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Dual-energy computed tomography of the abdomen: A reliable trouble-shooter
How to cite this article: Chidambaram V, Choong MC, Goud CD. Dual-energy computed tomography of the abdomen: A reliable troubleshooter. J Clin Imaging Sci 2023;13:12.
Dual-energy computed tomography (CT) systems have undergone significant evolution and advancements in technology since they came into clinical practice in 2006. The basic principle of dual-energy is comparing the attenuation of different materials when exposed to high and low energy levels. In this article, we provide a brief overview of the basics of dual-energy CT systems, a pictorial review of commonly encountered abdominal conditions, and its role as a trouble-shooter in various diagnostic difficulties.
Virtual unenhanced images
Routine use of dual-energy computed tomography (CT) in the abdomen can increase diagnostic confidence and lesion detection in oncological imaging for various liver, pancreas, and genitourinary malignancies. Improved detection of non-calcified gallstones, characterization of urinary calculi, differentiation of hyperdense renal cysts from solid renal lesions on single-phase CT by assessment of density on virtual unenhanced images and contrast enhancement on iodine maps, and increased sensitivity in detecting bowel ischemia and inflammation are some of the common instances where dual-energy CT provides ample advantages compared to single-energy CT. In this article, we provide a pictorial review of various abdominal conditions where dual-energy CT can provide valuable information in daily clinical practice.
PRINCIPLES OF DUAL-ENERGY COMPUTED TOMOGRAPHY
Conventional single-energy CT uses the principle of X-ray attenuation to distinguish different materials. It is represented as CT numbers in Hounsfield units (HU), which are calibrated in reference to water. However, CT numbers are erratic and variable due to overlap in material attenuation, thus resulting in a lack of precision to differentiate various materials. On the other hand, dual-energy CT provides material-specific analysis based on the elemental constituents and attenuation differences at different energy levels.[1,2]
The basic principles of X-ray attenuation in any material depend on the photoelectric effect, which causes photon absorption, and the Compton effect, which is responsible for photon scattering. The photoelectric effect is dominant at low energy levels and directly proportional to the atomic number of the material. Compton scattering is dominant at high energy levels and affects substances with low atomic numbers.[2,3] In low atomic number materials, such as soft tissue and muscle, there is only minimal difference in attenuation values with variation in energy levels. Conversely, in high atomic number materials, such as iodine and calcium, the attenuation value increases with a reduction in energy level. By making use of this principle of comparing the attenuation of different materials when exposed to high and low energy levels, dual-energy CT can help to improve material differentiation.
TYPES OF DUAL-ENERGY CT
The commonly used principles of dual-energy CT systems are:
Dual-source dual-energy CT
In this method, the two different tubes (sources) operating at two different voltage settings (high kVp and low kVp) enable the acquisition of datasets with two different energy levels. There are also two sets of corresponding detectors, which are mounted in the gantry orthogonally oriented to the tubes.
Fast voltage switching
This principle utilizes a single source CT which is capable of rapid voltage switching (commonly between 80 and 140 kVp) to obtain two different datasets. These CTs are equipped with a single-layer detector.
Detector-based spectral imaging
This technique uses a single source CT with fixed voltage coupled with a dual-layered detector, which consists of a thin inner layer to absorb the low-energy photons and a thick outer layer to absorb the high-energy photons [Figure 1].[5-7]
DUAL-ENERGY CT ANALYSIS
For routine diagnostic interpretation, non-material-specific images can be generated by blending data from high- and low-energy images (1). The other commonly used postprocessed images are:
Virtual unenhanced images
The acquired final image minus all contrast-enhanced structures results in the virtual unenhanced images. Virtual unenhanced images may substitute a pre-contrast scan in some situations and reduce radiation exposure. However, the difference in attenuation values may vary up to 15 HU in virtual unenhanced compared with true unenhanced images.[5,6] Furthermore, the suppression of iodine may not be complete in all situations.
