Faculty Bibliography

OBJECTIVE: The full diagnostic value of diffusion-weighted (DW) MRI in the evaluation of liver metastases remains uncertain. The aim of the present study was to assess the diagnostic accuracy of DW-MRI and contrast-enhanced MRI (CE-MRI) using extracellular gadolinium chelates, with the reference standard established by consensus interpretation of confirmatory imaging and histopathologic data. METHODS: MR examinations of 51 patients with extrahepatic malignancies were retrospectively reviewed by two independent observers who assessed DW-MRI and CE-MRI for detection of liver metastases. RESULTS: By reference standard, 93 liver lesions (49 metastases and 44 benign lesions) were identified in 27 patients, 11 patients had no liver lesions, and 13 patients had innumerable metastatic and/or benign lesions. There was no difference in diagnostic performance between the two methods for either observer for the diagnosis of metastatic lesions per patient. For per-lesion analysis, sensitivity of DW-MRI was equivalent to CE-MRI for observer 1 (67.3% vs. 63.3%, p = 0.67), but lower for observer 2 (65.3% vs. 83.7%, p = 0.007). By pooling data from both observers, the sensitivity of DW-MRI was 66.3% (65/98) and 73.5% (72/98) for CE-MRI, with no significant difference (p = 0.171). CONCLUSION: DW-MRI is a reasonable alternative to CE-MRI for the detection of liver metastases

Purpose: To investigate whether variability in reported renal apparent diffusion coefficient (ADC) values in literature can be explained by the use of different diffusion weightings (b values) and the use of a monoexponential model to calculate ADC. Materials and Methods: This prospective study was approved by institutional review board and was HIPAA-compliant, and all subjects gave written informed consent. Diffusion-weighted (DW) imaging of the kidneys was performed in three healthy volunteers to generate reference diffusion decay curves. In a literature meta-analysis, the authors resampled the reference curves at the various b values used in 19 published studies of normal kidneys (reported ADC = [2.0-4.1] x 10(-3) mm(2) / sec for cortex and [1.9-5.1] x 10(-3) mm(2) / sec for medulla) and then fitted the resampled signals by monoexponential model to produce 'predicted' ADC. Correlation plots were used to compare the predicted ADC values with the published values obtained with the same b values. Results: Significant correlation was found between the reported and predicted ADC values for whole renal parenchyma (R(2) = 0.50, P = .002), cortex (R(2) = 0.87, P = .0002), and medulla (R(2) = 0.61, P = .0129), indicating that most of the variability in reported ADC values arises from limitations of a monoexponential model and use of different b values. Conclusion: The use of a monoexponential function for DW imaging analysis and variably sampled diffusion weighting plays a substantial role in causing the variability in ADC of healthy kidneys. For maximum reliability in renal apparent diffusion coefficient quantification, data for monoexponential analysis should be acquired at a fixed set of b values or a biexponential model should be used. (c) RSNA, 2010

Purpose: Renal angiomyolipoma (AML) is a benign neoplasm with a propensity to bleed proportional to tumor size. Transarterial embolization prevents hemorrhage by decreasing the angiogenic component of AML. We sought to determine whether baseline vascularity and lipid content of AML measured by MRI techniques can predict embolization response, as measured by changes in volume and vascularity on MRI. Materials and Methods: A retrospective review using an electronic database, over a consecutive 3 year period, identified 38 AMLs that underwent embolization in 22 patients. 15 AMLs had both preembolization and postembolization MRIs. 11 AMLs were in females, 4 in males. Median age was 29 years, range 21-73 years. 13 AMLs occurred in the setting of tuberous sclerosis (TS), and 2 occurred sporadically. Mean interval between baseline MRI and embolization was 86 days (range 7, 324), and mean interval between embolization and follow-up MRI was 331 days (range 35, 876). Baseline vascularity was measured by percent enhancement, with higher enhancement signifying higher vascularity. Baseline lipid content was measured by AML to psoas signal ratio on T1 fat saturation images, with a higher ratio signifying lower lipid content. Response characteristics were percent change in volume and percent change in enhancement. Results: No correlation was seen between change in volume and change in enhancement (R=0.104). Embolization resulted in a mean change in volume of -28% (range -82,+10), and a mean change in enhancement of -41% (range -97,+17). Poor correlations were seen between baseline enhancement and change in volume (R=-0.033), and between baseline enhancement and change in enhancement (R=-0.345). Moderately good correlations were seen between baseline lipid content and change in volume (R=0.625), and between baseline lipid content and change in enhancement (R=0.463). Conclusion: Embolization is effective in decreasing AML size and vascularity, which can be regarded as independent MRI markers of response. Lower baseline lipid content on MRI may predict greater response to embolization. No significant correlation was detected between baseline vascularity and response

