From the Department of Radiology, Vancouver General Hospital, University of British Columbia, 899 W 12th Ave, Vancouver, BC, Canada V5Z 1M9 (N.M., K.E.D., F.E.W., P.D.M., S.N.); and the Medical Imaging Department, King Saud bin Abdulaziz University for Health Sciences, King Abdullah International Medical Research Center, Ministry of the National Guard, Health Affairs, Riyadh, Saudi Arabia (M.F.M.).
Evaluation of the nontraumatic acute abdomen with multidetector CT has long been accepted and validated as the reference standard in the acute setting. Dual-energy CT has emerged as a promising tool, with multiple clinical applications in abdominal imaging already demonstrated. With its ability to allow characterization of materials on the basis of their differential attenuation when imaged at two different energy levels, dual-energy CT can help identify the composition of internal body constituents. Therefore, it is possible to selectively identify iodine to assess the enhancement pattern of an organ, including the identification of hyperenhancement in cases of inflammatory processes, or ischemic changes secondary to vascular compromise. Quantification of iodine uptake with contrast material–enhanced dual-energy CT is also possible, and this quantification has been suggested to be useful in differentiating inflammatory from neoplastic conditions. Dual-energy CT can help determine the composition of gallstones and urolithiasis and can be used to accurately differentiate uric acid urinary calculi from non–uric acid urinary calculi. Moreover, dual-energy CT is capable of substantially reducing artifacts caused by metallic prostheses, to improve the imaging evaluation of abdominopelvic organs. The possibility of creating virtual nonenhanced images in the evaluation of acute aortic syndrome, gastrointestinal hemorrhage and ischemia, or pancreatic pathologic conditions substantially reduces the radiation dose delivered to the patient, by eliminating a true nonenhanced acquisition. Finally, by increasing the iodine conspicuity, contrast-enhanced dual-energy CT can render an area of free active extravasation or endoleak more visible, compared with conventional single-energy CT. This article reviews the basics of dual-energy CT and highlights its main clinical applications in evaluation of the nontraumatic acute abdomen.
After completing this journal-based SA-CME activity, participants will be able to:
■ Recognize the utility of dual-energy CT in evaluating the acute abdomen.
■ Describe the basic principles of CT and dual-energy CT, including acquisition and postprocessing techniques.
■ Describe the most commonly used dual-energy CT applications in pathologic conditions of the nontraumatic acute abdomen.
Evaluation of the abdomen with multidetector CT has long been accepted and validated as the reference standard in the acute setting (1,2). Its accessibility, high diagnostic accuracy, and effect on management have made multidetector CT a first-line tool in all major emergency departments around the world (3,4). Although investigators experimented with dual-energy CT in the early years of CT at the end of the 1970s (5,6), dual-energy CT has been used in clinical practice only since 2006, with the advances in CT technology targeting the early weaknesses of dual-energy CT, such as the disproportionate radiation dose compared with conventional single-energy CT, the excessive noise, the poor spatial resolution, and the long time of acquisition (7). Dual-energy CT has emerged as a promising tool, with multiple clinical applications already demonstrated in evaluation of acute abdominal processes.
This article focuses on the principles and main clinical applications of dual-energy CT in imaging the nontraumatic acute abdomen. The added value of dual-energy CT in evaluating the gastrointestinal, genitourinary, and vascular systems, as well as its potential for decreasing artifacts from metallic prostheses, is reviewed.
CT and Dual-Energy CT Principles
The images produced with CT depend on the attenuation of emitted x-ray photons by the different body constituents, with attenuation expressed in Hounsfield units, as calculated for each voxel of the imaged parts of the body (8). The attenuation of a specific material is a function of multiple factors but is mainly related to the material’s density, its effective atomic number (Z), and the energy level of the incoming x-ray beam (8). Every material has its own linear attenuation coefficient describing the fraction of an incoming x-ray beam attenuated per unit of thickness, a function of the level of energy to which it is exposed (8). Because the attenuation is also dependent on the density of the material, this coefficient can be normalized to what is called the mass attenuation coefficient (8).
