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Applications of dual energy computed tomography in abdominal imaging
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Diagnostic and Interventional Imaging (2016) xxx, xxx—xxx
PICTORIAL REVIEW /Abdominal imaging
Applications of dual energy computed tomography in abdominal imaging T. Lestra a,∗, S. Mulé a, I. Millet b, A. Carsin-Vu a, P. Taourel b, C. Hoeffel a a
Department of radiology, hôpitaux universitaires de Reims, avenue du Général-Koenig, 51092 Reims cedex, France b Department of medical imaging, hôpital Lapeyronie, hôpitaux universitaires de Montpellier, 191, avenue du Doyen-Gaston-Giraud, 34090 Montpellier, France
KEYWORDS Computed tomography; Dual energy CT; Liver tumors; Renal tumors; Urinary stones
Abstract Dual energy computed tomography (CT) is an imaging technique based on data acquisition at two different energy settings. Recent advances in CT have allowed data acquisition and almost simultaneously analysis of two spectra of X-rays at different energy levels resulting in novel developments in the field of abdominal imaging. This technique is widely used in cardiovascular imaging, especially for pulmonary embolism work-up but is now also increasingly developed in the field of abdominal imaging. With dual-energy CT it is possible to obtain virtual unenhanced images from monochromatic reconstructions as well as attenuation maps of different elements, thereby improving detection and characterization of a variety of renal, adrenal, hepatic and pancreatic abnormalities. Also, dual-energy CT can provide information regarding urinary calculi composition. This article reviews and illustrates the different applications of dual-energy CT in routine abdominal imaging. © 2016 Éditions franc ¸aises de radiologie. Published by Elsevier Masson SAS. All rights reserved.
Introduction Dual energy computed tomography (DECT) is based on CT data acquisition at two different energy settings. X-ray photons interact differently with matter at different energy settings resulting in different attenuation values. At two different energy settings, information about the composition of a given tissue may be obtained by analyzing how it interacts with
Abbreviations: CT, computed tomography; DECT, dual-energy computed tomography; HU, Hounsfield unit; HCC, hepatocellular carcinoma; GIST, gastrointestinal stromal tumor; NET, neuroendocrine tumor. ∗ Corresponding author. E-mail address: [email protected] (T. Lestra). http://dx.doi.org/10.1016/j.diii.2015.11.018 2211-5684/© 2016 Éditions franc ¸aises de radiologie. Published by Elsevier Masson SAS. All rights reserved.
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the photons [1,2]. DECT has been known for many years. Studies published in the 1970s have shown that this technique was useful for tissue characterization, but applications remained limited because of technical difficulties, especially prolonged data acquisition at different energy settings thereby decreasing spatial and temporal resolution [3]. Recent advances in computed tomography (CT) technology allow now acquisition and almost simultaneously analysis of two X-ray spectra at different energy settings. For these reasons, a new interest in DECT has emerged. In cardiovascular imaging, DECT is used in patients with suspected pulmonary embolism, because iodine is better visualized in distal, small pulmonary arteries and perfusion imaging offers excellent correlation with scintigraphy [4,5]. In musculoskeletal imaging, DECT helps characterize monosodium urate deposits in patients with gout and can also improve the assessment of metallic implants by reducing metal artifacts [6,7]. Abdominal applications of DECT are less well-known and still under development. With DECT it is possible to obtain virtual unenhanced images from monochromatic reconstructions as well as attenuation maps of different elements, thereby improving detection and characterization of a variety of renal, adrenal, hepatic and pancreatic abnormalities. Finally, using DECT is possible to determine urinary calculus composition. This article reviews and illustrates the different applications of DECT in abdominal imaging.
Physical and technical considerations General principles With CT, attenuation is directly related to the interaction between X-rays and matter. A matter’s attenuation coefficient depends on the matter’s composition and on the energy of the incident X-ray photons [8]. At the energy levels used in diagnostic imaging, X-ray photons interact with matter either by a photoelectric effect or by Compton scattering. At
Figure 1. Example of energy level distribution (simplified) of a beam of X-ray photons in conventional single source CT at 120 kVp. Note the asymmetry of the curve. At low energy setting, X photons are partially filtered at the exit of the tube (beam hardening).
lower kilovoltage, the photoelectric effect prevails. Here, attenuation, which is tissue-density dependent (i.e., atomic number), is high. This results in high contrast and noise. On the other hand, at higher kilovoltage, the attenuation is primarily driven by the Compton scattering mechanism. Here, attenuation is lower, resulting in lower image noise and contrast. With CT, tissue contrast depends on tube voltage (in kVp), which is an adjustable parameter. This parameter determines the upper limit of the energy spectrum of the X-rays produced by the tube. With conventional single-source CT using polychromatic X-rays, images are usually acquired at an energy setting between 80 and 120 kVp, to optimize the balance between contrast and noise (Fig. 1). DECT is based on the simultaneous acquisition at low (80 kVp) and high (140 kVp) energy levels (Fig. 2). Depending on the manufacturer, several technical approaches are available to acquire double datasets. Dualsource CT consists of two tubes and two detectors, simultaneously rotating around the patient, with an angle of 90◦ between both measuring systems with different fields of view (FOV) (Fig. 3). The single-source system consists of
Figure 2. Example of energy level distribution (simplified) of a beam of X-ray photons in dual energy computed tomography (DECT) at 80 and 140 keV.
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Monochromatic images Monochromatic images are reconstructed images obtained at fixed voltage, possibly ranging across a broad spectrum of energies (40 to 140 keV). They help improve the contrast between the different structures, based on the characteristics of the examination, of the patient, and of the area under investigation. These reconstructions are particularly useful to improve the iodine contrast and thus the assessment of contrast uptake in a given lesion (Fig. 5) [9].
