating digestive material or intestinalforeign bodies. Equally, low-keV VMI can identify active hemo rhage by promoting theconspicuity of iodine. Another advantage of contrast-enhanced DECT in the setting ofgastrointestinal bleeding and bowel ischemia is its ability to create VMI derived fromcontrast-enhanced phases, allowing up to 30% reduction in radiation dose when com-Diagnostics 2024, 14, 775 7 of 21pared to the triphasic studies usually performed as standard in suspected gastrointestinalbleeding or bowel ischemia [39].5.1.2. AppendixIn acute appendicitis, IOM can help increase the confidence of diagnosis by detectingareas of mural hyperenhancement owing to hyperemia; a feature that can be subtle onconventional CT in the early stages of inflammation [11]. Likewise, IOM can be utilized todepict complications of appendicitis including ischemia, gangrene, or perforation demon-strated by discontinuity of enhancement of the bowel wall. This finding can be subtle oreven undetectable on conventional SECT.5.2. Examples in the Genitourinary System5.2.1. UrolithiasisPatients presenting to the emergency department complaining of acute flank pain arefrequently investigated with a CT of the kidney, ureters, and bladder (CT KUB) to excludeurinary tract calculi, considering its high sensitivity (97%) and specificity (96%) [40,41]. Thecommonest composition of urinary tract calculi is calcium oxalate with uric acid stonesconstituting up to 10% of renal calculi [42]. It is of the essence to differentiate uric acid fromnon-uric acid stones as the former can be treated medically with urinary alkalinization [42].Non-uric acid stones, nevertheless, are treated with fragmentation or extraction. Calciumoxalate monohydrate, cystine, and brushite calculi may require percutaneous lithotomy,considering their increased resistance to fragmentation [43].SECT is restricted in establishing the chemical composition of urinary tract calculi,depending solely on attenuation of calculi with considerable overlap among differenttypes of calculi [44]. DECT differentiates uric acid from non-uric acid stones with anaccuracy of 90–100% [45] (Figure 2). Calculus characterization is achieved by calculatinga dual-energy index after analyzing the differential attenuation of the calculus at lowand high-energy levels. Second and third-generation DECT scanners employ additionalfiltration of high-energy photons allowing for a better spectral separation and more detailedanalysis of stone composition [46]. This allows differentiation of cystine, struvite, calciumoxalate dihydrate or monohydrate, and apatite stones [47]. Moreover, VNC images canbe derived from a contrast-enhanced DECT scan to identify urinary stones masked bycontrast opacification [48]. It is worth noting that DECT is limited in the assessment ofsmall stones (<3 mm) in patients with a high body mass index because of the reducedsignal-to-noise ratio [49].Diagnostics 2024, 14, 775 8 of 21Diagnostics 2024, 14, x FOR PEER REVIEW 8 of 21 5.2. Examples in the Genitourinary System 5.2.1. Urolithiasis Patients presenting to the emergency department complaining of acute flank pain are frequently investigated with a CT of the kidney, ureters, and bladder (CT KUB) to exclude urinary tract calculi, considering its high sensitivity (97%) and specificity (96%) [40,41]. The commonest composition of urinary tract calculi is calcium oxalate with uric acid stones constituting up to 10% of renal calculi [42]. It is of the essence to differentiate uric acid from non-uric acid stones as the former can be treated medically with urinary alka-linization [42]. Non-uric acid stones, nevertheless, are treated with fragmentation or ex-traction. Calcium oxalate monohydrate, cystine, and brushite calculi may require percu-taneous lithotomy, considering their increased resistance to fragmentation [43]. SECT is restricted in establishing the chemical composition of urinary tract calculi, depending solely on attenuation of calculi with considerable overlap among different types of calculi [44]. DECT differentiates uric acid from non-uric acid stones with an ac-curacy of 90–100% [45] (Figure 2). Calculus characterization is achieved by calculating a dual-energy index after analyzing the differential attenuation of the calculus at low and high-energy levels. Second and third-generation