Dual-energy computed tomography of the abdomen: A reliable trouble-shooter

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]

Close

Figure 1:

Various dual-energy computed tomography systems (DECT) are currently available on the market. (a) Dual source DECT: two source X-ray tubes are placed orthogonally to each other with corresponding detectors. (b) Rapid voltage switching DECT: single source which is capable of rapid voltage switching between low and high energies with a corresponding single detector. (c) Dual layer DECT: Single source and single detector, which consists of two different layers of detectors to separate low- and high-energy spectra.

Export to PPT

Virtual unenhanced images

The acquired final image minus all contrast-enhanced structures results in the virtual unenhanced images.^[5] 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.^[2]

Iodine-enhanced images

Contrast-enhanced dual-energy CT images minus water create iodine-enhanced images.^[2] For better visualization, the iodine concentration may be superimposed on the grayscale anatomic image as a color map.^[1] 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]

Close

Figure 2:

Various post-processing images from routine contrast-enhanced dual-energy computed tomography. (a) Axial blended image is derived from the blending of data from low- and high-energy images. (b) Virtual unenhanced image depicts the subtraction of contrast from the contrast-enhanced image. (c) In iodine overlay image, iodine-containing voxels are colour coded, usually in orange. (d) Virtual monochromatic low-energy image demonstrates increased contrast resolution with a better depiction of vascular structures.

Export to PPT

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.^[2] The difference in attenuation of soft-tissue structures, like muscles and organs, at various energy levels is minimal [Figure 3].^[4]

Close

Figure 3:

This graph shows the attenuation of various materials (y-axis) and their changes with differing energy levels (X-axis). The insert image demonstrates the location of the various regions of interest for various body constituents. Calcium (white curve) and iodine (yellow curve) show increased attenuation at low energy levels. Muscle (orange curve) and bile (green curve) do not show a significant difference in attenuation with differing energy levels. Fat (purple curve) shows reduced attenuation as the energy level decreases.

Export to PPT

Gallbladder

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]

Close

Figure 4:

A 68-year-old male presented with non-specific abdominal pain. (a and b) Axial unenhanced and portal venous phase of the upper abdomen demonstrates no calcified gallstone. (c) Axial virtual monoenergetic image at low-energy (40 keV) demonstrates two large non-calcified gallstones (arrows), which are hypoattenuating compared to bile. (d and e) On axial virtual monoenergetic image at high-energy (190 keV) and virtual unenhanced image the gallstones are hyperattenuating (arrows) compared to bile. These were not appreciable on routine CT images. (f) Ultrasound confirms two large gallstones (arrows).

Export to PPT

Close

Figure 5:

A 74-year-old female, known case of ovarian malignancy, computed tomography (CT) performed for staging. (a) Axial CT in the portal venous phase demonstrates a soft-tissue density lesion in the gallbladder (arrow) which was indeterminate for a solid lesion versus a non-calcified gallstone. (b) The axial virtual unenhanced image demonstrates a large soft tissue density lesion (arrow). (c) The axial iodine overlay map demonstrates iodine uptake within the lesion (arrow), confirming a solid lesion. The lesion was proven to be carcinoma of the gallbladder after surgery.

Export to PPT

Bowel

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].^[11] Active hemorrhage and hyperenhancement of the bowel wall in inflammation are also well demonstrated on low-keV virtual monoenergetic images.^[4,12]

Close

Figure 6:

A 73-year-old male clinically presented with intestinal obstruction. (a) Axial contrast-enhanced computed tomography demonstrates multiple dilated small bowel loops secondary to adhesions, causing intestinal obstruction. (b) The axial iodine overlay map demonstrates reduced mural enhancement of the small bowel loops on the left side (arrows), consistent with bowel ischemia, which was not readily apparent in the routine images. The patient also had a prior right hemicolectomy with a stoma in the right upper abdomen (not shown).

Export to PPT

Oncology

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]

Close

Figure 7:

A 72-year-old male presented with non-specific loss of weight. (a and b) Axial virtual monoenergetic images at low-energy (50 keV) demonstrate asymmetric enhancement in the left peripheral zone in the posterolateral aspect. (c and d) The lesions were not apparent in the routine axial images. The patient was later investigated to have elevated prostate-specific antigen levels, and prostatic carcinoma was detected on histopathology.

Export to PPT

Close

Figure 8:

A 55-year-old male was referred for upper abdominal pain. (a and b) Axial virtual monoenergetic image at low-energy (50 keV) and Iodine overlay images demonstrate an ill-defined enhancing mass in the neck of the pancreas (long arrow in a and b). The non-dilated main pancreatic duct (small arrows in a and b) is also better demonstrated and obscuration of the main pancreatic duct can be seen at the level of mass. (c) On the routine axial computed tomography of the abdomen in the arterial phase, the mass was inconspicuous. (d and e) Axial magnetic resonance imaging confirmed the suspicious mass in the neck of the pancreas, which was hypointense on the T1-weighted fat-suppressed images (arrow in d) and demonstrated diffusion restriction, appearing bright on diffusion-weighted images (arrow in e) and dark on apparent diffusion coefficient (ADC) maps (not shown). The lesion was proven to be a well-differentiated neuroendocrine tumor in histopathology.