Metal artifacts reduction
Figure 3. Dual-source system with two tubes and two detectors with an angle of 90◦ between both measuring systems.
a single tube and detector combination that alternates very quickly between high and low kilovoltage and has detectors with very short afterglow (Fig. 4). Another approach is to use a CT unit equipped with 320 rows of detectors that allow an area of 16 cm to be scanned in a single rotation, to perform two subsequent scans, with a switch in tube voltage (80 and 140 kVp) in-between. This technique has limited volume coverage. Finally, another technique, under development, consists in a dual layer detector placed directly on top of the other that receives separately low (up to 80 keV) then high (up to120 keV) energy photons. Post-processing of DECT data: With DECT, data processing processing yields several types of images.
Metal artifact is mainly caused by photon beam hardening resulting in streak artifacts that radiate away from an extremely dense element. When crossing dense structures, low-energy photons are more attenuated than high-energy photons (Fig. 6) [10]. DECT makes it possible to reduce the artifacts and provide satisfactory visualization of metallic implants and surrounding bony structures on monochromatic reconstructions at higher levels of energy. By using a metal artifact reduction software (MARs)-type reconstruction algorithm, and the DECT-acquired data, it is possible to improve the quality of the images of adjacent soft tissues, for instance in the pelvis in patients with metal hip prostheses (Fig. 7) [11].
Attenuation maps of different elements The Hounsfield (HU) attenuation curve of an element at a given intensity of the beam of X-rays depends on the element’s atomic number (Fig. 8). Dual energy acquisition makes it possible to distinguish some elements such as iodine, calcium, water, and uric acid on the basis of their atomic number and to create a map (Fig. 9). Finally, iodine quantification mapping makes it possible to reconstruct in retrospect virtual unenhanced images from the enhanced images, by removing the iodine, whatever the time after injection.
Radiation dose
Figure 4. Single source system. The tube alternates very quickly between 80 and 140 keV.
There is increasing concern for potential long-term risks regarding exposure to radiation in medical diagnostic imaging, especially for young patients and for patients who require follow-up examinations on a regular basis. Some experts believe that, compared to single energy, radiation from DECT is 20% greater, but it remains below the diagnostic reference levels [12,13]. The radiation dose varies depending on manufacturers and protocols. Recent studies in liver and urinary tract imaging have shown that with improved protocols, the radiation dose can be similar to the dose in single energy CT, and even lower [14,15]. For instance, one study that included patients with suspected hepatocellular carcinoma showed a highly significant reduction in dose-length product (DLP) and 37% reduction in effective dose (P < 0.001) for an adapted dual energy acquisition protocol compared to single energy acquisition [14]. Moreover, virtual unenhanced image acquisition by omitting true unenhanced images reduces the radiation dose up to 47%, depending on the protocol, while obtaining images of equivalent quality [16].
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Figure 5. Monochromatic reconstructions obtained at different level of energy: a: at 40 keV; b: at 60 keV; c: at 80 keV; d: at 100 keV; e: at 120 keV; f: at 140 keV. Reconstructions at a low energy setting have a high contrast but also a high noise level, while reconstructions at a high level of energy have low contrast but also low noise.
Applications in abdominal imaging Hepatic applications DECT is used to visualize and characterize liver lesions and assess tumor response to therapy. DECT has also been used to quantify the degree of hepatic steatosis or hepatic fibrosis. Virtual unenhanced images could replace true unenhanced images, based on acceptable image quality and good correlation between the attenuation values of both image categories [17]. However, as far as bile duct stones are concerned, these virtual unenhanced images do not accurately depict small stones (i.e., surface area <9 mm2 ) or stones with little attenuation values (i.e., < 78 HU) [18]. By using monochromatic images during the portal venous phase after IV administration of iodinated contrast material, it is possible to increase the contrast between lesion and adjacent healthy liver parenchyma, at an optimal energy level of 70 to 80 keV, while maintaining an acceptable level of noise and thereby improving conspicuity of hypovascular hepatic metastases (Fig. 10) [19].
DECT is even more useful to visualize hypervascular hepatic lesions. For instance, monochromatic reconstructions at a low energy setting (i.e., < 70 keV) combined to an iodine-based map during the late arterial phase (45 seconds after IV) improves the diagnostic accuracy for hypervascular lesions (detection, conspicuity and diagnostic confidence), especially for small hepatocellular carcinomas (Figs. 11 and 12) [9]. In addition, the use of DECT improves the characterization of hypervascular lesions. Lv et al. showed that small hepatocellular carcinomsa and small hepatic hemangiomas can be differentiated with high degrees of sensitivity and specificity, by quantifying the iodine density of the lesions, first during the arterial and then during the portal phase [20]. According to Wang et al., the analysis of the spectral curve slope of a lesion helps discriminate between hemangiomas, hepatocellular carcinoma, metastases and biliary cysts with sensitivities and specificities of 87% and 100% for hemangiomas, respectively, 82.1% and 65.9% for hepatocellular carcinomas, 65.9% and 59% for metastases and 44.4% and 100% for biliary cysts [21].
Figure 6. Modifications of the photon spectrum after beam attenuation in dense matter. When crossing dense structures, low-energy photons are halted.
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Figure 7. DECT image of the pelvis in the coronal plane obtained after IV administration of iodinated contrast material performed in 77year-old woman with acute abdominal pain: a: monochromatic reconstructions obtained without metal artifact reduction software (MARs) reconstruction algorithm at 70 keV; b: monochromatic reconstructions without MARs reconstruction algorithm at 140 keV; c: monochromatic reconstructions with MARs at 70 keV; d: monochromatic reconstructions with MARs at 140 keV. There are fewer artifacts with reconstructions at high energy levels. MARs with dual energy acquisition substantially reduces metal artifacts (c and d) (DLP = 1063.7 mGy/cm and CTDI vol = 25.53 Gy). There are fewer artifacts with reconstructions at high energy levels (b and d). MARs with dual energy acquisition substantially reduces metal artifacts (c and d). The DLP was 1063.7 mGy/cm and the CTDI vol = 25.53 Gy.
currently investigated to quantify hepatic steatosis and liver fibrosis. Compared to MRI data, encouraging preliminary results have been obtained in this area [24,25].
