Faculty Bibliography

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.

OBJECTIVE: The purpose of this study was to monitor iron clearance from the liver by means of T2 and T2* mapping after administration of an ultrasmall superparamagnetic iron oxide (USPIO) agent. MATERIALS AND METHODS: The study was performed using ferumoxytol (Feraheme), a USPIO agent that has been approved by the US Food and Drug Administration for the treatment of iron deficiency anemia in adult patients with chronic kidney disease. Six healthy human participants without anemia or preexisting iron overload were prospectively included. The cohort comprised 4 men and 2 postmenopausal women, aged 22 to 57 years. T2 and T2* mapping of the liver were performed at 1.5 T using multiple spin echo and multiple gradient echo sequences, respectively. After baseline imaging, ferumoxytol was injected intravenously at a dose of 5 mg Fe/kg body weight. Imaging was repeated at 3 days, 1 month, and every 2 months thereafter for up to 11 months or until liver T2* had recovered to 24 milliseconds, the threshold used to define iron deposition. For each examination, maps of the relaxation rates R2 (= 1/T2) and R2* (= 1/T2*) were generated by fitting the signal intensity data as a function of echo time to a monoexponential decay. RESULTS: No adverse reactions to ferumoxytol injection occurred. The magnetic resonance (MR) responses to ferumoxytol varied widely among the participants. Liver R2* increased from a mean value of 35.6 s (range, 28.7-40.9 s) at baseline to a mean value of 241 s (range, 161-417 s) 3 days after administration. Liver R2 increased from 19.4 s (range, 16.6-23.8 s) at baseline to 45.3 s (range, 34.4-58.5 s) at 3 days. There was also a large variation in iron clearance times. In 1 participant, MR relaxation rates had recovered to baseline by 3 months, whereas, in 3 participants, liver R2* remained elevated at 11 months (R2* > 55 s, ie, T2* < 18 milliseconds). In these 3 participants, liver R2 also remained marginally higher at 11 months than corresponding baseline values. CONCLUSIONS: Iron deposition in the liver after a 5 mg Fe/kg dose of ferumoxytol may alter signal contrast on MR images for several months after administration. This is an important consideration in the use of USPIO agents for diagnostic purposes.

Purpose: To compare histogram analysis of voxel-based whole-lesion (WL) enhancement to qualitative assessment and region-of-interest (ROI)-based enhancement analysis in discriminating the renal cell cancer (RCC) subtype clear cell RCC (ccRCC) from papillary RCC (pRCC). Materials and Methods: In this institutional review board-approved, HIPAA-compliant retrospective study, 73 patients underwent magnetic resonance (MR) imaging prior to surgery for RCC between January 2007 and January 2010. Three-dimensional fat-suppressed T1-weighted gradient-echo corticomedullary phase acquisitions, obtained before and after contrast agent administration, were transferred to a workstation at which automated registration followed by semiautomated segmentation of the RCC was performed. Percent enhancement was computed on a per-voxel basis: (SI(post) - SI(pre))/SI(pre) .100, where SI(pre) and SI(post) indicate signal intensity before and after contrast enhancement, respectively. The WL quantitative parameters of mean, median, and third quartile enhancement and histogram distribution parameters kurtosis and skewness were computed for each lesion. WL enhancement parameters were compared with ROI-based analysis and qualitative assessment with regards to diagnostic accuracy and interreader agreement in differentiating ccRCC from pRCC. Results: There were 19 pRCCs and 55 ccRCCs at pathologic examination. ccRCC had significantly higher WL mean, median, and third quartile enhancement compared with pRCC and hade significantly lower kurtosis and skewness (all P < .001). Third quartile enhancement had the highest accuracy (94.6%; area under the curve, 0.980) in discriminating ccRCC from pRCC, which was significantly higher than the accuracy of qualitative assessment (86.0%; P = .04) but not significantly higher than that of ROI enhancement (89.2%; P = .52). WL enhancement parameters had higher interreader agreement (kappa = 0.91-1.0) compared with ROI enhancement or qualitative assessment (kappa = 0.83 and 0.7, respectively) in discriminating ccRCC from pRCC. Conclusion: WL enhancement histogram analysis is feasible and can potentially be used to differentiate ccRCC from pRCC with high accuracy. (c) RSNA, 2012 Supplemental material: http://radiology.rsna.org/lookup/suppl/doi:10.1148/radiol.12111281/-/DC1.

PURPOSE: The aim of this study was to use intravoxel incoherent motion diffusion-weighted imaging to discriminate subtypes of renal neoplasms and to assess agreement between intravoxel incoherent motion (perfusion fraction, fp) and dynamic contrast-enhanced magnetic resonance imaging (MRI) metrics of tumor vascularity. SUBJECTS AND METHODS: In this Health Insurance Portability and Accountability Act-compliant, institutional review board-approved prospective study, 26 patients were imaged at 1.5-T MRI using dynamic contrast-enhanced MRI with high temporal resolution and diffusion-weighted imaging using 8 b values (range, 0-800 s/mm). Perfusion fraction (fp), tissue diffusivity (Dt), and pseudodiffusivity (Dp) were calculated using biexponential fitting of the diffusion data. Apparent diffusion coefficient (ADC) was calculated with monoexponential fit using 3 b values of 0, 400, and 800 s/mm. Dynamic contrast-enhanced data were processed with a semiquantitative method to generate model-free parameter cumulative initial area under the curve of gadolinium concentration at 60 seconds (CIAUC60). Perfusion fraction, Dt, Dp, ADC, and CIAUC60 were compared between different subtypes of renal lesions. Perfusion fraction was correlated with CIAUC60. RESULTS: We examined 14 clear cell, 4 papillary, 5 chromophobe, and 3 cystic renal cell carcinomas (RCCs). Although fp had higher accuracy (area under the curve, 0.74) for a diagnosis of clear cell RCC compared with Dt or ADC, the combination of fp and Dt had the highest accuracy (area under the curve, 0.78). The combination of fp and Dt diagnosed papillary RCC and cystic RCC with 100% accuracy, and clear cell RCC and chromophobe RCC, with 86.5% accuracy. There was significant strong correlation between fp and CIAUC60 (r = 0.82; P < 0.001). CONCLUSION: Intravoxel incoherent motion parameters fp and Dt can discriminate renal tumor subtypes. Perfusion fraction demonstrates good correlation with CIAUC60 and can assess degree of tumor vascularity without the use of exogenous contrast agent.

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.