Export to PPT

Hemorrhage

Virtual unenhanced images can confirm blood within the abdominopelvic cavity due to its hyperattenuating nature.

This may be beneficial in cases of acute intraabdominal hemorrhage or hemorrhagic lesions [Figure 9].^[4]

Close

Figure 9:

A 26-year-old female presented with lower abdominal pain. (a) Axial computed tomography of the pelvis in the portal venous phase demonstrates a circumscribed hyperdense lesion (arrow) closely abutting the posterior wall of the uterus (b) On the axial virtual unenhanced image, the lesion was hyperdense (arrow). (c) The lesion showed no iodine uptake on the axial iodine overlay map (arrow). These findings were consistent with a haemorrhagic lesion. (d and e) On the magnetic resonance imaging, the lesion was hyperintense on the axial T1-weighted image (arrow in d) and hypointense on the T2-weighted image (arrow in e). The imaging features were consistent with hemorrhage into a sub-serosal uterine fibroid.

Export to PPT

Adrenal

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]

Close

Figure 10:

A 79-year-old male presented with the left adrenal incidentaloma. (a) Axial unenhanced computed tomography demonstrates a circumscribed hypodense left adrenal nodule (arrow). The unenhanced attenuation was + 12 hounsfield units within the nodule. (b) The spectral curve within the lesion is suggestive of fat content within, which is consistent with adenoma.

Export to PPT

Renal

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].^[20]

Close

Figure 11:

A 63-year-old male with non-specific abdominal pain. (a) Axial computed tomography of the abdomen in the portal venous phase demonstrates an incidental circumscribed hyperattenuating (+68 hounsfield units) lesion in the upper pole of the right kidney (arrow). The lesion was indeterminate for a solid enhancing lesion versus a hyperdense cyst. (b) The lesion was also hyperattenuating (+ 65 hounsfield units) on the axial virtual unenhanced image (arrow). (c) There was absence of iodine uptake on the axial iodine overlay map (arrow). The imaging features were suggestive of hyperdense cyst, and the possibility of an enhancing solid lesion was ruled out.

Export to PPT

Close

Figure 12:

A 67-year-old male, a known case of autosomal dominant polycystic kidney disease (ADPKD), presented with hematuria. (a) Axial computed tomography of the abdomen in the portal venous phase demonstrates diffusely enlarged kidneys with multiple cysts of varying sizes replacing the entire renal parenchyma consistent with the known ADPKD. No suspicious enhancing lesion is apparent. (b) However, a focal enhancing lesion was readily identified on the axial iodine overlay map (arrow). (c) The lesion was isodense on the axial virtual unenhanced image (arrow), confirming contrast uptake on iodine overlay map. The imaging features were suspicious for renal cell carcinoma, which was confirmed on histopathology

Export to PPT

Bone

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].^[21]

Close

Figure 13:

An 81-year-old female presented with persistent left hip pain. (a) The color-coded map demonstrates bone marrow edema (coded in green) in the left femur extending from the lesser trochanter to the greater trochanter along the posterior aspect (arrow). (b) There was corresponding faint sclerosis (arrow) on the axial bone window, which was not readily apparent without the aid of colour-coded maps. This was consistent with an undisplaced intertrochanteric fracture of the femur.

Export to PPT

Limitations

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]

Close

Figure 14:

A 71-year-old male with a history of hepatocellular carcinoma post-chemoembolization. (a) Axial true unenhanced computed tomography (CT) demonstrates focal hyperdensity in the caudate lobe of the liver (arrow), due to lipiodol uptake within the hepatocellular carcinoma. (b) On the axial virtual unenhanced image, the lipiodol appears hypodense (arrow) due to the presence of iodine in lipiodol. (c) The arterial phase demonstrates focal hyperdensity (arrow), due to the lipiodol uptake and not due to arterial enhancement. If the true unenhanced image was unavailable in this study, there is a possibility of misinterpretation of hyperdensity in the arterial phase to be an arterial enhancement. This potential pitfall of suppression of lipiodol on the virtual unenhanced image must be remembered to avoid misinterpretation. Hence, we recommend obtaining true unenhanced CT in all cases of post-chemoembolization imaging of the liver.

Export to PPT

Virtual unenhanced images are not sensitive for the detection of renal calculi <3 mm because calcification tends to be smaller [Figure 15]. ^[14]

Close

Figure 15:

A 52-year-old male presented with hematuria. (a) The axial contrast-enhanced image demonstrates a tiny calculus in the left kidney (arrow). (b) However, the calculus was not visible on the axial virtual unenhanced image.

Export to PPT