Daniel T. Boll

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.

Renal Stone Assessment with Dual-Energy Multidetector CT and Advanced Postprocessing Techniques: Improved Characterization of Renal Stone Composition—Pilot Study

PURPOSE To prospectively evaluate the capability of noninvasive, simultaneous dual-energy (DE) multidetector computed tomography (CT) to improve characterization of human renal calculi in an anthropomorphic DE renal phantom by introducing advanced postprocessing techniques, with ex vivo renal stone spectroscopy as the reference standard. MATERIALS AND METHODS Fifty renal calculi were assessed: Thirty stones were of pure crystalline composition (uric acid, cystine, struvite, calcium oxalate, calcium phosphate, brushite), and 20 were of polycrystalline composition. DE CT was performed with a 64-detector CT unit. A postprocessing algorithm (DECT(Slope)) was proposed as a pixel-by-pixel approach to generate Digital Imaging and Communications in Medicine dataset gray-scale-encoding ratios of relative differences in attenuation values of low- and high-energy DE CT. Graphic analysis, in which clusters of equal composition were identified, was performed by sorting attenuation values of color composition-encoded calculi in an ascending sequence. Multivariate general linear model analysis was used to determine level of significance to differentiate composition on native and postprocessed DE CT images. RESULTS Graphic analysis of native DE CT images was used to identify clusters for uric acid (453-629 HU for low-energy CT, 443-615 HU for high-energy CT), cystine (725-832 HU for low-energy CT, 513-747 HU for high-energy CT), and struvite (1337-1530 HU for low-energy CT, 1007-1100 HU for high-energy CT) stones; high-energy clusters showed attenuation value overlap. Polycrystalline calcium oxalate and calcium phosphate calculi were found throughout the entire spectrum, and dense brushite had attenuation values of more than 1500 HU for low-energy CT and more than 1100 HU for high-energy CT. The DE CT algorithm was used to generate specific identifiers for uric acid (77-80 U(Slope), one outlier), cystine (70-71 U(Slope)), struvite (56-60 U(Slope)), calcium oxalate and calcium phosphate (17-59 U(Slope)), and brushite (4-15 U(Slope)) stones. Statistical analysis showed that all compositions were identified unambiguously with the DECT(Slope) algorithm. CONCLUSION DE multidetector CT with advanced postprocessing techniques improves characterization of renal stone composition beyond that achieved with single-energy multidetector CT acquisitions with basic attenuation assessment.

Short- and Midterm Reproducibility of Apparent Diffusion Coefficient Measurements at 3.0-T Diffusion-weighted Imaging of the Abdomen

PURPOSE To test the hypothesis that there is no significant variability in apparent diffusion coefficients (ADCs) at assessment of the short- and midterm reproducibility of ADC measurements in a healthy population. MATERIALS AND METHODS Twenty healthy male volunteers were enrolled in this prospective institutional review board-approved study after they provided written informed consent. A 3.0-T magnetic resonance (MR) system was used to perform five axial diffusion-weighted (DW) abdominal acquisitions (session 1). A mean of 147 days +/- 20 (standard deviation) later, 16 of the 20 volunteers were imaged again with use of the same protocol and MR system (session 2). The mean ADCs for three regions of interest (ROIs) in five anatomic locations (right hepatic lobe, spleen, and head, body, and tail of pancreas) were calculated. The coefficient of variation (CV, equal to standard deviation divided by mean) was calculated for each subject, session, and anatomic location. The ADC and CV data were then analyzed by using repeated-measures analysis of variance. RESULTS There were significant differences (P < .001) in mean ADCs among the five anatomic locations. There were no significant differences in ADCs among the various repeated sequence acquisitions or the three ROIs. There were no significant differences in ADCs between imaging sessions 1 and 2. ADCs were fairly stable over the midterm within a given individual. Finally, there were no significant differences in resulting CVs between the imaging sessions or anatomic locations. The mean CV for ADC measurement reproducibility was 14% (95% confidence interval: 13%, 15%). CONCLUSION The mean CV for short- and midterm ADC reproducibility was 14% at abdominal DW imaging. Treatment effects of less than approximately 27% (change in ADC divided by pretreatment ADC) will not be clinically detectable with confidence with one acquisition in a single individual.

State of the Art: Dual-Energy CT of the Abdomen

Recent technologic advances in computed tomography (CT)--enabling the nearly simultaneous acquisition of clinical images using two different x-ray energy spectra--have sparked renewed interest in dual-energy CT. By interrogating the unique characteristics of different materials at different x-ray energies, dual-energy CT can be used to provide quantitative information about tissue composition, overcoming the limitations of attenuation-based conventional single-energy CT imaging. In the past few years, intensive research efforts have been devoted to exploiting the unique and powerful opportunities of dual-energy CT for a variety of clinical applications. This has led to CT protocol modifications for radiation dose reduction, improved diagnostic performance for detection and characterization of diseases, as well as image quality optimization. In this review, the authors discuss the basic principles, instrumentation and design, examples of current clinical applications in the abdomen and pelvis, and future opportunities of dual-energy CT.

Field strength and diffusion encoding technique affect the apparent diffusion coefficient measurements in diffusion-weighted imaging of the abdomen.