DECT scanners employ additional filtra-tion of high-energy photons allowing for a better spectral separation and more detailed analysis of stone composition [46]. This allows differentiation of cystine, struvite, calcium oxalate dihydrate or monohydrate, and apatite stones [47]. Moreover, VNC images can be derived from a contrast-enhanced DECT scan to identify urinary stones masked by con-trast opacification [48]. It is worth noting that DECT is limited in the assessment of small stones (<3 mm) in patients with a high body mass index because of the reduced signal-to-noise ratio [49]. Figure 2. From left to right—Conventional CT KUB and DECT renal stone analysis axial images of the abdomen. The right midpole calculus (*) is identified on the color-coded stone analysis DECT application as a uric acid calculus (*) which can be treated medically with urine alkalinization. 5.2.2. Inflammation, Neoplasm and Hemorrhage IOM increases the conspicuity of the hyperemic urothelium by accentuating iodine in cases of urinary tract infections. IOM can also be utilized to identify focal areas of de-creased iodine uptake in an inflamed organ such as the prostate raising concern for a Figure 2. From left to right—Conventional CT KUB and DECT renal stone analysis axial images ofthe abdomen. The right midpole calculus (*) is identified on the color-coded stone analysis DECTapplication as a uric acid calculus (*) which can be treated medically with urine alkalinization.5.2.2. Inflammation, Neoplasm and HemorrhageIOM increases the onspicuity of the hyperemic urothelium by accentuating iodine incases of urinary tract infectio s. IOM can also be utilized to identify focal areas of decreasediodine uptake in an inflamed organ such as the prostate raising concern for a phlegmonor abscess formation [11]. VNC images can identify hyperattenuating blood products inkeeping with hematoma within the collecting system and IOM can be used to identify areasof active extravasation in keeping with acute hemorrhage.DECT analysis of renal masses is sensitive, specific and accurate at differentiatingenhancing masses when using an iodine-specific postprocessing application [50]. Thiscan be helpful in differentiating hemorrhagic or proteinaceous renal cysts from neoplasm(Figure 3).Diagnostics 2024, 14, 775 9 of 21Diagnostics 2024, 14, x FOR PEER REVIEW 9 of 21 phlegmon or abscess formation [11]. VNC images can identify hyperattenuating blood products in keeping with hematoma within the collecting system and IOM can be used to identify areas of active extravasation in keeping with acute hemorrhage. DECT analysis of renal masses is sensitive, specific and accurate at differentiating enhancing masses when using an iodine-specific postprocessing application [50]. This can be helpful in differentiating hemorrhagic or proteinaceous renal cysts from neoplasm (Fig-ure 3). Figure 3. From left to right—Contrast-enhanced (portal venous) and Iodine overlay map (IOM) axial images of the abdomen. Right exophytic hyperattenuating lesion (*) is further characterized as lack-ing internal enhancement on the IOM (*) image and is therefore likely to represent a benign hyper-dense cyst rather than an enhancing lesion. 5.3. Examples in the Biliary System 5.3.1. Cholelithiasis Ultrasound is distinguished for its sensitivity in the evaluation of cholelithiasis and acute cholecystitis. Despite that, CT is frequently used for the evaluation of right upper quadrant pain especially in the acute setting. Up to 57% of gallstones isoattenuate to the surrounding bile and are therefore inconspicuous on conventional SECT [51]. Unlike water, which demonstrates minimal change in attenuation when imaged by various X-ray spectra, heavy atoms such as cholesterol-containing gallstones possess a CT value that increases with increasing X-ray tube voltage [52]. Therefore, VMI can be used to identify noncalcified gallstones that appear hypoattenuating to surrounding bile at low-keV (40 keV) and hyperattenuating relative to bile at high-keV reconstruction (≥140 keV) [53] (Figure 4). As described earlier, DECT’s material decomposition techniques allow the differen-tiation of materials of various chemical compositions that have similar attenuation values on SECT. These techniques with a calcium and lipid base, or a calcium, lipid, and water (three-material