Faculty Bibliography

OBJECTIVES: : To obtain intravoxel incoherent motion (IVIM) parameters with biexponential analysis of multiple b-value diffusion-weighted imaging (DWI) and compare these parameters to apparent diffusion coefficient (ADC) obtained with monoexponential modeling in their ability to discriminate enhancing from nonenhancing renal lesions. MATERIALS AND METHODS: : Twenty-eight patients were imaged at 1.5 T utilizing contrast-enhanced (CE) magnetic resonance imaging (MRI) and breath-hold DWI using 8 b values (range: 0-800 s/mm). Perfusion fraction (fp), tissue diffusivity (Dt), and pseudo-diffusion coefficient (Dp) were calculated using segmented biexponential analysis. ADCtotal and ADC0-400-800 were calculated with monoexponential fitting of the DWI data. fp, Dt, Dp, ADCtotal, and ADC0-400-800 were compared between enhancing and nonenhancing renal lesions. Receiver operating characteristic analysis was performed for all DWI parameters. fp was correlated with percent enhancement. RESULTS: : There were a total of 31 renal lesions (15 enhancing and 16 nonenhancing) in 28 patients on CE-MRI. fp of enhancing masses was significantly higher (27.9 vs. 6.1) and Dt was significantly lower (1.47 vs. 2.40 x10 mm/s). IVIM parameters fp and Dt demonstrated higher accuracy in differentiating enhancing from nonenhancing renal lesions compared with monoexponential parameters ADC0-400-800 and ADCtotal, with area under the curve of 0.946, 0.896, 0.854, and 0.675, respectively. There was a good correlation between fp and percent enhancement (r = 0.7; P < 0.001). CONCLUSION: : IVIM parameters fp and Dt obtained with biexponential fitting of multi-b value DWI have higher accuracy compared with ADC (obtained with monoexponential fit) in discriminating enhancing from nonenhancing renal lesions. Furthermore, fp demonstrates good correlation with percent enhancement and can provide information regarding lesion vascularity without the use of exogenous contrast agent

Purpose: To evaluate mixed ground glass nodules (GGNs) utilizing 3 Tesla (T) MRI and 32-channel torso-array-coil and to correlate non-echo planar diffusion weighted imaging (DWI) and T2^* measurements with pathologic findings. Materials and Methods: Twelve patients with 13 GGNs >1cm in diameter detected on computed tomography were prospectively recruited for this Institutional Review Board approved study. T1-weighted 2D gradient echo (GRE), T2-weighted 2D turbo spin echo with fat saturation, T2^*-weighted multiple GRE, and diffusion weighted single shot twice-refocused spin echo axial images of the GGNs were acquired at end inspiration without intravenous contrast. Apparent diffusion coefficient (ADC) and T2^* values were determined and correlated to pathology. Results: All GGNs were visualized with the T2-weighted 2D TSE sequence providing the best morphologic delineation. Pathology was available for 9 of 13 lesions. ADC ranged from 1.19 to 1.78 mum²/ms (mean 1.45+/-0.19) and T2^* ranged from 6.78 to 27.81 mum²/ms (median 16.13, mean 16.68+/- 7.19) for the 7 malignant lesions. ADC was 1.59 and 1.42 and T2^* was 6.78 and 20.24 for the 2 malignant lesions with positive epidermal growth factor receptors. ADC ranged from 0.9 to 1.47 mum²/ms (mean 1.18 +/-0.4) and T2^* ranged from 6.87 to 10.93 (mean 8.9 +/-2.87) for the 2 benign lesions. Conclusion: 3T MRI with a 32-channel torso-array-coil provides a radiation free means of GGN evaluation. The T2-weighted 2D TSE with fat saturation sequence yields the best lesion visibility. DWI and T2^* measurements may provide quantitative measures for distinguishing malignant from benign nodules

Many goals and challenges of research in natural or synthetic porous media are mirrored in quantitative medical MRI. This review will describe examples where MR techniques used in porous media (particularly diffusion-weighted imaging (DWI)) are applied to physiological pathologies. Tissue microstructure is one area with great overlap with porous media science. Diffusion-weighting (esp. in neurological tissue) has motivated models with explicit physical dimensions, statistical parameters, empirical descriptors, or hybrids thereof. Another clinically relevant microscopic process is active flow. Renal (kidney) tissue possesses significant active vascular / tubular transport that manifests as "pseudodiffusion." Cancerous lesions involve anomalies in both structure and flow. The tools of magnetic resonance and their interpretation in porous media has had great impact on clinical MRI, and continued cross-fertilization of ideas can only enhance the progress of both fields.