Contrast-enhanced dual-energy CT images minus water create iodine-enhanced images. For better visualization, the iodine concentration may be superimposed on the grayscale anatomic image as a color map. The enhancement can also be assessed quantitatively by measuring the amount of iodine in a specific region of interest (ROI).[4,7]
Virtual monoenergetic images
In low-energy images, the increased attenuation of materials with a high atomic number, like iodine and calcium, helps to enhance the contrast-to-noise ratio [Figure 2]. In high-energy images, beam hardening artifacts are reduced and can be used to reduce artifacts from metal hardware.[4,5]
Spectral HU curve
Spectral HU curves can be obtained using virtual monochromatic images by placing an ROI on the tissue and measuring the average CT number of the tissue at each monochromatic energy (from low to high keV). At lower energies, the attenuation of bone and high atomic number materials, like iodine and calcium, increases, whereas it decreases for fat. For water, the attenuation is zero at all energies. The difference in attenuation of soft-tissue structures, like muscles and organs, at various energy levels is minimal [Figure 3].
Non-calcified cholesterol containing gallstones are isoattenuating with bile on conventional CT images. Since the energy dependent X-ray attenuation curve is different for bile and fat, these cholesterol containing gallstones appear hypoattenuating to bile on low-keV images and hyperattenuating to bile on high-keV images. These stones are appreciated as hyperattenuating foci on virtual unenhanced images. The reason for the increased attenuation of cholesterol stones on virtual unenhanced images is attributed to increased high keV information on the dual-energy post-processing technique [Figures 4 and 5].[8-10]
The use of low-keV virtual monoenergetic imaging improves the accuracy of detecting bowel ischemia in the setting of obstruction and vascular occlusion by increasing the difference in attenuation of ischemic and non-ischemic segments. The iodine overlay map depicts the absence of mural iodine uptake in the bowel that helps to confirm ischemia [Figure 6]. Active hemorrhage and hyperenhancement of the bowel wall in inflammation are also well demonstrated on low-keV virtual monoenergetic images.[4,12]
Monochromatic low-keV images and iodine maps also increase the contrast between the tumor and the adjacent normal parenchyma, improving lesion detection, the relationship of the tumor with adjacent structures, and distant metastases [Figures 7 and 8].[3,13-15]
Virtual unenhanced images can confirm blood within the abdominopelvic cavity due to its hyperattenuating nature.
Lipid-rich adenomas show lower attenuation at low keV due to the presence of fat, demonstrating higher specificity and sensitivity for the presence of intracellular lipids in some studies [Figure 10].[16,17]
Dual-energy CT provides a better evaluation of the unenhanced attenuation of renal lesions. This helps in the diagnosis of incidentally detected renal lesions, such as hyperdense cysts [Figure 11].[18,19] In patients with polycystic kidney disease, dual-energy CT expedites the unearthing of malignancy with increased diagnostic reliability [Figure 12].
Dual-energy CT-generated edema images are used in detecting early bone fractures. Water density images demonstrating bone marrow edema can be generated by suppressing the calcium signal from the bone [Figure 13].
Lipiodol, made up of iodine combined with ethyl esters of fatty acids of poppyseed oil, is used in chemoembolization of hepatocellular carcinoma. The lipiodol uptake in hepatocellular carcinoma appears hyperdense on true non-contrast images; however, it is suppressed on virtual unenhanced images and appears iso to hypodense which is attributed to the presence of iodine in lipiodol [Figure 14].[22,23]
Dual-energy CT of the abdomen provides ample added information to radiologists as compared to single-energy CT. Radiologists’ awareness of the potential applications of dual-energy CT is essential to minimize the read time of the increased volume of post-processed images in routine practice. Limitations of the post-processed images must also be kept in mind to avoid misinterpretation.
Declaration of patient consent
Patient’s consent not required as patient’s identity is not disclosed or compromised.
Conflicts of interest
There are no conflicts of interest.
Financial support and sponsorship
- Lipiodol [Package Insert] Princeton, NJ: Guerbet LLC; 2020.
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