Pancreatic applications DECT, with monochromatic reconstructions at 50—70 keV, improves the contrast between tumor and surrounding parenchyma [26]. This optimized contrast improves the detection of pancreatic adenocarcinomas, especially isoattenuating adenocarcinomas, that are generally small lesions and with a better prognosis than more advances tumors. Iodine maps also improve the detection of these lesions (Fig. 13) [27]. Although no studies have yet been published, DECT could also be useful to assess other types of pancreatic diseases. Figure 8. Hounsfield attenuation curve (HU) of calcium and iodine at a given level of energy.
DECT showed promising results in the assessment of tumor response in patients undergoing targeted therapy for hypervascular liver tumors such as hepatocellular carcinoma, metastases from gastrointestinal stromal tumors, neuroendocrine tumors, or melanoma [22,23]. DECT is also
Gastrointestinal tract applications Acute mesenteric ischemia is a serious condition that conveys high morbidity and mortality and requires early diagnosis for best treatment. CT is currently the reference technique for patients with suspected mesenteric ischemia [28]. However early in the disease process, signs
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Figure 9. Examples of different maps obtained in a 45-year-old man, acquired at the level of right renal hilum in the transverse plane: a: virtual unenhanced DECT image in the transverse plane; b: corresponding native DECT image after IV administration of iodinated contrast material during the portal venous phase; c: corresponding iodine map.
Figure 10. DECT images of a biliary cyst (arrow) in the transverse plane: a: native image during the portal venous phase; b: iodine maps shows no uptake of iodinated contrast material by the lesion.
of ischemia may be difficult to identify on CT, especially because decrease parietal enhancement of the small bowel wall may be subtle and hardly visible in case of arterial ischemia. An experimental study, on animal model, shows that DECT, notably through iodine mapping, improves the visualization of ischemic small bowel compared to single energy CT (Fig. 14) [29].
Applications in urologic imaging Renal tumors Graser et al. have shown that a series of virtual unenhanced images allows characterizing renal masses without the need for a true unenhanced CT acquisition [30]. Moreover, monochromatic images, like for hepatic and pancreatic imaging, are reconstructions at a low dose of energy that show improved contrast between tumor and healthy parenchyma and therefore enable optimal detection of hypervascular renal tumors and tumor recurrences after radiofrequency ablation [31,32]. Iodine maps also improve the detection of tissular portions in cystic lesions (Figs. 15 and 16). Iodine quantification is also possible, which seems more accurate than the measurement of attenuation values during the different phases. Lesion characterization is thus improved [33]. Indeed, Milet et al. reported in a study that included 21 papillary tumors and 67 renal clear cell carcinomas, that an iodine concentration of 0.9 g/cm3 is an optimal threshold value to
discriminate between papillary tumors than show lower degrees of enhancement and clear cell adenocarcinomas, with a sensitivity of 98.2% and a specificity of 86.3% (Fig. 16) [34].
Characterization of urinary calculi Treatment of urinary calculi depends on stone composition. For instance, urine alkalization is the cornerstone of treatment for patients with uric acid containing urinary calculi [35]. Using DECT, it is possible to assess in vivo the composition of the stones, and thus tailor the treatment, by analyzing the attenuation curve of the stone and to compare it with reference values and by mapping elements [36]. Based on the analysis of the attenuation values, it is possible to discriminate between uric acid containing from non-uric acid containing calculi with DECT in 93% of the cases, against 40% with conventional CT (Fig. 17) [37]. Takahashi et al. have shown that the detection of small calculi (1 to 2 mm) was limited on virtual unenhanced images generated during the pyelographic phase (i.e., obtained approximately 10 minutes after IV) [38].
Characterization of adrenal nodules Adrenal incidentalomas have an incidence ranging between 4.4% and 9% on CT examinations [39,40]. Characterization of these nodules requires unenhanced CT images. Virtual unenhanced images allow the radiologist to determine that an
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Figure 11. DECT angiography of the abdomen performed for the follow-up of aortic dissection in a 60-year-old man at an arterial phase. A hypervascular liver metastasis (arrow) from neuroendocrine tumor was incidentally detected in right liver: a: native polychromatic image; b: virtual unenhanced image; c: monochromatic reconstruction at 55 keV; d: monochromatic reconstruction at 100 keV; e: iodine mapping. Virtual unenhanced reconstruction images are similar to unenhanced images. The visualization of the lesion is good with reconstructions at a low level of energy (on c) and iodine mapping (on e). At a high-energy setting the lesion is less visible due to the sharp reduction of contrast-to-noise ratio (contrast reduction is more important than noise reduction).
adrenal mass is an adenoma if its attenuation value is < 10 HU [41,42]. Although further large-scale studies are needed, preliminary studies have shown that adrenal adenomas would be more accurately diagnosed by analyzing attenuation variations at energy levels between 80 keV and 140 KeV. Unlike for adrenal metastases, the attenuation values for
adenomas at 80 keV appear to be lower than at 140 keV, indicating the presence of intracellular lipid with 100% specificity and 50% sensitivity (Fig. 18) [43]. The low sensitivity may be explained by the fact that some adenomas contain little fat, thereby modifying their attenuation values and their interaction with the X-ray photons at different levels of energy.