In the range of energies used for diagnostic CT, two dominant physics principles account for the x-ray attenuation in the human body. First, the photoelectric effect results in photon absorption and increases with higher effective atomic numbers and lower energy levels. Second, the Compton effect is associated with photon scattering, which is relatively independent from the photon’s energy level, but is also accentuated with higher atomic numbers (8).
The attenuation value of a material submitted to a photon beam is at its maximum close to what is called the k-edge value, which represents the binding energy of the innermost shell of electrons, caused by a peak in photoelectric absorption. The k-edge value of iodine is 33.2 keV. On the opposite side of the spectrum, soft-tissue structures have little variation in their attenuation values on the diagnostic range of energy, with k-edge values ranging from 0.01 keV to 0.53 keV (9). Although calcium has a relatively low k-edge value of 4 keV, its value remains superior to those of soft tissues, with markedly higher attenuation of low-energy photons.
Depending on their relative density, two totally different materials such as calcium and iodine may present the same CT attenuation value at conventional single-energy CT when subjected to a single radiation beam. However, these two materials interact differently when exposed to different energy levels, independently of their density. Because of their high effective atomic numbers, they are more susceptible to the photoelectric effect (10). In opposition, the soft-tissue structures constituting the human body, such as the muscles or organs, demonstrate weak photoelectric effect and less variation of their attenuation values with different energy levels (10) (Fig 1).
Dual-Energy CT Technology
The principle behind dual-energy CT technology implies interrogating the attenuation of material when submitted to two different levels of energy, usually at high energy (140–150 kVp) and low energy (80–100 kVp). The term dual-energy CT is somewhat a misnomer because each beam is polyenergetic in nature, containing photons of multiple energies; and the technique is sometimes referred to as spectral CT.
Different scanning techniques have been developed by vendors to allow dual-energy CT, with the most commonly used technologies being dual-source dual-energy CT (Siemens Healthineers, Forchheim, Germany), fast-kilovoltage switching dual-energy CT (GE Healthcare, Milwaukee, Wis), and dual-layer detector dual-energy CT (Philips Medical Systems, Bothell, Wash). Although the acquisition technique differs among vendors, the common principle for dual-energy CT requires the acquisition of two datasets at separate energy levels for each part of the body. After acquiring these two datasets of images, postprocessing manipulations can be realized to create three different types of images at reconstruction, namely, mixed images, material-specific images, and virtual monoenergetic images.
First, images representing a combination of the high- and low-energy datasets are created, which are called mixed, blended, or weighted images. They are equivalent to the conventional single-energy CT images and are sent to the picture archiving and communication system (PACS) for interpretation. These images are usually made of variable percentages of the high- and low-kVp image datasets, simulating single-energy CT images acquired at 120 kVp (11,12).
Second, after evaluating the interaction of all body constituents with the high and low energy levels, dual-energy CT material-specific images can be created by analyzing the differential attenuation to obtain a dual-energy index. Software is used to calculate the attenuation properties of each voxel at low and high energy and, with a mathematical algorithm, can be used to determine the proportion of dominant materials within the voxel. Depending on the dual-energy CT technology, it is possible to evaluate in each voxel the content of calcium, iodine, and fat (13). Once the proportions are known, different sets of images can be generated after subtracting or isolating each constituent. The virtual nonenhanced images are created after the pixels containing iodine are identified and excluded from the image. Similarly, iodine-only images and virtual noncalcium images can be generated. It is also possible to superimpose a color-coded iodine overlay map on the mixed images (Fig 2) or to quantify the amount of iodine in a specific ROI to assess for enhancement. By allowing identification of the tissue composition, dual-energy CT provides a definitive advantage, compared with conventional single-energy CT.
