Lisa M. Ho

Dual-energy multidetector CT: how does it work, what can it tell us, and when can we use it in abdominopelvic imaging?

Dual-energy CT provides information about how substances behave at different energies, the ability to generate virtual unenhanced datasets, and improved detection of iodine-containing substances on low-energy images. Knowing how a substance behaves at two different energies can provide information about tissue composition beyond that obtainable with single-energy techniques. The term K edge refers to the spike in attenuation that occurs at energy levels just greater than that of the K-shell binding because of the increased photoelectric absorption at these energy levels. K-edge values vary for each element, and they increase as the atomic number increases. The energy dependence of the photoelectric effect and the variability of K edges form the basis of dual-energy techniques, which may be used to detect substances such as iodine, calcium, and uric acid crystals. The closer the energy level used in imaging is to the K edge of a substance such as iodine, the more the substance attenuates. In the abdomen and pelvis, dual-energy CT may be used in the liver to increase conspicuity of hypervascular lesions; in the kidneys, to distinguish hyperattenuating cysts from enhancing renal masses and to characterize renal stone composition; in the adrenal glands, to characterize adrenal nodules; and in the pancreas, to differentiate between normal and abnormal parenchyma.

Pneumatosis intestinalis in the adult: benign to life-threatening causes.

OBJECTIVE The frequency of detection of pneumatosis intestinalis (PI) appears to be increasing. This increase may be the result of increased CT use. New medications and surgical procedures have been reported to be associated with an increase in the incidence of PI. The purpose of this review is to provide an update on the imaging features and clinical conditions associated with PI. CONCLUSION This article illustrates the imaging findings of PI due to benign and life-threatening causes, with emphasis placed on describing newly associated conditions and also the imaging appearance on CT.

Detection of Pancreatic Tumors, Image Quality, and Radiation Dose during the Pancreatic Parenchymal Phase: Effect of a Low-Tube-Voltage, High-Tube-Current CT Technique—Preliminary Results

PURPOSE To intraindividually compare a low-tube-voltage (80 kVp), high-tube-current (675 mA) computed tomographic (CT) technique with a high-tube-voltage (140 kVp) CT protocol for the detection of pancreatic tumors, image quality, and radiation dose during the pancreatic parenchymal phase. MATERIALS AND METHODS This prospective, single-center, HIPAA-compliant study was approved by the institutional review board, and written informed consent was obtained. Twenty-seven patients (nine men, 18 women; mean age, 64 years) with 23 solitary pancreatic tumors underwent dual-energy CT. Two imaging protocols were used: 140 kVp and 385 mA (protocol A) and 80 kVp and 675 mA (protocol B). For both protocols, the following variables were compared during the pancreatic parenchymal phase: contrast enhancement for the aorta, the pancreas, and the portal vein; pancreas-to-tumor contrast-to-noise ratio (CNR); noise; and effective dose. Two blinded, independent readers qualitatively scored the two data sets for tumor detection and image quality. Random-effect analysis of variance tests were used to compare differences between the two protocols. RESULTS Compared with protocol A, protocol B yielded significantly higher contrast enhancement for the aorta (508.6 HU vs 221.5 HU, respectively), pancreas (151.2 HU vs 67.0 HU), and portal vein (189.7 HU vs 87.3 HU), along with a greater pancreas-to-tumor CNR (8.1 vs 5.9) (P < .001 for all comparisons). No statistically significant difference in tumor detection was observed between the two protocols. Although standard deviation of image noise increased with protocol B (11.5 HU vs 18.6 HU), this protocol significantly reduced the effective dose (from 18.5 to 5.1 mSv; P < .001). CONCLUSION A low-tube-voltage, high-tube-current CT technique has the potential to improve the enhancement of the pancreas and peripancreatic vasculature, improve tumor conspicuity, and reduce patient radiation dose during the pancreatic parenchymal phase.

Dual-Energy CT Applications in the Abdomen

OBJECTIVE The purpose of this article is to give a brief overview of the technical background of dual-energy CT (DECT) imaging and to review various DECT applications in the abdomen that are currently available for clinical practice. In a review of the recent literature, specific DECT applications available for abdominal organs, liver, pancreas, kidneys including renal stones, and adrenal glands, will be discussed in light of reliability and clinical usefulness in replacing true unenhanced imaging, increased lesion conspicuity, iodine extraction, and improved tissue/material characterization (e.g., renal stone composition). Radiation dose considerations will be addressed in comparison with standard abdominal imaging protocols. CONCLUSION Modern DECT applications for the abdomen expand the use of CT and enable advanced quantitative methods in the clinical routine on the basis of differences in material attenuation observed by imaging at two different distinct photon energies.

Hepatocellular carcinoma in a North American population: Does hepatobiliary MR imaging with Gd‐EOB‐DTPA improve sensitivity and confidence for diagnosis?

To evaluate the value of hepatobiliary phase imaging for detection and characterization of hepatocellular carcinoma (HCC) in liver MRI with Gd‐EOB‐DTPA, in a North American population.