Figure 12. DECT image of the abdomen in the transverse plane shows an hypervascular liver metastasis of gastric stromal tumor (arrow) in a 64-year-old woman: a:arterial phase with monochromatic reconstruction at 70 keV; b:iodine mapping. The increased contrast-to-noise ratio on monochromatic images and iodine mapping provide a good visualization of the hypervascular metastasis and a more accurate identification of the contrast uptake by the lesion.
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Figure 13. DECT image of the abdomen in the transverse plane during the late arterial phase after IV, shows adenocarcinoma of the pancreatic head and biliary stent (arrows) in a 56-year-old man: a: monochromatic reconstruction at 70 keV; b: iodine mapping. The visualization of the tumor, which is hypovascular relative to the healthy pancreatic parenchyma, is good due to the contrast-to-noise ratio in monochromatic imaging and the iodine mapping.
Figure 14. 73-year-old man with several cardiovascular risk factors presenting with acute abdominal pain, inflammatory syndrome and increased level of serum lactates. Mesenteric ischemia was strongly suspected: a: DECT image of the abdomen in the transverse plane obtained with polychromatic reconstruction; b: iodine mapping. Compared to polychromatic image, iodine mapping provides better visualization of the ischemic loops (white arrows) in relation to the vascularized loops (black arrowhead)
Figure 15. Abdominal DECT image in the transverse plane obtained in a patient with Bosniak I left renal cyst (arrow) during the arterial phase: a: native DECT image; b: corresponding iodine map definitely confirms absence of enhancement of the left renal cyst.
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Figure 16. Abdominal DECT image in the transverse plane obtained in a 67-year-old man, with left renal adenocarcinoma (arrow) during the arterial phase: a: unenhanced DECT image; b: monochromatic reconstruction at 70 keV during the arterial phase; c: iodine mapping. Due to excellent contrast-to-noise ratio, hypervascular tumor shows high degrees of conspicuity on monochromatic image and iodine mapping. On the iodine map, the concentration of iodine is measured at 2.01 g/cm3 indicating a clear cell renal cancer (concentration of iodine > 0.9 g/cm3 ).
Figure 17. Unenhanced DECT images of the right kidney in the transverse plane obtained in a 48-year-old woman with renal calculus: a: on polychromatic unenhanced image, right pyelic stone (arrow) has attenuation value of 480 HU; b: on calcium mapping image, the stone is not visible; c: uric acid mapping image; d: combined unenhanced/uric acid image. High value of the stone on the uric acid map (c and d) indicates the uric component.
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Figure 18. Unenhanced abdominal DECT images in the transverse plane obtained in a 52-year-old woman with left adrenal adenoma: a: on monochromatic reconstructions at 80 keV, the attenuation value of left adrenal adenoma (arrow) is 2.7 HU; b: on monochromatic reconstructions at 140 keV, the attenuation value of left adrenal adenoma (arrow) is 6.7 HU. The increase in attenuation value with increased energy setting indicates the adenomatous nature of the adrenal lesion.
Conclusion In recent years, DECT has been subjected to marked refinements resulting in a variety of applications, including abdominal imaging. DECT helps radiologists to better detect and characterize abdominal lesions, thus providing optimal patient care. DECT enables the reconstruction of monochromatic images with increased contrast to noise ratio, allows obtaining virtual unenhanced images in retrospect and maps for elements such as iodine and calcium, and reducing metal artifacts. DECT is mainly used in abdominal imaging to detect and characterize hyper- or hypovascular liver lesions, diagnose and determine the extent of pancreatic cancers, characterize some renal and adrenal lesions, and to assess in vivo urinary stones. DECT is an innovating technique, with applications, which are still under development. In the future, DECT could provide quantitative perfusion mapping of focal abdominal lesions, especially liver lesions, to improve characterization and also predict prognosis. DECT may also be a useful technique in patient with acute digestive diseases such as mesenteric ischemia and digestive obstructions, by showing signs of bowel wall compromise.
Disclosure of interest The authors declare that they have no competing interest.
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11 [28] Mbengue A, Diallo I, Ndiaye AR, et al. Acute intestinal ischaemia revealing a metastatic ileal endocrine tumour. Diagn Interv Imaging 2014;95:1109—10. [29] Potretzke TA, Brace CL, Lubner MG, Sampson LA, Willey BJ, Lee Jr FT. Early small-bowel ischemia: dual-energy CT improves conspicuity compared with conventional CT in a swine model. Radiology 2015;275:119—26. [30] Graser A, Johnson TR, Hecht EM, et al. Dual-energy CT in patients suspected of having renal masses: can virtual nonenhanced images replace true nonenhanced images? Radiology 2009;252:433—40. [31] Mileto A, Marin D, Nelson RC, Ascenti G, Boll DT. Dual energy MDCT assessment of renal lesions: an overview. Eur Radiol 2014;24:353—62. [32] Park SY, Kim CK, Park BK. Dual-energy CT in assessing therapeutic response to radiofrequency ablation of renal cell carcinomas. Eur J Radiol 2014;83:73—9. [33] Ascenti G, Mileto A, Krauss B, et al. Distinguishing enhancing from nonenhancing renal masses with dual-source dual energy CT: iodine quantification versus standard enhancement measurements. Eur Radiol 2013;23:2288—95. [34] Mileto A, Marin D, Alfaro-Cordoba M, et al. Iodine quantification to distinguish clear cell from papillary renal cell carcinoma at dual-energy multidetector CT: a multireader diagnostic performance study. Radiology 2014;273:813—20. [35] Shekarriz B, Stoller ML. Uric acid nephrolithiasis: current concepts and controversies. J Urol 2002;168:1307—14. [36] Hidas G, Eliahou R, Duvdevani M, et al. Determination of renal stone composition with dual-energy CT: in vivo analysis and comparison with X-ray diffraction. Radiology 2010;257:394—401. [37] Wisenbaugh ES, Paden RG, Silva AC, Humphreys MR. Dualenergy vs conventional computed tomography in determining stone composition. Urology 2014;83:1243—7. [38] Takahashi N, Vrtiska TJ, Kawashima A, et al. Detectability of urinary stones on virtual non enhanced images generated at pyelographic-phase dual-energy CT. Radiology 2010;256:184—90. [39] Bovio S, Cataldi A, Reimondo G, et al. Prevalence of adrenal incidentaloma in a contemporary computerized tomography series. J Endocrinol Invest 2006;29:298—302. [40] Song JH, Chaudhry FS, Mayo-Smith WW. The incidental adrenal mass on CT: prevalence of adrenal disease in 1049 consecutive adrenal masses in patients with no known malignancy. AJR Am J Roentgenol 2008;190:1163—8. [41] Botsikas D, Triponez F, Boudabbous S, Hansen C, Becker CD, Montet X. Incidental adrenal lesions detected on enhanced abdominal dual-energy CT: can the diagnostic workup be shortened by the implementation of virtual unenhanced images? Eur J Radiol 2014;83:1746—51. [42] Glazer DI, Maturen KE, Kaza RK, et al. Adrenal incidentaloma triage with single-source (fast-kilovoltage switch) dual-energy CT. AJR Am J Roentgenol 2014;203:329—35. [43] Gupta RT, Ho LM, Marin D, Boll DT, Barnhart HX, Nelson RC. Dual-energy CT for characterization of adrenal nodules: initial experience. AJR Am J Roentgenol 2010;194:1479—83.