decomposition) base algorithm can be utilized to identify non-calcified gallstones with excellent sensitivity and specificity ranging from 80–95% and 95–100% re-spectively [54]. Figure 3. Fro left to right—Contrast-enhanced (portal venous) an Iodine overlay map (IOM)axial images of the abdomen. Right exophytic hyperattenuating lesion (*) is further characterizedas lacking internal enhancement on the IOM (*) image and is therefore likely to represent a benignhyperdense cyst rather than an enhancing lesion.5.3. Examples in the Biliary Syste5.3.1. CholelithiasisUltrasound is distin i ti f lelithiasis andacute cholecystitis. al ation of right upperquadrant pain especial y i t e ac t s tt s is attenuate to thesur ounding bile and are therefore incons icUnlike water, which de o t tt ation hen i aged byvarious X-ray spectra, heav t i i allstones posse s a CTvalue that increases with increasing X-ray tube voltage [52]. Therefore, VMI can be used toidentify noncalcified gallstones tha appear hypoattenuating to surroundi g bile at low-keV(40 keV) and hyperattenuati g relative to bile at high-keV reconstruction (≥140 keV) [53](Figure 4).As described earlier, DECT’s material decomposition techniques allow the differentiationof materials of various chemical co positions that have similar attenuation values on SECT.These techniques wit a calcium and lipid base, or a calcium lipid, nd water (three-materialdecomposition) base alg rithm can be utilized to identify non-calcified gallstones withexcellent sensi ivity and sp cif city ranging from 80–95% and 95–100% respectively [54].Diagnostics 2024, 14, 775 10 of 21Diagnostics 2024, 14, x FOR PEER REVIEW 10 of 21 Figure 4. From left to right—Conventional mixed 120 kVp equivalent, 40 keV virtual monoenergetic and gallstone analysis application axial images through the abdomen. Gallstones are not visible on the conventional image as they isoattenuate to the surrounding bile. However, they are visible on the DECT images and appear hypodense (filling defect) on the virtual monoenergetic image (*) and hyperdense (*) on the gallstone analysis application. 5.3.2. Cholecystitis Gallbladder hyperemia concordant with acute cholecystitis is easily discernable us-ing low-keV VMI and IOM reconstruction images. These techniques also improve detec-tion of gallbladder wall gangrene by depiction of areas of decreased enhancement in cases of complicated cholecystitis [55]. 5.4. Examples in the Musculoskeletal System 5.4.1. Bone Marrow Edema (BME) Accurate and early identification of acute fractures is important to avoid complica-tions including malunion, osteonecrosis, degenerative arthritis, persistent pain, and func-tional compromise [56]. BME typically occurs in response to a fracture and is attributable to the accumulation of fluid or blood within the bone marrow. In acute settings, BME can be key in the identification of non-displaced fractures which may be subtle or even im-possible to identify on SECT. Magnetic Resonance Imaging (MRI) is the preferred method for identifying BME [57]. However, due to potential contraindications and limited availability of MRI especially in the acute setting, CT remains the most frequent modality used in trauma. DECT enables BME detection using the three-material decomposition algorithm, de-termining the contributions of fat, soft tissue, and calcium within each voxel. The calcium is removed from the data to produce a virtual non-calcium image (VNCa) representative of underlying bone marrow. Increased attenuation of the bone marrow can then be qual-itatively assessed using color-coded VNCa maps [58] (Figures 5 and 6). Comparing color signals with the adjacent region of the same bone or with the contralateral side is im-portant for the detection of an abrupt change in attenuation raising concern for BME. i . Fr left to rig t ti l i p equivalent, 40 ke virtual onoener tiand gallstone analysis application axial i ages through the abdo en. allstones are not visible onthe conventional image as they isoattenuate to the surrounding bile. However, they are visible onthe DECT images and appear hypodense (filling defect) on the virtual monoenergetic image (*) andhyperdense (*) on the gallstone analysis application.5.3.2. lec stitisll l r r i co cordant with acute cholecystitis is easily discernable usinglow-keV VMI and IOM reconstruction