Third, with use of two different energy levels, dual-energy CT allows the extrapolation of images for multiple single-energy levels. The created virtual monoenergetic images simulate the resultant image that would be obtained if the CT scanner were able to emit a true monoenergetic beam of x-rays, expressed in kiloelectron volts. Most types of postprocessing software programs are capable of extrapolating energy levels ranging from 40 keV to 200 keV, beyond the effective energy level of both x-ray tubes, by using variable fractions of the data acquired by each tube (14). Use of low-energy images helps to improve the contrast-to-noise ratio by increasing the attenuation of materials with a high atomic number, such as iodine and calcium, at the expense of a decreased spatial resolution and increased noise. On the other hand, the high-energy images are suitable to improve the signal-to-noise ratio, which is notably useful to overcome streak artifacts; but the high-energy images are associated with an overall decreased attenuation of the body constituents, limiting the soft-tissue contrast resolution between the imaged structures. Identification of gallstones isoattenuating to bile is also possible by using the virtual monoenergetic images, as described later in the “Biliary System” section.
At the beginning of the clinical application of dual-energy CT, the radiation dose associated with the modality was a concern, with substantially increased radiation exposure compared with that from single-energy CT (15). After optimization of the technology, including newer iterative reconstruction techniques, improved detector efficiency, and additional spectral filtration,
multiple investigators have shown that dual-energy CT is able to deliver an equal or smaller radiation dose, compared with conventional single-energy CT, especially with the newer-generation dual-energy CT scanners (16–18).
The false assumption that dual-energy CT is associated with twice the radiation dose is also erroneous because the total radiation dose used to obtain the two datasets is split between the two x-ray beams when using the dual-source or fast-kV switching techniques. Although technique varies among dual-energy CT technologies, utilization of a fraction of the data from the high- and low-kVp datasets allows one to obtain an image similar to that from conventional single-energy CT for the same level of noise. Dual-energy CT, with its ability to allow creation of virtual nonenhanced images, may obviate the need for a true nonenhanced acquisition, substantially lowering the radiation dose for multiphase examinations.
Given the advantages of CT scanning, patients who are suspected of having pathologic conditions of the bowel are usually evaluated with CT in the acute setting, owing to the rapid acquisition time, high resolution, and widespread availability of CT. However, single-energy CT scanning may have many limitations when it comes to evaluation of the unprepared bowel, some of which can be overcome with use of dual-energy CT. Imaging the bowel primarily relies on the principles of mural enhancement and nonenhancement on dual-energy CT images of the abdomen and pelvis obtained with administration of intravenous contrast material (19). It is also important to note that enteric iodinated contrast material should not be used when contrast-enhanced dual-energy CT postprocessing techniques are used, because the quantity of iodine within the enteric contrast material can compromise the assessment of mural enhancement. In this section, use of dual-energy CT for problem solving and diagnosing pathologic conditions of the bowel in the acute setting is discussed.
Inflammatory and Neoplastic Conditions of the Bowel.—Single-energy CT has been shown to be sensitive for the detection and assessment of intestinal mucosal enhancement and pericolonic fat changes in patients with inflammatory conditions of the bowel, including colitis, diverticulitis, and appendicitis (20). The subtle differences in mural attenuation can be accentuated with iodine quantification postprocessing techniques at dual-energy CT. In a recent publication, Lee et al (21) showed that bowel wall changes such as mucosal hyperemia related to active Crohn disease were most conspicuous on virtual monoenergetic images at 40 keV, improving the sensitivity and negative predictive value with regard to the depiction of active Crohn disease, compared with conventional polychromatic images obtained at 120 kVp by using iterative reconstruction (Fig 3). Additional research is required to determine if this finding is specific to Crohn disease or can be seen in other inflammatory conditions of the bowel, such as infection, because these diagnoses are associated with different management strategies and prognoses.
In addition to monoenergetic imaging, iodine mapping can be used to detect areas of hyperenhancement within the intestinal wall. One of the imaging features of acute appendicitis is mural enhancement owing to hyperemia, which can be subtle earlier in the course of the illness. Clinical experience at our institution has shown that dual-energy CT can be useful in the evaluation of acute appendicitis, but this use has not been validated in dedicated imaging series. Iodine overlay maps can help increase confidence in the diagnosis of acute appendicitis (Fig 4). Similarly, iodine quantification techniques can be used to identify complications of appendicitis, such as early ischemic or gangrenous changes and perforation that is indicated by discontinuity of the enhancement of the appendiceal wall (22), a finding that is often subtle or impossible to detect at conventional single-energy CT (Fig 5).