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PICTORIAL REVIEW /Abdominal imaging
Applications of dual energy computed tomography in abdominal imaging T. Lestra a,∗, S. Mulé a, I. Millet b, A. Carsin-Vu a, P. Taourel b, C. Hoeffel a a
Department of radiology, hôpitaux universitaires de Reims, avenue du Général-Koenig, 51092 Reims cedex, France b Department of medical imaging, hôpital Lapeyronie, hôpitaux universitaires de Montpellier, 191, avenue du Doyen-Gaston-Giraud, 34090 Montpellier, France
KEYWORDS Computed tomography; Dual energy CT; Liver tumors; Renal tumors; Urinary stones
Abstract Dual energy computed tomography (CT) is an imaging technique based on data acquisition at two different energy settings. Recent advances in CT have allowed data acquisition and almost simultaneously analysis of two spectra of X-rays at different energy levels resulting in novel developments in the field of abdominal imaging. This technique is widely used in cardiovascular imaging, especially for pulmonary embolism work-up but is now also increasingly developed in the field of abdominal imaging. With dual-energy CT it is possible to obtain virtual unenhanced images from monochromatic reconstructions as well as attenuation maps of different elements, thereby improving detection and characterization of a variety of renal, adrenal, hepatic and pancreatic abnormalities. Also, dual-energy CT can provide information regarding urinary calculi composition. This article reviews and illustrates the different applications of dual-energy CT in routine abdominal imaging. © 2016 Éditions franc ¸aises de radiologie. Published by Elsevier Masson SAS. All rights reserved.
Introduction Dual energy computed tomography (DECT) is based on CT data acquisition at two different energy settings. X-ray photons interact differently with matter at different energy settings resulting in different attenuation values. At two different energy settings, information about the composition of a given tissue may be obtained by analyzing how it interacts with
Abbreviations: CT, computed tomography; DECT, dual-energy computed tomography; HU, Hounsfield unit; HCC, hepatocellular carcinoma; GIST, gastrointestinal stromal tumor; NET, neuroendocrine tumor. ∗ Corresponding author. E-mail address: [email protected] (T. Lestra). http://dx.doi.org/10.1016/j.diii.2015.11.018 2211-5684/© 2016 Éditions franc ¸aises de radiologie. Published by Elsevier Masson SAS. All rights reserved.
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the photons [1,2]. DECT has been known for many years. Studies published in the 1970s have shown that this technique was useful for tissue characterization, but applications remained limited because of technical difficulties, especially prolonged data acquisition at different energy settings thereby decreasing spatial and temporal resolution [3]. Recent advances in computed tomography (CT) technology allow now acquisition and almost simultaneously analysis of two X-ray spectra at different energy settings. For these reasons, a new interest in DECT has emerged. In cardiovascular imaging, DECT is used in patients with suspected pulmonary embolism, because iodine is better visualized in distal, small pulmonary arteries and perfusion imaging offers excellent correlation with scintigraphy [4,5]. In musculoskeletal imaging, DECT helps characterize monosodium urate deposits in patients with gout and can also improve the assessment of metallic implants by reducing metal artifacts [6,7]. Abdominal applications of DECT are less well-known and still under development. With DECT it is possible to obtain virtual unenhanced images from monochromatic reconstructions as well as attenuation maps of different elements, thereby improving detection and characterization of a variety of renal, adrenal, hepatic and pancreatic abnormalities. Finally, using DECT is possible to determine urinary calculus composition. This article reviews and illustrates the different applications of DECT in abdominal imaging.