images. These techniques al o improve d tectionof gallbl dder wall gan rene by depiction f are s of decr as d nhancement in case ofcomplicated cholecystitis [55].5.4. xa ples i the sc loskeletal yste5.4.1. Bone Marrow Edema (BME)Accurate and early identification of acute fractures is important to avoid complicationsincluding malunion, osteonecrosis, degenerative arthritis, persistent pain, and functionalcompromise [56]. BME typically occurs in response to a fracture and is attributable to theaccumulation of fluid or blood within the bone marrow. In acute settings, BME can be keyin the identification of non-displaced fractures which may be subtle or even impossible toidentify on SECT.Magnetic Resonance Imaging (MRI) is the preferred method for identifying BME [57].However, due to potential contraindications and limited availability of MRI especially inthe acute setting, CT remains the most frequent modality used in trauma.DECT enables BME detection using the three-material decomposition algorithm, de-termining the contributions of fat, soft tissue, and calcium within each voxel. The calciumis removed from the data to produce a virtual non-calcium image (VNCa) representativeof underlying bone marrow. Increased attenuation of the bone marrow can then be quali-tatively assessed using color-coded VNCa maps [58] (Figures 5 and 6). Comparing colorsignals with the adjacent region of the same bone or with the contralateral side is importantfor the detection of an abrupt change in attenuation raising concern for BME.Diagnostics 2024, 14, 775 11 of 21Diagnostics 2024, 14, x FOR PEER REVIEW 11 of 21 Figure 5. From left to right—Conventional mixed 120 kVp equivalent and bone marrow edema (BME) overlay sagittal images of the lumbar spine. Subtle L3 superior endplate compression fracture is difficult to detect on the conventional images(*). BME is, however, confidently demonstrated on the overlay map (orange arrow) Figure 5. From left to right—Conventional mixed 120 kVp equivalent and bone marrow edema (BME)overlay sagittal images of the lumbar spine. Subtle L3 up rior endplate compression fracture isdifficult to detect on the conventional images(*). BME is, however, confidently demonstrated on theoverlay map (orange arrow).Diagnostics 2024, 14, x FOR PEER REVIEW 11 of 21 Figure 5. From left to right—Conventional mixed 120 kVp equivalent and bone marrow edema (BME) overl y sagittal images of the lu bar spine. Subtle L3 superior l te compression fracture is difficult to detect on the conventional images(*). BME is, howeve , c fi tl de onstrated on the overlay map (orange arrow) Figure 6. From left to right—Conventional mixed 120 kVp equivalent and 3D bone marrow edemaoverlay coronal images of the right knee. Subtle intercondylar tibial fracture extending into the tibialcondyle (*) is confidently diagnosed when appreciating the associated BME on the overlay image(orange arrow).Diagnostics 2024, 14, 775 12 of 215.4.2. GoutGout is the commonest and the most validated DECT application in the musculoskele-tal system with multiple studies showing sensitivities ranging between 78–100% andspecificities of 89–100% [59–61]. In fact, a positive DECT scan is part of the 2015 AmericanCollege of Radiology classification criteria for gout [62]. DECT utilizes two-material (uricacid and calcium) or three-material (uric acid, calcium and soft tissue) decompositiontechniques for the characterization of monosodium urate (MSU) crystals. The separationof materials is feasible because the attenuation of the low atomic weight MSU crystals isdifferent from that of the high atomic weight calcium when exposed to different energylevels [63]. The MSU crystals are color-coded and overlaid on gray-scale images as illus-trated in Figure 7. The quantification of the volume of crystal burden can be accuratelyassessed to estimate the severity of the disease, prognosis, and monitoring response totarget therapy [59,64].Diagnostics 2024, 14, x FOR PEER REVIEW 12 of 21 Figure 6. From left to right—Conventional mixed 120 kVp equivalent and 3D bone marrow edema overlay coronal images of the right knee. Subtle intercondylar tibial fracture extending into the tibial condyle (*) is confidently diagnosed when appreciating the associated BME on the overlay image (orange arrow). 