A bowel neoplasm can initially manifest as an acute abdominal condition when complicated by bowel obstruction, perforation, or intussusception (Fig 6). Emergency radiologists therefore have an important role in the correct identification of the causative factor for such a manifestation, because the cause may change the clinical and surgical management. An increasing amount of literature exists on the advantages of dual-energy CT for the detection of colorectal carcinoma (19,23,24). This detection has been primarily proposed through use of iodine maps, which rely on mural accumulation of intravenous contrast material. Although investigators have reported differentiation of malignancy from stool (25), discrimination of malignancy from nonneoplastic inflammatory processes such as acute diverticulitis has not been reported. The results of preliminary work at our institution suggest that the iodine concentration on iodine maps may be helpful in differentiating perforated colon cancer from acute diverticulitis, but this use is yet to be confirmed, and additional research studies are required to further define an appropriate threshold.
Ischemic and Hemorrhagic Conditions of the Bowel.—Bowel ischemia can result from hypoperfusion in low-flow states, arterial thromboembolic phenomena, and venous congestion and occlusion, or as a complication of intestinal obstruction and strangulation. Conventional multiphase contrast-enhanced single-energy CT has been shown to be accurate in the diagnosis of bowel ischemia resulting from primary vascular causes, with a reported sensitivity of 93.3% and a specificity of 95.9% (26). Segmental hypoenhancement or nonenhancement of the bowel wall is the most reliable sign for detecting ischemic and nonviable segments, but these changes are often subtle on conventional single-energy CT images and difficult to identify with confidence, with a sensitivity of 63.2% most recently reported in the setting of small-bowel obstruction (27,28).
Investigators have demonstrated increased sensitivity for detecting small-bowel ischemia in the setting of obstruction or vascular occlusion with use of low-keV virtual monoenergetic imaging by increasing the difference in attenuation between ischemic and nonischemic segments (29). By using the principle of iodine quantification, iodine mapping also increases the conspicuity of nonenhancing bowel, thereby providing the radiologist with increased diagnostic confidence (30) (Fig 7).
In the later stages of ischemia, as a result of damage to the wall and the microvasculature, or secondary to vascular congestion, bowel ischemia can be associated with submucosal hemorrhage. Because the resultant mural hyperattenuation may mimic enhancement, dual-energy CT virtual nonenhanced images and the iodine overlay map can be used to respectively demonstrate intramural hemorrhage and decreased iodine uptake and to support the diagnosis of late bowel ischemia (Fig 8). Similar principles can be applied to the detection of ischemia secondary to thrombotic phenomena (20). In the setting of arterial occlusion, the diagnosis may even be subtler than in the case of intestinal obstruction, because the remainder of the bowel may appear unremarkable, with no associated congestive edema (Fig 9).
Gastrointestinal bleeding is commonly encountered in the emergency setting and can be caused by many conditions, including anticoagulant therapy, bleeding disorders, neoplasms, inflammatory processes, or vascular conditions (31). Dual-energy CT analysis is helpful in confirming the intrinsic hyperattenuating nature of the intramural or intraluminal blood products on virtual nonenhanced images and by excluding the presence of an underlying enhancing lesion on the iodine-specific images (Fig 10). Similarly, dual-energy CT iodine-specific images can increase the conspicuity of small areas of active extravasation of intravenous iodinated contrast material within the intestinal wall or lumen (32) and can allow differentiation of these areas from hyperattenuating digestive material or intestinal foreign bodies (Fig 11). By increasing the conspicuity of iodine, utilization of the low-keV virtual monoenergetic images is also helpful in the identification of areas of active hemorrhage (33).
CT evaluation of suspected bowel ischemia and gastrointestinal hemorrhage usually requires a multiphase protocol, most commonly composed of nonenhanced and arterial and venous phases of acquisition. Another advantage of contrast-enhanced dual-energy CT is the possibility of creating virtual nonenhanced images derived from the contrast-enhanced phases, allowing up to a 30% reduction in the radiation dose, compared with that for conventional triphasic protocols (34).