Physical and technical considerations General principles With CT, attenuation is directly related to the interaction between X-rays and matter. A matter’s attenuation coefficient depends on the matter’s composition and on the energy of the incident X-ray photons [8]. At the energy levels used in diagnostic imaging, X-ray photons interact with matter either by a photoelectric effect or by Compton scattering. At
Figure 1. Example of energy level distribution (simplified) of a beam of X-ray photons in conventional single source CT at 120 kVp. Note the asymmetry of the curve. At low energy setting, X photons are partially filtered at the exit of the tube (beam hardening).
lower kilovoltage, the photoelectric effect prevails. Here, attenuation, which is tissue-density dependent (i.e., atomic number), is high. This results in high contrast and noise. On the other hand, at higher kilovoltage, the attenuation is primarily driven by the Compton scattering mechanism. Here, attenuation is lower, resulting in lower image noise and contrast. With CT, tissue contrast depends on tube voltage (in kVp), which is an adjustable parameter. This parameter determines the upper limit of the energy spectrum of the X-rays produced by the tube. With conventional single-source CT using polychromatic X-rays, images are usually acquired at an energy setting between 80 and 120 kVp, to optimize the balance between contrast and noise (Fig. 1). DECT is based on the simultaneous acquisition at low (80 kVp) and high (140 kVp) energy levels (Fig. 2). Depending on the manufacturer, several technical approaches are available to acquire double datasets. Dualsource CT consists of two tubes and two detectors, simultaneously rotating around the patient, with an angle of 90◦ between both measuring systems with different fields of view (FOV) (Fig. 3). The single-source system consists of
Figure 2. Example of energy level distribution (simplified) of a beam of X-ray photons in dual energy computed tomography (DECT) at 80 and 140 keV.
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Monochromatic images Monochromatic images are reconstructed images obtained at fixed voltage, possibly ranging across a broad spectrum of energies (40 to 140 keV). They help improve the contrast between the different structures, based on the characteristics of the examination, of the patient, and of the area under investigation. These reconstructions are particularly useful to improve the iodine contrast and thus the assessment of contrast uptake in a given lesion (Fig. 5) [9].
Metal artifacts reduction
Figure 3. Dual-source system with two tubes and two detectors with an angle of 90◦ between both measuring systems.
a single tube and detector combination that alternates very quickly between high and low kilovoltage and has detectors with very short afterglow (Fig. 4). Another approach is to use a CT unit equipped with 320 rows of detectors that allow an area of 16 cm to be scanned in a single rotation, to perform two subsequent scans, with a switch in tube voltage (80 and 140 kVp) in-between. This technique has limited volume coverage. Finally, another technique, under development, consists in a dual layer detector placed directly on top of the other that receives separately low (up to 80 keV) then high (up to120 keV) energy photons. Post-processing of DECT data: With DECT, data processing processing yields several types of images.
Metal artifact is mainly caused by photon beam hardening resulting in streak artifacts that radiate away from an extremely dense element. When crossing dense structures, low-energy photons are more attenuated than high-energy photons (Fig. 6) [10]. DECT makes it possible to reduce the artifacts and provide satisfactory visualization of metallic implants and surrounding bony structures on monochromatic reconstructions at higher levels of energy. By using a metal artifact reduction software (MARs)-type reconstruction algorithm, and the DECT-acquired data, it is possible to improve the quality of the images of adjacent soft tissues, for instance in the pelvis in patients with metal hip prostheses (Fig. 7) [11].
Attenuation maps of different elements The Hounsfield (HU) attenuation curve of an element at a given intensity of the beam of X-rays depends on the element’s atomic number (Fig. 8). Dual energy acquisition makes it possible to distinguish some elements such as iodine, calcium, water, and uric acid on the basis of their atomic number and to create a map (Fig. 9). Finally, iodine quantification mapping makes it possible to reconstruct in retrospect virtual unenhanced images from the enhanced images, by removing the iodine, whatever the time after injection.
Radiation dose
Figure 4. Single source system. The tube alternates very quickly between 80 and 140 keV.
There is increasing concern for potential long-term risks regarding exposure to radiation in medical diagnostic imaging, especially for young patients and for patients who require follow-up examinations on a regular basis. Some experts believe that, compared to single energy, radiation from DECT is 20% greater, but it remains below the diagnostic reference levels [12,13]. The radiation dose varies depending on manufacturers and protocols. Recent studies in liver and urinary tract imaging have shown that with improved protocols, the radiation dose can be similar to the dose in single energy CT, and even lower [14,15]. For instance, one study that included patients with suspected hepatocellular carcinoma showed a highly significant reduction in dose-length product (DLP) and 37% reduction in effective dose (P < 0.001) for an adapted dual energy acquisition protocol compared to single energy acquisition [14]. Moreover, virtual unenhanced image acquisition by omitting true unenhanced images reduces the radiation dose up to 47%, depending on the protocol, while obtaining images of equivalent quality [16].
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Figure 5. Monochromatic reconstructions obtained at different level of energy: a: at 40 keV; b: at 60 keV; c: at 80 keV; d: at 100 keV; e: at 120 keV; f: at 140 keV. Reconstructions at a low energy setting have a high contrast but also a high noise level, while reconstructions at a high level of energy have low contrast but also low noise.
Applications in abdominal imaging Hepatic applications DECT is used to visualize and characterize liver lesions and assess tumor response to therapy. DECT has also been used to quantify the degree of hepatic steatosis or hepatic fibrosis. Virtual unenhanced images could replace true unenhanced images, based on acceptable image quality and good correlation between the attenuation values of both image categories [17]. However, as far as bile duct stones are concerned, these virtual unenhanced images do not accurately depict small stones (i.e., surface area <9 mm2 ) or stones with little attenuation values (i.e., < 78 HU) [18]. By using monochromatic images during the portal venous phase after IV administration of iodinated contrast material, it is possible to increase the contrast between lesion and adjacent healthy liver parenchyma, at an optimal energy level of 70 to 80 keV, while maintaining an acceptable level of noise and thereby improving conspicuity of hypovascular hepatic metastases (Fig. 10) [19].