Polycystic kidney disease (PKD) is a disorder characterized by the growth of numerous fluid filled cysts in the kidneys. Measuring cystic kidney volume is thus crucial to monitoring the evolution of the disease. While T2-weighted MRI delineates the organ, automatic segmentation is very difficult due to highly variable shape and image contrast. The interactive stereology methods used currently involve a compromise between segmentation accuracy and time. We have investigated semi-automated methods: active contours and a sub-voxel morphology based algorithm. Coronal T2-weighted images of 17 patients were acquired in four breath-holds using the HASTE sequence on a 1.5 Tesla MRI unit. The segmentation results were compared to ground truth kidney masks obtained as a consensus of experts. Automatic active contour algorithm yielded an average 22% +/- 8.6% volume error. A recently developed method (Bridge Burner) based on thresholding and constrained morphology failed to separate PKD from the spleen, yielding 37.4% +/- 8.7% volume error. Manual post-editing reduced the volume error to 3.2% +/- 0.8% for active contours and 3.2% +/- 0.6% for Bridge Burner. The total time (automated algorithm plus editing) was 15 min +/- 5 min for active contours and 19 min +/- 11 min for Bridge Burner. The average volume errors for stereology method were 5.9%, 6.2%, 5.4% for mesh size 6.6, 11, 16.5 mm. The average processing times were 17, 7, 4 min. These results show that nearly two-fold improvement in PKD segmentation accuracy over stereology technique can be achieved with a combination of active contours and post-editing.

OBJECTIVE: The purpose of this study was to determine whether the conspicuity of malignant tumors of the pancreas at dual-source dual-energy CT is better with 80-kVp acquisition than with 120-kVp acquisition simulated with a weighted average. MATERIALS AND METHODS: Fifteen patients with pancreatic adenocarcinoma underwent contrast-enhanced dual-source dual-energy CT. The abdominal diameter of all patients was 35 cm or less. Data were reconstructed as a weighted average of the 140- and 80-kVp acquisitions, simulating 120 kVp, and as a pure 80-kVp data set. A region-of-interest cursor was placed within the tumor and the adjacent normal parenchyma, and attenuation differences and contrast-to-noise ratios were calculated for pancreatic tumors at 80 kVp and with the weighted-average acquisition. The 80-kVp and weighted-average images were subjectively compared in terms of lesion conspicuity, image quality, and duct visualization. An exact Wilcoxon's matched pairs signed rank test was used to test whether differences in attenuation, contrast-to-noise ratio, and subjective assessment were greater at 80 kVp. RESULTS: The mean difference in attenuation for each pancreatic tumor and adjacent portion of normal pancreas was 83.27+/-29.56 (SD) HU at 80 kVp and 49.40+/-23.00 HU at weighted-average 120 kVp. Adenocarcinoma attenuation differences were significantly greater at 80 kVp than at 120 kVp (p=0.00006). Contrast-to-noise ratio was significantly higher at 80 kVp than at 120 kVp (p=0.00147). Subjective analysis showed lesion conspicuity (p=0.001) and duct visualization (p=0.0156) were significantly better on the 80-kVp images. CONCLUSION: At portal venous phase dual-source dual-energy CT, the conspicuity of malignant tumors of the pancreas is greater at 80 kVp than with weighted-average acquisition

OBJECTIVE: The purpose of this study was to conduct, using histopathologic examination as the reference standard, a preliminary evaluation of the use of a breath-hold multiecho T2(*)-weighted MRI sequence in the detection and quantification of hepatic iron deposition in patients with liver disease. MATERIALS AND METHODS: The images of 43 patients with liver disease who underwent 1.5-T MRI of the liver that included a multiecho T2(*)-weighted sequence who also underwent concomitant liver biopsy or liver transplantation were assessed. Two independent observers measured hepatic T2(*) by placing regions of interest in the hepatic parenchyma. Hepatic T2(*) values were compared between patients stratified by hepatic iron grade and were correlated with histopathologic iron grade. Receiver operating characteristics analysis was performed to assess the accuracy of images obtained with the hepatic T2(*)-weighted sequence in the diagnosis of iron deposition. RESULTS: Patients with iron deposition had shorter hepatic T2(*) values than did patients without iron deposition (mean T2(*), 17.7 vs 32.3 milliseconds with pooled data from both observers; p < 0.0001). Patients with iron grade 3 or greater had shorter T2(*) values than those with iron grade 2 or less (10.1 vs 20.8 milliseconds; p < 0.0001). There was a strong negative correlation between hepatic T2(*) and histopathologic iron grade (r = -0.849; p < 0.0001). For the prediction of iron grades 1 or greater and 3 or greater, area under the curve, sensitivity, and specificity were 0.968-0.982, 90.5-100%, and 100-97.3% at T2(*) cutoffs of less than 24 and less than 14 milliseconds, respectively. CONCLUSION: Hepatic iron overload in patients with liver disease can be assessed rapidly and accurately with MRI performed with a breath-hold T2(*)-weighted sequence

OBJECTIVE: We review the basics of functional renal imaging and highlight a few clinical applications. CONCLUSION: Techniques such as contrast-enhanced MR renography, diffusion-weighted imaging, and blood oxygen level-dependent MRI have been investigated in animal models and in a few human studies. Functional renal imaging is a rapidly growing field that has the potential to provide new insight into the pathophysiology of renal disease