Gallstone disease affects 10%–20% of the population and is a common reason that patients present with right upper quadrant abdominal pain at emergency departments (35). Although US is more sensitive in the evaluation of cholelithiasis and acute cholecystitis, CT is frequently used for the evaluation of right upper quadrant pain in the emergency department. However, up to 57% of gallstones are isoattenuating to the surrounding bile and therefore are not depicted on single-energy CT images (36).
Material decomposition techniques for dual-energy CT have enabled the identification and differentiation of materials of differing chemical compositions that happen to have similar attenuation values on CT images (eg, isoattenuating gallstones). Although the CT number of water, as well as that of most soft tissues, demonstrates minimal change in attenuation when imaged with varying x-ray spectra, this observation does not apply to other body materials that possess an effective atomic number that is substantially above or below the atomic number for water. This means that for heavy atoms (such as iodine in contrast agents), the CT number decreases with increasing tube voltage; and for carbon- and hydrogen-rich materials such as cholesterol-containing gallstones, the CT value increases with increasing x-ray tube voltage (37,38).
In recent years, much work has been done with regard to use of dual-energy CT to assess noncalcified gallstones.
Although energy-specific monoenergetic reconstruction algorithms have shown promise in the detection of gallstones isoattenuating to bile—with noncalcified gallstones having an attenuation lower than that of bile on images obtained with low-keV reconstruction (40 keV) and higher than that of bile on images obtained with high-keV reconstruction (≥140 keV) (39,40)—the sensitivity and specificity of these reconstruction algorithms have yet to be determined.
Alternatively, material decomposition techniques with a calcium base, lipid base, or a calcium, lipid, and water base algorithm (three-material decomposition) provide excellent values for sensitivity and specificity, ranging from 80% to 95% and from 95% to 100%, respectively (41). The three-material decomposition technique is used routinely at our institution (Figs 12, 13).
Low-keV virtual monoenergetic image reconstruction and iodine overlay images also improve assessment of wall enhancement, enabling improved assessment of gallbladder wall hyperemia in cases of acute cholecystitis, as well as accentuating regional hepatic parenchymal hyperemia (Fig 14). These techniques also improve detection of gallbladder wall gangrene by improving depiction of areas of decreased enhancement in cases of complicated cholecystitis (7) (Fig 15) or gallbladder volvulus (Fig 16). The combination of the ability of dual-energy CT to depict noncalcified gallstones and its improved depiction of wall enhancement defects makes it ideal in the assessment of gallstone ileus in which (a) the impacted stone may not be readily depicted on conventional single-energy CT images and (b) areas of interruption of the gallbladder wall may be difficult to detect (Fig 17).
In the setting of acute pancreatitis, CT is the modality of choice to direct management, assess disease evolution, and identify complications. The utility of dual-energy CT in acute nontraumatic emergencies of the pancreas has not been thoroughly studied, but many applications can be used to assess complications of acute pancreatitis such as ischemia, necrosis, pseudocyst formation, and vascular complications, including thrombosis and pseudoaneurysms (42,43).
On conventional contrast-enhanced single-energy CT images, the inflamed pancreas will usually demonstrate hypoenhancement or hypoattenuation, but distinguishing among edema, ischemia, or necrosis can be challenging. With use of low-keV virtual monoenergetic imaging, dual-energy CT can increase the attenuation difference between the normal parenchyma and the affected parenchyma, improving the detection of hypoenhancement or a complete lack of enhancement in cases of pancreatic ischemia or necrosis (7,44) (Fig 18). Necrotizing pancreatitis can also be diagnosed with the iodine overlay map by showing a lack of iodine uptake in necrotic areas (43) (Fig 18). Complex pancreatic and peripancreatic collections in cases of acute pancreatitis can also be evaluated with dual-energy CT to help differentiate necrotic debris, hematoma, or residual parenchyma with preserved enhancement (43,45). Silva et al (7) argued that spectral dual-energy CT may be able to help differentiate between acute hemorrhage and enhancing pancreatic parenchyma when the water material density display is used. Similarly, use of the virtual nonenhanced images is helpful in identifying hematoma related to hemorrhagic pancreatitis and in differentiating it from enhancing parenchyma on the contrast-enhanced images (Fig 18).