DECT is even more useful to visualize hypervascular hepatic lesions. For instance, monochromatic reconstructions at a low energy setting (i.e., < 70 keV) combined to an iodine-based map during the late arterial phase (45 seconds after IV) improves the diagnostic accuracy for hypervascular lesions (detection, conspicuity and diagnostic confidence), especially for small hepatocellular carcinomas (Figs. 11 and 12) [9]. In addition, the use of DECT improves the characterization of hypervascular lesions. Lv et al. showed that small hepatocellular carcinomsa and small hepatic hemangiomas can be differentiated with high degrees of sensitivity and specificity, by quantifying the iodine density of the lesions, first during the arterial and then during the portal phase [20]. According to Wang et al., the analysis of the spectral curve slope of a lesion helps discriminate between hemangiomas, hepatocellular carcinoma, metastases and biliary cysts with sensitivities and specificities of 87% and 100% for hemangiomas, respectively, 82.1% and 65.9% for hepatocellular carcinomas, 65.9% and 59% for metastases and 44.4% and 100% for biliary cysts [21].
Figure 6. Modifications of the photon spectrum after beam attenuation in dense matter. When crossing dense structures, low-energy photons are halted.
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Figure 7. DECT image of the pelvis in the coronal plane obtained after IV administration of iodinated contrast material performed in 77year-old woman with acute abdominal pain: a: monochromatic reconstructions obtained without metal artifact reduction software (MARs) reconstruction algorithm at 70 keV; b: monochromatic reconstructions without MARs reconstruction algorithm at 140 keV; c: monochromatic reconstructions with MARs at 70 keV; d: monochromatic reconstructions with MARs at 140 keV. There are fewer artifacts with reconstructions at high energy levels. MARs with dual energy acquisition substantially reduces metal artifacts (c and d) (DLP = 1063.7 mGy/cm and CTDI vol = 25.53 Gy). There are fewer artifacts with reconstructions at high energy levels (b and d). MARs with dual energy acquisition substantially reduces metal artifacts (c and d). The DLP was 1063.7 mGy/cm and the CTDI vol = 25.53 Gy.
currently investigated to quantify hepatic steatosis and liver fibrosis. Compared to MRI data, encouraging preliminary results have been obtained in this area [24,25].
Pancreatic applications DECT, with monochromatic reconstructions at 50—70 keV, improves the contrast between tumor and surrounding parenchyma [26]. This optimized contrast improves the detection of pancreatic adenocarcinomas, especially isoattenuating adenocarcinomas, that are generally small lesions and with a better prognosis than more advances tumors. Iodine maps also improve the detection of these lesions (Fig. 13) [27]. Although no studies have yet been published, DECT could also be useful to assess other types of pancreatic diseases. Figure 8. Hounsfield attenuation curve (HU) of calcium and iodine at a given level of energy.
DECT showed promising results in the assessment of tumor response in patients undergoing targeted therapy for hypervascular liver tumors such as hepatocellular carcinoma, metastases from gastrointestinal stromal tumors, neuroendocrine tumors, or melanoma [22,23]. DECT is also
Gastrointestinal tract applications Acute mesenteric ischemia is a serious condition that conveys high morbidity and mortality and requires early diagnosis for best treatment. CT is currently the reference technique for patients with suspected mesenteric ischemia [28]. However early in the disease process, signs
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Figure 9. Examples of different maps obtained in a 45-year-old man, acquired at the level of right renal hilum in the transverse plane: a: virtual unenhanced DECT image in the transverse plane; b: corresponding native DECT image after IV administration of iodinated contrast material during the portal venous phase; c: corresponding iodine map.
Figure 10. DECT images of a biliary cyst (arrow) in the transverse plane: a: native image during the portal venous phase; b: iodine maps shows no uptake of iodinated contrast material by the lesion.
of ischemia may be difficult to identify on CT, especially because decrease parietal enhancement of the small bowel wall may be subtle and hardly visible in case of arterial ischemia. An experimental study, on animal model, shows that DECT, notably through iodine mapping, improves the visualization of ischemic small bowel compared to single energy CT (Fig. 14) [29].
Applications in urologic imaging Renal tumors Graser et al. have shown that a series of virtual unenhanced images allows characterizing renal masses without the need for a true unenhanced CT acquisition [30]. Moreover, monochromatic images, like for hepatic and pancreatic imaging, are reconstructions at a low dose of energy that show improved contrast between tumor and healthy parenchyma and therefore enable optimal detection of hypervascular renal tumors and tumor recurrences after radiofrequency ablation [31,32]. Iodine maps also improve the detection of tissular portions in cystic lesions (Figs. 15 and 16). Iodine quantification is also possible, which seems more accurate than the measurement of attenuation values during the different phases. Lesion characterization is thus improved [33]. Indeed, Milet et al. reported in a study that included 21 papillary tumors and 67 renal clear cell carcinomas, that an iodine concentration of 0.9 g/cm3 is an optimal threshold value to
discriminate between papillary tumors than show lower degrees of enhancement and clear cell adenocarcinomas, with a sensitivity of 98.2% and a specificity of 86.3% (Fig. 16) [34].
Characterization of urinary calculi Treatment of urinary calculi depends on stone composition. For instance, urine alkalization is the cornerstone of treatment for patients with uric acid containing urinary calculi [35]. Using DECT, it is possible to assess in vivo the composition of the stones, and thus tailor the treatment, by analyzing the attenuation curve of the stone and to compare it with reference values and by mapping elements [36]. Based on the analysis of the attenuation values, it is possible to discriminate between uric acid containing from non-uric acid containing calculi with DECT in 93% of the cases, against 40% with conventional CT (Fig. 17) [37]. Takahashi et al. have shown that the detection of small calculi (1 to 2 mm) was limited on virtual unenhanced images generated during the pyelographic phase (i.e., obtained approximately 10 minutes after IV) [38].