Dual-Energy CT for Characterization of Adrenal Nodules: Initial Experience

OBJECTIVE The purpose of this study was to determine whether use of dual-energy technique can improve the diagnostic performance of CT in the differential diagnosis of adrenal adenomas and metastatic lesions. SUBJECTS AND METHODS Thirty-one adrenal nodules were prospectively identified in 17 patients who underwent dual-energy CT at 140 and 80 kVp. Attenuation measurements were performed for each nodule at both tube voltages. The mean attenuation change (increase or decrease) between 140 kVp and 80 kVp was determined for each adrenal nodule. RESULTS Twenty-six adrenal nodules were benign adenomas (attenuation less than +10 HU or stability for at least 1 year). Five adrenal nodules were classified as metastatic (rapid growth in 1 year and history of extraadrenal malignancy). The mean attenuation change between 140 kVp and 80 kVp was 0.4 +/- 7.1 HU for adenomas and 9.2 +/- 4.3 HU for metastatic lesions (p < 0.003). Fifty percent of adenomas had an attenuation decrease at 80 kVp. All metastatic lesions had an attenuation increase at 80 kVp. With a decrease in attenuation at 80 kVp as an indicator of intracellular lipid within an adenoma, dual-energy CT has 50% sensitivity, 100% specificity, 100% positive predictive value, and 28% negative predictive value in the diagnosis of adenoma. CONCLUSION A decrease in attenuation of an adrenal lesion between 140 kVp and 80 kVp is a highly specific sign of adrenal adenoma. However, because an increase in attenuation at 80 kVp is seen with metastatic lesions and some adenomas, the sensitivity of this test is low. These data suggest that dual-energy CT can be used to help differentiate some lipid-poor adrenal adenomas from metastatic lesions.

Dual energy versus single energy MDCT: measurement of radiation dose using adult abdominal imaging protocols

RATIONALE AND OBJECTIVES The aim of this study was to measure the radiation dose of dual-energy and single-energy multidetector computed tomographic (CT) imaging using adult liver, renal, and aortic imaging protocols. MATERIALS AND METHODS Dual-energy CT (DECT) imaging was performed on a conventional 64-detector CT scanner using a software upgrade (Volume Dual Energy) at tube voltages of 140 and 80 kVp (with tube currents of 385 and 675 mA, respectively), with a 0.8-second gantry revolution time in axial mode. Parameters for single-energy CT (SECT) imaging were a tube voltage of 140 kVp, a tube current of 385 mA, a 0.5-second gantry revolution time, helical mode, and pitch of 1.375:1. The volume CT dose index (CTDI(vol)) value displayed on the console for each scan was recorded. Organ doses were measured using metal oxide semiconductor field-effect transistor technology. Effective dose was calculated as the sum of 20 organ doses multiplied by a weighting factor found in International Commission on Radiological Protection Publication 60. Radiation dose saving with virtual noncontrast imaging reconstruction was also determined. RESULTS The CTDI(vol) values were 49.4 mGy for DECT imaging and 16.2 mGy for SECT imaging. Effective dose ranged from 22.5 to 36.4 mSv for DECT imaging and from 9.4 to 13.8 mSv for SECT imaging. Virtual noncontrast imaging reconstruction reduced the total effective dose of multiphase DECT imaging by 19% to 28%. CONCLUSION Using the current Volume Dual Energy software, radiation doses with DECT imaging were higher than those with SECT imaging. Substantial radiation dose savings are possible with DECT imaging if virtual noncontrast imaging reconstruction replaces precontrast imaging.

Adrenal Nodules at FDG PET/CT in Patients Known to Have or Suspected of Having Lung Cancer: A Proposal for an Efficient Diagnostic Algorithm

PURPOSE To develop an algorithm to maximize the diagnostic yield of positron emission tomography (PET)/computed tomography (CT) by using defined attenuation and standardized uptake value (SUV) criteria. MATERIALS AND METHODS An IRB-approved, HIPAA-compliant retrospective review with waiver of informed consent of data in 1388 consecutive patients who underwent PET/CT for known or suspected lung cancer was completed, and 187 adrenal nodules were identified in 147 patients. Nodules were defined histologically or by size change (malignant, n = 37) or stability for more than 1 year (benign, n = 58). Nodules not sampled for biopsy and with less than 1 year of follow-up were considered indeterminate (n = 92). Diameter, mean attenuation, SUV(max), and SUV ratio (nodule SUV(max)/liver SUV(avg)) were compared with t test and receiver operating characteristic analyses. Sensitivity, specificity, positive predictive value, and negative predictive value were calculated for diameter > 3 cm, mean attenuation > 10 HU, nodule SUV(max) > 3.1, and SUV ratio > 1.0. These were also calculated for higher SUV(max) and SUV ratio thresholds that were found to exclude all false-positives. Diagnostic accuracy was compared by using the McNemar test (P < .05). RESULTS In the study group of 147 patients (aged 42-88 years; mean, 65.5 years; 59 women), combined PET/CT with mean attenuation > 10 HU and SUV(max) > 3.1 had 97.3% sensitivity and 86.2% specificity. Combined PET/CT with mean attenuation > 10 HU and SUV ratio > 1.0 had 97.3% sensitivity and 74.1% specificity. The accuracies of these threshold combinations (90.5% and 83.2%, respectively) were significantly different (P = .008). Applying a further cutoff of SUV ratio > 2.5 enabled identification of 22 of 37 metastatic lesions and exclusion of all fluorodeoxyglucose-avid benign nodules. CONCLUSION Definitive identification of many metastases can be accomplished by applying an SUV ratio cutoff of greater than 2.5, allowing pragmatic management of adrenal nodules that initially test positive with the combined PET/CT criteria SUV(max) > 3.1 and mean attenuation > 10 HU. SUPPLEMENTAL MATERIAL http://radiology.rsnajnls.org/cgi/content/full/250/2/523/DC1.