Objectives:The purpose of this study is to determine what effects a variety of diffusion encoding techniques at 1.5 T and 3 T have on measured abdominal apparent diffusion coefficient (ADC) values obtained in a healthy population. Materials and Methods:Sixteen healthy male volunteers were enrolled in this prospective Institutional Review Board-approved study following written informed consent. Imaging was performed on a 1.5 T and a 3 T magnetic resonance system (Siemens, Erlangen) with several abdominal axial diffusion weighted imaging (DWI) acquisitions: an orthogonal diffusion encoding with b-values of 0/400 seconds/mm2, and a series of four 3-scan trace weighted acquisitions with b-values of 0/50, 0/400, 0/800, 0/50/400/800 seconds/mm2, respectively. The mean ADC values were calculated for 3 regions of interest (ROI) in 5 locations (right hepatic lobe, spleen, pancreatic head, body, and tail). The ADC data were analyzed using a repeated-measures analysis of variance. Results:There was a significant difference between measured ADC values at 1.5 T and 3 T for liver (P < 0.001), but not for pancreas (P = 0.427) or spleen (P = 0.167). There was no significant difference (P > 0.999) in the measured ADC values between the orthogonal encodings and the 3-scan trace weighted encoding with the same b-value. There were significant differences (P < 0.001) between all 4 weighting schemes for the 3-scan trace with the measured ADC decreasing with increasing b-value. Conclusion:Measured abdominal ADC values depend on the exact selection of b-value used for encoding for liver, pancreas, and spleen. In addition, the measured ADC values depend on the field strength of the scanner for liver.

Slice-to-volume registration and its potential application to interventional MRI-guided radio-frequency thermal ablation of prostate cancer

In this study, we registered live-time interventional magnetic resonance imaging (iMRI) slices with a previously obtained high-resolution MRI volume that in turn can be registered with a variety of functional images, e.g., PET, SPECT, for tumor targeting. We created and evaluated a slice-to-volume (SV) registration algorithm with special features for its potential use in iMRI-guided radio-frequency (RF) thermal ablation of prostate cancer. The algorithm features included a multiresolution approach, two similarity measures, and automatic restarting to avoid local minima. Imaging experiments were performed on volunteers using a conventional 1.5-T MR scanner and a clinical 0.2-T C-arm iMRI system under realistic conditions. Both high-resolution MR volumes and actual iMRI image slices were acquired from the same volunteers. Actual and simulated iMRI images were used to test the dependence of SV registration on image noise, receive coil inhomogeneity, and RF needle artifacts. To quantitatively assess registration, we calculated the mean voxel displacement over a volume of interest between SV registration and volume-to-volume registration, which was previously shown to be quite accurate. More than 800 registration experiments were performed. For transverse image slices covering the prostate, the SV registration algorithm was 100% successful with an error of <2 mm, and the average and standard deviation was only 0.4 mm /spl plusmn/ 0.2 mm. Visualizations such as combined sector display and contour overlay showed excellent registration of the prostate and other organs throughout the pelvis. Error was greater when an image slice was obtained at other orientations and positions, mostly because of inconsistent image content such as that from variable rectal and bladder filling. These preliminary experiments indicate that MR SV registration is sufficiently accurate to aid image-guided therapy.

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.

Detection of Renal Lesion Enhancement with Dual-Energy Multidetector CT

PURPOSE To determine whether dual-energy multidetector CT enables detection of renal lesion enhancement by using calculated nonenhanced images with spectral-based extraction in a non-body weight-restricted patient population. MATERIALS AND METHODS Between January 2008 and December 2009, 139 patients were enrolled in this prospective HIPAA-compliant, institutional review board-approved study. Written informed consent was obtained from all patients. After single-energy nonenhanced 120-kVp CT images were acquired, contrast material-enhanced dual-energy multidetector CT images were acquired at 80 and 140 kVp. Calculated nonenhanced images were generated by using spectral-based iodine extraction. Lesion attenuation was measured on the acquired nonenhanced, calculated nonenhanced, and 140-kVp contrast-enhanced nephrographic images. Enhancement, defined as a 15-HU or greater increase in attenuation on the nephrographic images, was assessed by using the baseline attenuation on the acquired and calculated nonenhanced images. Acquired nonenhanced versus calculated nonenhanced image attenuation, as well as enhancement values, were compared by using paired Student t tests and Bland-Altman plots. RESULTS Hypoattenuating (n = 66) and hyperattenuating (n = 28) cysts, angiomyolipomas (n = 18), and solid enhancing lesions (n = 27) were detected. Mean attenuation values for hypoattenuating cysts on the acquired and calculated nonenhanced CT images were 6.5 HU ± 5.8 (standard deviation) and 8.1 HU ± 3.1 (P = .13), respectively, with corresponding enhancement values of 1.1 HU ± 5.2 and -0.5 HU ± 6.2 (P = .12), respectively. Mean values for hyperattenuating cysts were 29.4 HU ± 5.6 on acquired images and 31.7 HU ± 5.1 on calculated images (P = .39) (corresponding enhancement, 4.7 HU ± 3.3 and 2.3 HU ± 4.1, respectively; P = .09). Mean values for fat-containing enhancing lesions were -90.6 HU ± 24.7 on acquired images and -85.9 HU ± 23.7 on calculated images (P = .57) (corresponding enhancement, 18.2 HU ± 10.1 and 13.6 HU ± 10.7, respectively; P = .19). Mean attenuation values for solid enhancing lesions were 26.0 HU ± 15.0 on acquired images and 27.7 HU ± 14.9 on calculated images (P = .45) (corresponding enhancement, 60.3 HU ± 13.1 and 58.3 HU ± 15.5, respectively; P = .38). CONCLUSION Dual-energy CT acquisitions with spectral-based postprocessing enabled accurate detection of renal lesion enhancement across the attenuation spectrum of frequently encountered renal lesions in a non-body habitus-restricted patient population.