Characterization of adrenal nodules Adrenal incidentalomas have an incidence ranging between 4.4% and 9% on CT examinations [39,40]. Characterization of these nodules requires unenhanced CT images. Virtual unenhanced images allow the radiologist to determine that an
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Figure 11. DECT angiography of the abdomen performed for the follow-up of aortic dissection in a 60-year-old man at an arterial phase. A hypervascular liver metastasis (arrow) from neuroendocrine tumor was incidentally detected in right liver: a: native polychromatic image; b: virtual unenhanced image; c: monochromatic reconstruction at 55 keV; d: monochromatic reconstruction at 100 keV; e: iodine mapping. Virtual unenhanced reconstruction images are similar to unenhanced images. The visualization of the lesion is good with reconstructions at a low level of energy (on c) and iodine mapping (on e). At a high-energy setting the lesion is less visible due to the sharp reduction of contrast-to-noise ratio (contrast reduction is more important than noise reduction).
adrenal mass is an adenoma if its attenuation value is < 10 HU [41,42]. Although further large-scale studies are needed, preliminary studies have shown that adrenal adenomas would be more accurately diagnosed by analyzing attenuation variations at energy levels between 80 keV and 140 KeV. Unlike for adrenal metastases, the attenuation values for
adenomas at 80 keV appear to be lower than at 140 keV, indicating the presence of intracellular lipid with 100% specificity and 50% sensitivity (Fig. 18) [43]. The low sensitivity may be explained by the fact that some adenomas contain little fat, thereby modifying their attenuation values and their interaction with the X-ray photons at different levels of energy.
Figure 12. DECT image of the abdomen in the transverse plane shows an hypervascular liver metastasis of gastric stromal tumor (arrow) in a 64-year-old woman: a:arterial phase with monochromatic reconstruction at 70 keV; b:iodine mapping. The increased contrast-to-noise ratio on monochromatic images and iodine mapping provide a good visualization of the hypervascular metastasis and a more accurate identification of the contrast uptake by the lesion.
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Figure 13. DECT image of the abdomen in the transverse plane during the late arterial phase after IV, shows adenocarcinoma of the pancreatic head and biliary stent (arrows) in a 56-year-old man: a: monochromatic reconstruction at 70 keV; b: iodine mapping. The visualization of the tumor, which is hypovascular relative to the healthy pancreatic parenchyma, is good due to the contrast-to-noise ratio in monochromatic imaging and the iodine mapping.
Figure 14. 73-year-old man with several cardiovascular risk factors presenting with acute abdominal pain, inflammatory syndrome and increased level of serum lactates. Mesenteric ischemia was strongly suspected: a: DECT image of the abdomen in the transverse plane obtained with polychromatic reconstruction; b: iodine mapping. Compared to polychromatic image, iodine mapping provides better visualization of the ischemic loops (white arrows) in relation to the vascularized loops (black arrowhead)
Figure 15. Abdominal DECT image in the transverse plane obtained in a patient with Bosniak I left renal cyst (arrow) during the arterial phase: a: native DECT image; b: corresponding iodine map definitely confirms absence of enhancement of the left renal cyst.
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Figure 16. Abdominal DECT image in the transverse plane obtained in a 67-year-old man, with left renal adenocarcinoma (arrow) during the arterial phase: a: unenhanced DECT image; b: monochromatic reconstruction at 70 keV during the arterial phase; c: iodine mapping. Due to excellent contrast-to-noise ratio, hypervascular tumor shows high degrees of conspicuity on monochromatic image and iodine mapping. On the iodine map, the concentration of iodine is measured at 2.01 g/cm3 indicating a clear cell renal cancer (concentration of iodine > 0.9 g/cm3 ).
Figure 17. Unenhanced DECT images of the right kidney in the transverse plane obtained in a 48-year-old woman with renal calculus: a: on polychromatic unenhanced image, right pyelic stone (arrow) has attenuation value of 480 HU; b: on calcium mapping image, the stone is not visible; c: uric acid mapping image; d: combined unenhanced/uric acid image. High value of the stone on the uric acid map (c and d) indicates the uric component.
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Figure 18. Unenhanced abdominal DECT images in the transverse plane obtained in a 52-year-old woman with left adrenal adenoma: a: on monochromatic reconstructions at 80 keV, the attenuation value of left adrenal adenoma (arrow) is 2.7 HU; b: on monochromatic reconstructions at 140 keV, the attenuation value of left adrenal adenoma (arrow) is 6.7 HU. The increase in attenuation value with increased energy setting indicates the adenomatous nature of the adrenal lesion.
Conclusion In recent years, DECT has been subjected to marked refinements resulting in a variety of applications, including abdominal imaging. DECT helps radiologists to better detect and characterize abdominal lesions, thus providing optimal patient care. DECT enables the reconstruction of monochromatic images with increased contrast to noise ratio, allows obtaining virtual unenhanced images in retrospect and maps for elements such as iodine and calcium, and reducing metal artifacts. DECT is mainly used in abdominal imaging to detect and characterize hyper- or hypovascular liver lesions, diagnose and determine the extent of pancreatic cancers, characterize some renal and adrenal lesions, and to assess in vivo urinary stones. DECT is an innovating technique, with applications, which are still under development. In the future, DECT could provide quantitative perfusion mapping of focal abdominal lesions, especially liver lesions, to improve characterization and also predict prognosis. DECT may also be a useful technique in patient with acute digestive diseases such as mesenteric ischemia and digestive obstructions, by showing signs of bowel wall compromise.
Disclosure of interest The authors declare that they have no competing interest.
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Please cite this article in press as: Lestra T, et al. Applications of dual energy computed tomography in abdominal imaging. Diagnostic and Interventional Imaging (2016), http://dx.doi.org/10.1016/j.diii.2015.11.018
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Please cite this article in press as: Lestra T, et al. Applications of dual energy computed tomography in abdominal imaging. Diagnostic and Interventional Imaging (2016), http://dx.doi.org/10.1016/j.diii.2015.11.018