In about 40%–70% of the cases of acute pancreatitis, the cause is related to gallstones (46). Dual-energy CT with virtual monoenergetic image analysis has been demonstrated to be helpful in the detection of noncalcified gallstones in the gallbladder, as discussed earlier in the “Biliary System” section (40,45). Similarly, use of virtual monoenergetic images can increase the conspicuity of isoattenuating cholesterol stones within the cystic duct or the common bile duct, which demonstrate higher and lower attenuation values compared with the surrounding bile at high and low keV values, respectively (40,41).
Mileto et al (47) concluded that dual-energy CT virtual nonenhanced images are comparable to true nonenhanced images at pancreatic abdominal CT in a cohort of patients with a known or suspected pancreatic mass. Use of the dual-energy CT virtual nonenhanced acquisition can obviate use of the true nonenhanced acquisition and thus result in reduction of the radiation dose. In the results of Mileto et al (47), radiation dose reduction of up to 46% was observed when a combined dual-energy CT–single-energy CT dual-phase protocol was used, compared with a single-energy CT standard triple-phase protocol. This potential for dose reduction is even more important because many patients with severe or complicated pancreatitis will require repeated follow-up examinations or imaging-guided interventions, which can markedly increase their cumulative radiation exposure (48).
The standard virtual nonenhanced algorithm can potentially subtract calcium, considering that the algorithm is based on three different components (soft tissue, fat, and iodine). This subtraction of calcium would result in an underestimation of duct calculi and parenchymal calcifications, and Mileto et al (47) suggested adjusting the postprocessing algorithm for soft tissue, iodine, and calcium. However, the findings from use of this adjustment in cases of acute or chronic pancreatitis have never before been published.
Dual-energy CT has potential for use in the acute pancreatic emergency. However, dedicated research needs to be done to further investigate its potential.
Urolithiasis is a frequently encountered problem in the general population, with a rate of occurrence of 12% in men and 6% in women (49). Many patients will present to the emergency department with an episode of acute flank pain, for which the cornerstone investigation is CT, considering its high sensitivity (97%) and specificity (96%) (50,51).
Definitive treatment of urolithiasis depends on the location, size, and chemical composition of the stone. The most common composition of urinary stones is calcium oxalate, and uric acid stones represent up to 10% of the renal calculi (52). In patients with urolithiasis, differentiating uric acid stones from non–uric acid stones is especially important because the former are treatable with urine alkalization (52). The non–uric acid stones usually require mechanical fragmentation or extraction, depending on their relative hardness. However, calcium oxalate monohydrate, cystine, and brushite calculi may require percutaneous lithotomy, considering their increased resistance to fragmentation (53).
Conventional single-energy CT has limited utility in the accurate determination of the chemical composition of urinary calculi solely on the basis of their attenuation, considering that an overlap exists among the different types of stones (54,55). On the other hand,
dual-energy CT has been shown to be highly accurate in the differentiation of uric acid stones from non–uric acid stones, with a diagnostic accuracy ranging from 90% to 100% (56–58) (Fig 19).
Urinary stone characterization is achieved by determination of a dual-energy index for the evaluated calculus after analyzing the differential attenuation at high and low energy levels. Moreover, second- and third-generation dual-energy CT scanners, with additional filtration of the high-energy photons, allow a better spectral separation and a more detailed analysis of the stone composition (59,60), with differentiation of cystine, struvite, calcium oxalate dihydrate or monohydrate, and apatite stones (59,61).
Investigators have also suggested that dual-energy CT is a useful predictor of the success of therapy with extracorporeal shock wave lithotripsy (62,63). Finally, dual-energy CT virtual nonenhanced images derived from an intravenous contrast material–enhanced abdominopelvic examination can also be used to identify urinary stones masked by the contrast opacification of the kidneys and collecting system (64,65). However, dual-energy CT has been shown to be limited in the assessment of the composition of small stones (≤3 mm) when one is imaging patients with a higher body mass index, because of a reduced signal-to-noise ratio (64,66), or in the presence of a urinary catheter causing artifacts (67).
