Faculty Bibliography

OBJECTIVE: The purpose of this study was to assess the effect of hepatic iron deposition on apparent diffusion coefficient (ADC) values measured with single-shot echo-planar imaging (EPI) diffusion-weighted MRI (DWI) in patients with liver cirrhosis and in vitro. MATERIALS AND METHODS: Fifty-two patients with liver cirrhosis who underwent breath-hold single-shot EPI DWI at 1.5 T before liver transplantation were retrospectively assessed. Estimated signal-to-noise ratio (SNR(est)) and ADC were measured in the right hepatic lobe (for b values of 50 and 500 s/mm(2)). SNR(est) and ADC were compared between patients stratified by pathologic iron grade using the Mann-Whitney test. Hepatic ADC values were correlated to T2(*) values using the Spearman correlation test in a subset of patients. In addition, a phantom consisting of solutions of varying iron concentrations was imaged with single-shot EPI DWI and T2(*) imaging, and iron concentration was correlated with ADC and T2(*). RESULTS: In phantoms, there was a decrease in ADC and T2(*) with increasing iron concentration (r = -0.95 and -0.92, respectively; p < 0.05). Patients with hepatic siderosis had significantly lower SNR(est) and ADC compared with patients without siderosis (p < 0.0001). SNR(est) at b = 50 s/mm(2) and b = 500 s/mm(2) and ADC had a significant negative correlation with pathologic iron grade (r = -0.67 to 0.77, p < 0.0001). There was a significant correlation between liver T2(*) and ADC (r = 0.83, p < 0.0001). CONCLUSION: Hepatic siderosis lowers liver ADC and should be taken into account when using ADC for diagnosing liver cirrhosis.

OBJECTIVE: Total renal volume and changes in kidney volume are markers of disease progression in autosomal-dominant polycystic kidney disease (ADPKD) but are not used in clinical practice in part because of the complexity of manual measurements. This study aims to assess the intra- and interobserver reproducibility of a semiautomated renal volumetric algorithm using fluid-sensitive MRI pulse sequences. SUBJECTS AND METHODS: Renal volumes of 17 patients with ADPKD were segmented from high-resolution coronal HASTE and true fast imaging with steady-state precession (FISP) MR acquisitions. Measurements performed independently by four readers were repeated, typically after 7 days. Intraobserver agreement indexes were calculated for total kidney volume for each patient. Interobserver agreement indexes were obtained for the six paired combinations of readers as well as for two readers after rigorous formalized training. Pearson and concordance correlation coefficients, coefficients of variation (CVs), and 95% limits of agreement were determined. RESULTS: The HASTE and true FISP sequences performed similarly with a median intraobserver agreement of greater than 98.1% and a CV of less than 2.4% across all readers. The median interobserver agreement was greater than 95.2% and the CV was less than 7.1%, across all reader pairs. Reader training further lowered interobserver CV. The mean total kidney volume was 1420 mL (range, 331-3782 mL) for HASTE imaging and 1445 mL (range, 301-3714 mL) for true FISP imaging, with mean image processing times per patient of 43 and 28 minutes, respectively. CONCLUSION: This semiautomated MR volumetric algorithm provided excellent intraobserver and very good interobserver reproducibility using fluid-sensitive pulse sequences that emphasize cyst conspicuity.

Purpose:To assess the reproducibility and the distribution of intravoxel incoherent motion (IVIM) and diffusion-tensor (DT) imaging parameters in healthy renal cortex and medulla at baseline and after hydration or furosemide challenges.Materials and Methods:Using an institutional review board-approved HIPAA-compliant protocol with written informed consent, IVIM and DT imaging were performed at 3 T in 10 volunteers before and after water loading or furosemide administration. IVIM (apparent diffusion coefficient [ADC], tissue diffusivity [D(t)], perfusion fraction [f(p)], pseudodiffusivity [D(p)]) and DT (mean diffusivity [MD], fractional anisotropy [FA], eigenvalues [lambda(i)]) imaging parameters and urine output from serial bladder volumes were calculated. (a) Reproducibility was quantified with coefficient of variation, intraclass correlation coefficient, and Bland-Altman limits of agreement; (b) contrast and challenge response were quantified with analysis of variance; and (c) Pearson correlations were quantified with urine output.Results:Good reproducibility was found for ADC, D(t), MD, FA, and lambda(i) (average coefficient of variation, 3.7% [cortex] and 5.0% [medulla]), and moderate reproducibility was found for D(p), f(p), and f(p) . D(p) (average coefficient of variation, 18.7% [cortex] and 25.9% [medulla]). Baseline cortical diffusivities significantly exceeded medullary values except D(p), for which medullary values significantly exceeded cortical values, and lambda(1,) which showed no contrast. ADC, D(t), MD, and lambda(i) increased significantly for both challenges. Medullary diffusivity increases were dominated by transverse diffusion (1.72 +/- 0.09 [baseline] to 1.79 +/- 0.10 [hydration] mum(2)/msec, P = .0059; or 1.86 +/- 0.07 [furosemide] mum(2)/msec, P = .0094). Urine output correlated with cortical ADC with furosemide (r = 0.7, P = .034) and with medullary lambda(1) (r = 0.83, P = .0418), lambda(2) (r = 0.85, P = .0301), and MD (r = 0.82, P = .045) with hydration.Conclusion:Diffusion MR metrics are sensitive to flow changes in kidney induced by diuretic challenges. The results of this study suggest that vascular flow, tubular dilation, water reabsorption, and intratubular flow all play important roles in diffusion-weighted imaging contrast.(c) RSNA, 2012.