5.4.2. Gout Gout is the commonest and the most validated DECT application in the musculoskel-etal system with multiple studies showing sensitivities ranging between 78–100% and specificities of 89–100% [59–61]. In fact, a positive DECT scan is part of the 2015 A erican College of Radiology classification criteria for gout [62]. DECT utilizes two-material (uric acid and calcium) or thr e-material (uric acid, calcium and soft tissue) decomposition techniques for the characterization of monosodium urate (MSU) crystals. The separation of materials is feasible because the attenuation of the low atomic weight MSU crystals is different from that of the high atomic weight calcium when exposed to different energy levels [63]. The S crystals are color-coded and overlai on gray-scale i a s as ill s-trated in Figure 7. The quantification of the volu e of crystal burden can be a curately a se sed to estimate the severity of the disease, prognosis, and monitoring response to target therapy [59,64]. Figure 7. DECT gout application readily identifies the color-coded monosodium urate (MSU) crys-tals (green color) and assesses disease burden. 5.5. Examples in Neuroradiology Post Endovascular Thrombectomy (EVT) Acute ischemic stroke (AIS) is the leading cause of disability and mortality world-wide [65]. EVT is recommended as first-line treatment for AIS with large artery occlusion Figure 7. DECT gout application readily identifies the color-coded monosodium urate (MSU) crystals(green color) and assesses disease burden.5.5. Examples in NeuroradiologyPost Endovascular Thrombectomy (EVT)Acute ische ic stroke (AIS) is the leading cause of disability and ortality orld-wide [65]. EVT is recommended as first-line treatment for AIS with large artery occlu-sion [66–68]. Blood–brain barrier injury secondary to ischemia commonly causes post-interventional cerebral hyperdensities (PICHs) on non-contrast CT Brain studies performedafter EVT [68]. PICHs are usually ascribed to contrast staining (CS) secondary to limitedvascular endothelial injury. Nevertheless, a serious complication of AIS is hemorrhagicDiagnostics 2024, 14, 775 13 of 21transformation (HT) which is associated with worsening prognosis or even death. Hence,it is crucial to identify the cause of PICHs after EVT.DECT is invaluable in differentiating the cause of PICHs as either due to CS ordangerous HT (Figure 8). Differentiating contrast from hemorrhage is also vital to gear thephysician’s decision-making as to whether and/or when to commence anti-platelet therapypost-EVT. Utilization of VNC and IOM images can differentiate the source of PICHs asdemonstrated in Figure 8.Diagnostics 2024, 14, x FOR PEER REVIEW 13 of 21 [66–68]. Blood–brain barrier injury secondary to ischemia commonly causes post-inter-ventional cerebral hyperdensities (PICHs) on non-contrast CT Brain studies performed after EVT [68]. PICHs are usually ascribed to contrast staining (CS) secondary to limited vascular endothelial injury. Nevertheless, a serious complication of AIS is hemorrhagic transformation (HT) which is associated with worsening prognosis or even death. Hence, it is crucial to identify the cause of PICHs after EVT. DECT is invaluable in differentiating the cause of PICHs as either due to CS or dan-gerous HT (Figure 8). Differentiating contrast from hemorrhage is also vital to gear the physician’s decision-making as to whether and/or when to commence anti-platelet ther-apy post-EVT. Utilization of VNC and IOM images can differentiate the source of PICHs as demonstrated in Figure 8. Figure 8. From left to right—Unenhanced conventional 120 kVp equivalent, iodine overlay map (IOM) and virtual non-contrast (VNC) axial images of the brain post Endovascular thrombectomy (EVT). Hyperdensity within the left basal ganglia on the conventional image (*) could be caused by contrast staining (CS) or hemorrhagic transformation (HT). The hyperdensity persists on the IOM (*) but is not detectable on the VNC image; confirming this to be secondary to CS and not HT. 5.6. Pulmonary Examples Utilizing low keV VMI achieves a significant increase in iodine attenuation thereby enhancing the visibility of iodine-containing structures and helps distinguish enhancing from non-enhancing lesions [69]. Employing VNC imaging may result in a notable de-crease in the overall administered contrast dose, as pre-contrast scans become