Inflammatory, Hemorrhagic, and Neoplastic Processes
Although dual-energy CT evaluation of the acute inflammatory and hemorrhagic conditions of the genitourinary system has not been studied in depth, it is routinely used in our clinical practice to increase the diagnostic confidence in interpreting examinations and as a problem-solving tool. With use of iodine-specific images such as the iodine overlay map, dual-energy CT can help increase the conspicuity of hyperemia and accentuated iodine uptake in cases of acute inflammatory processes involving the upper and lower urinary tracts. With use of the same postprocessing technique, focal areas of decreased iodine uptake within an inflamed organ such as the prostate can also raise concern about an evolving plegmon or abscess (Fig 20).
In cases of acute hemorrhage, the virtual nonenhanced images can be used to help confirm the intrinsic hyperattenuating nature of acute blood products within the collecting system or abdominopelvic cavity (Fig 21), and the iodine-specific images can be used to identify any underlying enhancing lesions or areas of active extravasation of intravenous contrast material (Fig 22).
In the literature, dual-energy CT analysis of renal masses has been shown to be sensitive, specific, and accurate at differentiating enhancing from nonenhancing renal masses when using the iodine-specific postprocessing analysis (68,69), either subjectively or by measuring the iodine concentration within the evaluated lesion. In the acute setting, this type of analysis is notably helpful in differentiating hyperattenuating hemorrhagic or proteinaceous renal cysts from renal neoplasms (Fig 23).
Imaging the aorta in the acute setting usually requires a nonenhanced acquisition and an angiographic acquisition. The nonenhanced evaluation allows characterization of increased mural attenuation as an acute intramural hematoma, not to be confused with mural enhancement after contrast material administration. The nonenhanced acquisition is also relevant when a previous intervention such as an endograft procedure or surgery has been performed, to differentiate postinterventional material from iodine.
Material-specific imaging with dual-energy CT can be used to differentiate intravascular iodinated contrast material from calcified atherosclerotic plaques or surgical materials on the basis of their differential attenuation values. By using postprocessing software, calcified plaques or iodine can be digitally subtracted from the image to generate virtual noncalcium images or virtual nonenhanced images. In this way, an intramural hematoma or acute vascular thrombosis can be diagnosed (Figs 24, 25). Radiologists should, however, be aware of a potential pitfall in detecting intramural hematoma with visual inspection of the virtual nonenhanced images, because there can be incomplete subtraction of the intravascular iodine content, leading to the attenuation of the wall being identical to that of the lumen (70). A substantial reduction in radiation dose may be achieved with dual-energy CT by eliminating the need for a true nonenhanced acquisition, which is replaced by the virtual nonenhanced reconstruction images (71,72). Similarly, calcified atherosclerotic plaques obscuring the vascular lumen can be removed to improve assessment of the degree of stenosis (73).
Taking advantage of the increased attenuation of iodine at low-kV imaging, use of virtual monoenergetic images at a low energy level has been shown to be helpful in reducing the amount of intravenous contrast material injected, with preservation of diagnostic accuracy for depicting endoleaks (71,72) (Fig 26). Stolzmann et al (72) demonstrated a preserved high sensitivity and specificity for depicting endoleaks with a single-phase delayed dual-energy CT evaluation with reconstruction of virtual nonenhanced images. Endoleaks can also be confirmed by using the iodine overlay images that distinguish between (a) leakage of intravenous iodinated contrast material and (b) postinterventional materials, both of which demonstrate hyperattenuation on conventional single-energy CT images.
An advantage of low-keV reconstruction is that it can also be used to salvage a poorly enhanced study secondary to a miscalculation of contrast material administration, patient physiology, poor bolus timing, or partial extravasation of contrast material, by avoiding the need to repeat an examination with suboptimal results. Low-keV reconstruction can also be used to decrease the rate and volume of contrast material injected into the patient without compromising the quality of the study (74), which is especially useful in the case of actual renal impairment or to prevent such an event. Marin et al (74) also demonstrated an improved homogeneity in aortic enhancement with virtual monoenergetic images.