Contemporary imaging techniques for renal mass evaluation are essential to clinical management and surgical planning. Ultrasonography can be used to distinguish cystic from solid lesions but is less sensitive and accurate in renal mass characterization than computed tomography (CT) and magnetic resonance imaging (MRI). Multiphase CT imaging before and after administration of contrast is the primary imaging modality for characterization and staging of renal lesions. MRI is increasingly used as a problem solving tool. Advanced MRI techniques such as diffusion-weighted imaging and perfusion-weighted imaging are being explored in assessment of renal lesions. These techniques are discussed in this article.

The introduction of dual-energy computed tomography systems (ie, scanners that can simultaneously acquire images at different energies) has significant and unique applications for urologists. Imaging data from these scanners can be used to evaluate composition of urinary calculi and, by 'removing' iodine from an image, significantly decrease radiation dose to patients referred for hematuria. Further, the ability to create a virtual noncontrast image obviates the need for repeated scanning in patients with incidentally detected renal and adrenal masses. Finally, the ability to quantify the regional concentration of iodine in a renal neoplasm may provide a method to monitor effectiveness of therapy before size changes become apparent

Computed tomography (CT) is considered the imaging modality of choice in evaluation of renal lesions. The advantages of magnetic resonance imaging (MRI) compared to CT include superior soft tissue contrast, avoidance of ionizing radiation and iodinated contrast media, and the possibility of performing functional and advanced imaging techniques such as diffusion-weighted (DWI) and perfusion-weighted imaging (PWI). Although the traditional role of MRI in the evaluation of renal mass is primarily that of a problem-solving tool, DWI and PWI are expanding the role of MRI in management of renal cell cancers. DWI and PWI have shown considerable promise not only in renal lesion detection and characterization as benign or malignant, but also in assessment of renal cell cancer subtype and nuclear grade. Furthermore, these techniques have the potential to assist with tailoring patient- and disease-specific management by providing surgical planning in patients with localized renal cell cancer and assessing treatment response in patients with advanced renal cell cancer undergoing targeted chemotherapy

Functional magnetic resonance imaging (fMRI) techniques enable noninvasive assessment of renal function. Diffusion-weighted imaging, diffusion tensor imaging, blood oxygen level-dependent MRI, magnetic resonance elastography, and arterial spin labeling are some of the emerging techniques that have potential to investigate renal function without the use of exogenous gadolinium contrast. This article discusses the principles of these techniques, as well as their possible applications and limitations. This will introduce the readers to these novel imaging tools, which appear to have promising futures.

Diffusion-weighted imaging (DWI) involves data acquisitions at multiple b values. In this paper, we presented a method of selecting the b values that maximize estimation precision of the biexponential analysis of renal DWI data. We developed an error propagation factor for the biexponential model, and proposed to optimize the b-value samplings by minimizing the error propagation factor. A prospective study of four healthy human subjects (eight kidneys) was done to verify the feasibility of the proposed protocol and to assess the validity of predicted precision for DWI measures, followed by Monte Carlo simulations of DWI signals based on acquired data from renal lesions of 16 subjects. In healthy subjects, the proposed methods improved precision (P = 0.003) and accuracy (P < 0.001) significantly in region-of-interest based biexponential analysis. In Monte Carlo simulation of renal lesions, the b-sampling optimization lowered estimation error by at least 20-30% compared with uniformly distributed b values, and improved the differentiation between malignant and benign lesions significantly. In conclusion, the proposed method has the potential of maximizing the precision and accuracy of the biexponential analysis of renal DWI. Magn Reson Med, 2011. (c) 2011 Wiley-Liss, Inc

PURPOSE: To assess the utility of a 3D two-point Dixon sequence with water-fat decomposition for quantification of fat content of renal angiomyolipoma (AML). METHODS: 84 patients underwent renal MRI including 2D in-and-opposed-phase (IP and OP) sequence and 3D two-point Dixon sequence that generates four image sets [IP, OP, water-only (WO), and fat-only (FO)] within one breath-hold. Two radiologists reviewed 2D and 3D images during separate sessions to identify fat-containing renal masses measuring at least 1cm. For identified lesions subsequently confirmed to represent AML, ROIs were placed at matching locations on 2D and 3D images and used to calculate 2D and 3D SI(index) [(SI(IP)-SI(OP))/SI(IP)] and 3D fat fraction (FF) [SI(FO)/(SI(FO)+SI(WO))]. 2D and 3D SI(index) were compared with 3D FF using Pearson correlation coefficients. RESULTS: 41 AMLs were identified in 6 patients. While all were identified using the 3D sequence, 39 were identified using the 2D sequence, with the remaining 2 AMLs retrospectively visible on 2D images but measuring under 1cm. Among 32 AMLs with a 3D FF of over 50%, both 2D and 3D SI(index) showed a statistically significant inverse correlation with 3D FF (2D SI(index): r=-0.63, p=0.0010; 3D SI(index): r=-0.97, p<0.0001). CONCLUSION: 3D two-point Dixon sequence may provide a reasonable alternative to 2D dual-echo sequence for detection of renal AML and may have additional value for quantification of fat content of these lesions given the observation that 3D FF, unlike 2D and 3D SI(index), is not limited by ambiguity of water or fat dominance. This may assist clinical management of AML given evidence that fat content predicts embolization response