Histogram Analysis of Small Solid Renal Masses: Differentiating Minimal Fat Angiomyolipoma From Renal Cell Carcinoma

OBJECTIVE The objective of our study was to retrospectively determine whether minimal fat renal angiomyolipoma can be differentiated from clear cell or papillary renal cell carcinoma (RCC) in small renal masses using attenuation measurement histogram analysis on unenhanced CT. MATERIALS AND METHODS Twenty minimal fat renal angiomyolipomas were compared with 22 clear cell RCCs and 23 papillary RCCs using an institutional database. All masses were histologically confirmed and all minimal fat renal angiomyolipomas lacked radiographic evidence of macroscopic fat. Using attenuation measurement histogram analysis, two blinded radiologists determined the percentage of negative pixels within each renal mass. The percentages of negative pixels below attenuation thresholds of 0, -5, -10, -15, -20, -25, and -30 HU were recorded. Sensitivity, specificity, positive predictive value, negative predictive value, and receiver operator characteristic curves for the diagnosis of minimal fat renal angiomyolipoma were generated for each threshold. The Student t test was used to compare radiologists and cohorts. Previously published attenuation and pixel-counting thresholds reported as having a specificity of near 100% for discriminating between minimal fat renal angiomyolipomas and RCCs were analyzed. RESULTS The mean maximal transverse lesion diameter was 1.8 cm for minimal fat renal angiomyolipomas (SD, 0.5 cm; range, 1.1-3.0 cm), 2.1 cm for clear cell RCCs (SD, 0.5 cm; range, 1.0-2.9 cm), and 2.1 cm for papillary RCCs (SD, 0.7 cm; range, 1.3-3.9 cm). No significant difference in the percentage of negative pixels was found between minimal fat renal angiomyolipomas and clear cell RCCs or between minimal fat renal angiomyolipomas and papillary RCCs at any of the selected attenuation thresholds for either radiologist (p = 0.210-0.499). Radiologist 1 and radiologist 2 used significantly different region-of-interest sizes (p < 0.001), but neither radiologist could differentiate minimal fat renal angiomyolipoma from RCC. No previously published threshold allowed discrimination between minimal fat renal angiomyolipoma and RCC with 100% specificity. CONCLUSION Attenuation measurement histogram analysis cannot reliably differentiate minimal fat renal angiomyolipoma from RCC.

Characterization of adrenal nodules with dual-energy CT: can virtual unenhanced attenuation values replace true unenhanced attenuation values?

OBJECTIVE The purpose of our study was to investigate whether virtual unenhanced adrenal nodule attenuation values can replace true noncontrast attenuation values. MATERIALS AND METHODS Twenty-three incidentally discovered adrenal nodules (19 adenomas and four metastases) were identified in 19 patients (11 men and eight women; mean age, 65 years; age range, 38-84 years) who underwent unenhanced single-energy CT followed by contrast-enhanced dual-energy CT on the same scanner. A virtual unenhanced imaging dataset was generated from each dual-energy CT dataset. CT attenuation of each adrenal nodule was measured at the same location on virtual unenhanced images and true unenhanced images by three radiologists and mean values compared using the Student t test. Correlation between virtual unenhanced and true unenhanced values was determined using linear regression analysis. The mean difference and percentage of diagnostic agreement were also determined. Interreader variability was assessed using the intraclass correlation coefficient (ICC). RESULTS The mean ± SD attenuation values for virtual unenhanced images and true unenhanced images were 14.7 ± 15.1 HU and 12.9 ± 13.4 HU, respectively (p = 0.2). Strong positive correlation was observed between virtual unenhanced images and true unenhanced images (R = 0.83-0.87). The mean difference between virtual unenhanced images and true unenhanced images was 1.8 ± 1.7 HU. Diagnostic agreement between virtual unenhanced images and true unenhanced images was 83-91% for three radiologists. No malignant nodules were misclassified as benign on virtual unenhanced images. The ICC was 0.88 and 0.96 for virtual unenhanced images and true unenhanced images, respectively, indicating high interreader agreement. CONCLUSION Virtual unenhanced and true unenhanced attenuation measurements of adrenal nodules were not significantly different and showed strongly positive linear correlation. This finding resulted in substantial diagnostic agreement between virtual unenhanced images and true unenhanced images for distinguishing benign from malignant nodules.