Another advantage of dual-energy CT with use of the virtual monoenergetic application, this time at a higher energy level, is to decrease streak and blooming artifacts caused by endovascular stents or embolization material (75). On the other hand, the contrast attenuation of the vessels will be reduced, and the inverse relationship exists at lower energy levels where contrast material attenuation and blooming or streak artifacts are increased. Further studies are, however, needed to define the optimal kiloelectron volt setting.
The use of metallic prostheses has markedly increased during the past few decades with the aging of the population and the increased life expectancy, with a 30%–50% increase in the rate of primary hip arthroplasty (76). Metallic prostheses are highly attenuating because of their high density and elevated attenuation coefficient causing substantial impairment of the image quality by creating artifacts related to beam hardening, photon starvation, scattered radiation, and excessive quantum noise (77,78).
As the polyenergetic x-ray beam passes through a metallic prosthesis, preferential attenuation of the low-energy photons will happen, with only the higher-energy photons being able to reach the detector. This preferential attenuation will result in an increased mean energy level of the resultant beam posterior to the prosthesis, which is called the beam hardening effect. Because the resultant higher-energy photons are not substantially attenuated by the normal constituents of the body, dark streaks will be created where no interaction has occurred. The limited number of photons traversing the prosthesis is also responsible for the photon starvation artifact (79), in which the overall reduced number of residual photons is insufficient to create an image with a respectable signal-to-noise ratio.
The virtual monoenergetic image postprocessing application of dual-energy CT can substantially reduce the artifacts caused by metallic prostheses when selecting higher-energy images (Fig 27).
In this way, a greater proportion of the information registered by the higher-energy photons is selected at reconstruction of the images, with greater photon penetration traversing the prosthesis to provide information about the surrounding tissues. Reduction in the degree of beam hardening artifacts and photon starvation artifacts is therefore possible with an improved image quality, without increasing the radiation dose (79–81). However, with a high energy level, the degree of contrast resolution between the different anatomic planes will be impaired. Many investigators have evaluated the optimal kiloelectron volt level to overcome metallic artifacts without compromising the image quality, usually set between 110 keV and 140 keV (80,81). The optimal level should be set up by the interpreting radiologist to provide the best diagnostic quality to answer a specific clinical question. Also, the energy level will vary depending on the type and number of prostheses, their size, and also the dual-energy CT technology used (82,83).
Dual-energy CT represents the most recent major advance in CT technology and is changing the face of CT. Many clinical applications have been demonstrated to represent added value in imaging the nontraumatic acute abdomen, notably for the evaluation of inflammatory and ischemic changes, hemorrhagic conditions and active bleeding, identification of gallstones isoattenuating to bile, characterization of renal stones, and reduction of artifacts. The technology of dual-energy CT is still evolving, and further research is needed to understand the full extent of its potential.
Disclosures of Conflicts of Interest.—M.F.M.Activities related to the present article: disclosed no relevant relationships. Activities not related to the present article: speaker fees from Siemens Healthineers. Other activities: disclosed no relevant relationships. P.D.M.Activities related to the present article: disclosed no relevant relationships. Activities not related to the present article: Chief Medical Officer for Change Healthcare; speaker fees from Siemens Healthineers. Other activities: disclosed no relevant relationships. S.N.Activities related to the present article: disclosed no relevant relationships. Activities not related to the present article: personal fees and institutional research grant from Siemens Healthineers. Other activities: disclosed no relevant relationships.
Recipient of a Certificate of Merit award at the 2017 RSNA Annual Meeting.
For this journal-based SA-CME activity, the authors M.F.M., P.D.M., and S.N. have provided disclosures (see end of article); all other authors, the editor, and the reviewers have disclosed no relevant relationships.
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Received: Mar 12 2018 Revision requested: May 4 2018 Revision received: July 4 2018 Accepted: July 5 2018 Published online: Jan 08 2019 Published in print: Jan 2019