unneces-sary [69]. Additionally, high-energy VMI can be utilized to mitigate beam hardening arti-facts typically observed in the thorax due to dorsal spine stabilization for scoliosis [70–72]. DECT is effective in identifying acute pulmonary embolism (PE). Its primary ad-vantage lies in its capability to detect perfusion abnormalities in the lung parenchyma resulting from pulmonary arterial occlusion and to directly visualize arterial filling defects [73–77]. The color-coded IOM is useful to identify dark wedge-shaped segments con-sistent with PE. DECT is particularly beneficial in identifying small vessel occlusive em-boli. One study emphasized the effectiveness of VMI in cases of suboptimal scans due to technical issues or mistimed contrast bolus injections [78]. DECT can aid in distinguishing between benign and malignant pulmonary nodules and masses [79]. It has been shown that subjective image quality for visualizing lung car-cinoma is enhanced when using energy levels between 55–70 keV as opposed to conven-tional CT scans [80,81]. Additionally, DECT can be employed for cases involving anterior mediastinal lesions to differentiate thymic cysts from thymic epithelial tumors [79]. Fig re 8. From left to right—Une hanced conventional 120 kVp equivalent, iodine overlay map (IOM)and virtual non-co trast (VNC) axial images of the brain post Endovascular thrombecto y (EVT).Hyperdensity within the left basal ganglia on the c venti al image (*) could be caused by contraststaining (CS) or hemorrhagic transformation (HT). The hyperdensity persists on the IOM (*) but is notdetectable on the VNC image; confirming this to be secondary to CS and not HT.ili i l i i ifi i i i i tt ii t i i ilit f i i - t i i t t l i ti i ifr on-enhancing lesions [69]. Employing VNC imaging may result in a notable decreasein the overall administered contrast dose, as pre-contrast scans become unnecessary [69].Additionally, high-energy VMI can be utilized to mitigate bea hardening artifacts typicallyobserved in the thorax due to dorsal spine stabilization for scoliosis [70–72].DECT is effective in identifying acute pulmonary embolism (PE). Its primary advan-tage lies in its capability to detect perfusion abnormalities in the lung parenchyma resultingfrom pulmonary arterial occlusion and to directly visualize arterial filling defects [73–77].The color-coded IOM is useful to identify dark wedge-shaped segments consistent withPE. DECT is particularly beneficial in identifying small vessel occlusive emboli. One studyemphasized the effectiveness of VMI in cases of suboptimal scans due to technical issues ormistimed contrast bolus injections [78].DECT can aid in distinguishing between benign and malignant pulmonary nodulesand masses [79]. It has been shown that subjective image quality for visualizing lungcarcinoma is enhanced when using energy levels between 55–70 keV as opposed to conven-tional CT scans [80,81]. Additionally, DECT can be employed for cases involving anteriormediastinal lesions to differentiate thymic cysts from thymic epithelial tumors [79].5.7. Cardiac EvaluationThe precise and swift diagnosis of patients presenting with chest pain is imperativefor initiation of appropriate acute management and minimizing risk of complications [82].Diagnostics 2024, 14, 775 14 of 21Innovations in cardiac CT imaging have facilitated prompt and precise, non-invasive,evaluation of various cardiac conditions. Despite its notable negative predictive power,obtaining high resolution cardiac images presents inherent challenges, most notably cardiacmotion artefact secondary to an elevated heart rate [82,83].Various applications of DECT in cardiac imaging have demonstrated clinical feasibilityand usefulness through utilization of Iodine myocardial perfusion maps, VMI and VNCreconstructions [84–86]. These techniques have been proven to enhance image quality anddiagnostic accuracy.Of note, only dual-layer and dual-source DECT technologies enable near completesimultaneous data acquisition, making them particularly valuable for cardiac imaging [87].Other techniques are at risk for misregistration posing diagnostic challenge for evaluationof atherosclerotic plaque in tiny and rapidly moving coronary arteries.Perfusion maps highlight iodine distribution within the myocardium. This allowsqualitative detection and delineation of myocardial perfusion defects [83]. Studies haveshown that perfusion maps increase the diagnostic accuracy for infarct detection comparedto coronary computed tomography angiography (CCTA) [88,89]. Furthermore, there isevidence that perfusion maps help radiologists predict if a coronary artery stenosis ishemodynamically significant, since it can give information regarding a potential decreasein iodine uptake in the respective supplied territory [88,90,91]. Other authors demonstratedadditional value of perfusion maps for detection of late-enhancing tissue, indicating chronicischemia or scarred myocardium and potentially allowing differentiation between chronicor reversible myocardial ischemia [92–94].Low KeV VMI (less than 80 KeV) improves conspicuity of iodine in comparison toconventional CT with objective increase in image quality [83]. Studies have reported arterialattenuation values exceeding 1000 HU in CT angiography when employing ultra-low keVVMI, leading to increased contrast-to-noise and signal-to-noise ratios [95,96].Reconstructions using low-energy VMI are particularly beneficial in scenarios wherecontrast bolus is suboptimal. This may occur due to factors such as inaccurate ROI placement,myocardial insufficiency, or difficulties in venous access during the injection process [83].Cardiac imagers can deliberately leverage VMI enhancing attenuation capability todecrease the necessary dosage of contrast media administered [97].VMI has the potential to alleviate various artifacts frequently encountered in car-diac CT e.g., beam-hardening artifacts, which mimic myocardial hypoattenuation seen inischemia, can be mitigated quantitively and qualitatively [98]. Additionally, calcium bloom-ing artifacts, commonly observed in patients with severe calcifications, often diminish thediagnostic accuracy of CCTA exams, frequently resulting in overestimation of coronaryartery stenosis [83]. There is supporting evidence indicating that high-energy VMI algo-rithms (such as those at 110 keV and higher) efficiently diminish blooming artifacts [99,100].Virtual calcium subtraction is another technique which has been demonstrated toenhance the reader’s diagnostic certainty when evaluating coronary artery stenosis inindividuals with extensively calcified coronary arteries [101].5.8. Vascular ExamplesAn especially enticing application of DECT is direct CT angiography. In this method,the dual-energy algorithm detects and eliminates bone, enabling direct visualization ofiodinated vessels [102–105]. Atherosclerotic calcified plaque can obscure the vessel lumen.Calcium subtraction application can assess vascular lumen patency and delineates areas ofstenosis on maximum intensity projections of CT angiograms [106–108].The typical CT arteriography protocol used to evaluate acute aortic syndromes in-volves acquiring unenhanced images to identify intramural hematomas or intimal calci-fications which lead to significant radiation exposure. Despite the fact that VNC imagesexhibit slightly more noise compared to conventional unenhanced images, they providediagnostic value in nearly 95% of cases [109].Diagnostics 2024, 14, 775 15 of 21DECT is also beneficial in aortic evaluation after endovascular stent repair. IOM can beutilized to distinguish high attenuation material caused by blood, bone, or contrast agentand increase confidence for endoleak detection particularly when subtle [110].VMI at 40 keV offers enhanced contrast attenuation of poorly opacified liver veinsin comparison to blended images [111]. Additionally, low keV VMI has demonstratedimprovements in assessing portal vein thrombosis [112]. Both IOM and VMI at 40 keVsignificantly elevate diagnostic confidence and accuracy in identifying and distinguishingvenous thrombosis from iodine reflux when compared to mixed (blended) images [113].5.9. Oncology ExamplesDECT proves valuable in distinguishing hemorrhagic brain metastasis from intracra-nial hemorrhage [114]. Employing a monoenergetic beam of 40 keV enhances tumordelineation. DECT can also discern hyperdense bleeding from enhancement within ahemorrhagic mass. It has been proposed that 65 keV VMI offer optimal overall imagequality, while 40 keV VMI images offer superior tumor delineation [115].Several studies have indicated that hyperattenuating and hypervascular liver lesionsare more conspicuous at lower keV levels compared to conventional CT scanners [116,117].In individuals with pancreatic adenocarcinoma, 55-keV VMI has been shown toenhance the visualization of tumors and vascular infiltration compared to blended recon-structions [118]. However, 70 keV was identified as the most favorable subjective energylevel overall [119].For delineating renal cell carcinoma tumors, the optimal keV is 40 [120]. Other stud-ies have aimed to establish optimal thresholds for distinguishing between vascular andnonvascular renal lesions, recommending energy levels of 40–60 keV [121].An iodine concentration of 2.0 mg/mL has been identified as the optimal thresholdfor discriminating between lymphoma and lymph node metastasis, with a sensitivity of87% and specificity of 89% [122].6. Challenges and PitfallsDECT has limitations despite its advantages. The technology is expensive and requiresspecialized hardware and software. Implementation of DECT requires training to staffradiologists and technologists. Additionally, the generated image datasets can be over-whelming and may turn into an incumbrance for the reporting radiologist. This challengecan be overcome by limiting data sent to PACS to the essential required for reporting.There is paucity of literature on the cost effectiveness of DECT [123,124]. Furthermore,challenges exist for both qualitative and quantitative interpretation of images producedby DECT [125,126]. For instance, calcium in tissues may appear smaller on VNC imagescompared to true non-contrast imaging; potentially leading to the oversight of smallcalcifications or calculi [127]. In patients with chronic parenchymal liver disease, thepresence of liver fat can complicate the quantification of liver iron deposition [127].Another significant limitation is the variability in technology, terminology, and imagedatasets among different vendors, which hampers the standardization of quantitativemetrics and poses difficulties for multi-center research studies.Finally, the emergence of photon-counting CT holds significant promise and presentsnumerous benefits compared to DECT. These advantages encompass enhanced spectralresolution, refined tissue characterization, minimized image artifacts, and improvedimage quality [128].7. ConclusionsAdvancing medical technology continuously revolutionizes and reforms our abilityto diagnose multiple disease processes. Various DECT applications have stood firm asproblem-solving tools supporting radiologists to confidently diagnose a myriad of patholo-gies and help physicians to select appropriate management plans to ensure delivery of bestquality care for their patients.Diagnostics 2024, 14, 775 16 of 21Funding: This research received no external funding.Data Availability Statement: No new data were created or analyzed in this study. Data sharing isnot applicable to this article.Conflicts of Interest: Faisal Khosa is the recipient of the Michael Smith Health research BC award(2023-2028). Vicenza Granata is involved with the following study: PRESTO-1 STUDY: precisiontherapeutic strategies for the treatment of colorectal cancer and HCC. The remaining authors declarethat the research was conducted in the absence of any commercial or financial relationships thatcould be construed as a potential conflict of interest.References1. Stoker, J.; van Randen, A.; Laméris, W.; Boermeester, M.A. Imaging patients with acute abdominal pain. Radiology 2009, 253,31–46. [CrossRef] [PubMed]2. Coursey, C.A.; Nelson, R.C.; Boll, D.T.; Paulson, E.K.; Ho, L.M.; Neville, A.M.; Marin, D.; Gupta, R.T.; Schindera, S.T. Dual-energymultidetector CT: How does it work, what can it tell us, and when can we use it in abdominopelvic imaging? Radiographics 2010,30, 1037–1055. [CrossRef] [PubMed]3. Brown, D.F.; Fischer, R.H.; Novelline, R.A.; Kim, J.; Nagurney, J.T. The role of abdominal computed tomography scanning inpatients with non-traumatic abdominal symptoms. Eur. J. Emerg. Med. 2002, 9, 330–333. [CrossRef] [PubMed]4. Macari, M.; Spieler, B.; Kim, D.; Graser, A.; Megibow, A.J.; Babb, J.; Chandarana, H. 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dual energy scanned projection radiography
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Utility of Dual-Energy Computed Tomography in Clinical Conundra
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