Functional MRI is a new and exciting tool enabling non-invasive assessment of renal function. Diffusion-weighted imaging (DWI), diffusion tensor imaging (DTI), blood oxygen level-dependent (BOLD) MRI, and magnetic resonance elastography (MRE) are some of the techniques under investigation. In this article we review the basic principles of these techniques, their possible applications, and their limitations

PURPOSE:: To report our preliminary experience with the use of intravoxel incoherent motion (IVIM) diffusion-weighted magnetic resonance imaging (DW-MRI) and dynamic contrast-enhanced (DCE)-MRI alone and in combination for the diagnosis of liver cirrhosis. MATERIALS AND METHODS:: Thirty subjects (16 with noncirrhotic liver, 14 with cirrhosis) were prospectively assessed with IVIM DW-MRI (n = 27) and DCE-MRI (n = 20). IVIM parameters included perfusion fraction (PF), pseudodiffusion coefficient (D*), true diffusion coefficient (D), and apparent diffusion coefficient (ADC). Model-free DCE-MR parameters included time to peak (TTP), upslope, and initial area under the curve at 60 seconds (IAUC60). A dual input single compartmental perfusion model yielded arterial flow (Fa), portal venous flow (Fp), arterial fraction (ART), mean transit time (MTT), and distribution volume (DV). The diagnostic performances for diagnosis of cirrhosis were evaluated for each modality alone and in combination using logistic regression and receiver operating characteristic analyses. IVIM and DCE-MR parameters were compared using a generalized estimating equations model. RESULTS:: PF, D*, D, and ADC values were significantly lower in cirrhosis (P = 0.0056-0.0377), whereas TTP, DV, and MTT were significantly increased in cirrhosis (P = 0.0006-0.0154). There was no correlation between IVIM- and DCE-MRI parameters. The highest Az (areas under the curves) values were observed for ADC (0.808) and TTP-DV (0.952 for each). The combination of ADC with DV and TTP provided 84.6% sensitivity and 100% specificity for diagnosis of cirrhosis. CONCLUSION:: The combination of DW-MRI and DCE-MRI provides an accurate diagnosis of cirrhosis. J. Magn. Reson. Imaging 2010;31:589-600. (c) 2010 Wiley-Liss, Inc

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: To compare the performance of apparent diffusion coefficient (ADC) measurement obtained with diffusion-weighted (DW) magnetic resonance (MR) imaging in the characterization of non-fat-containing T1 hyperintense renal lesions with that of contrast material-enhanced MR imaging, with histopathologic analysis and follow-up imaging as the reference standards. MATERIALS AND METHODS: Institutional review board approval was obtained for this HIPAA-compliant retrospective study, and the informed consent requirement was waived. Two independent observers retrospectively assessed MR images obtained in 41 patients with non-fat-containing T1 hyperintense renal lesions. The MR examination included acquisition of DW and contrast-enhanced T1-weighted images. For each index lesion, the observers assessed the (a) mean (+/- standard deviation) of ADC, (b) enhancement ratio, and (c) subtracted images for the presence of enhancement (confidence score, 1-5). Histopathologic analysis of renal cell carcinomas (RCCs) and follow-up imaging for benign lesions were the reference standards. ADCs of benign lesions and RCCs were compared. Receiver operating characteristic (ROC) curve analysis was performed to assess the accuracy of DW imaging, enhancement ratio, and subtraction for the diagnosis of RCC. Results: A total of 64 lesions (mean diameter, 3.9 cm), including 38 benign T1 hyperintense cysts and 26 RCCs, were assessed. Mean ADCs of RCCs were significantly lower than those of benign cysts ([1.75 +/- 0.57] x 10(-3) mm(2)/sec vs [2.50 +/- 0.53] x 10(-3) mm(2)/sec, P < .0001). ADCs of solid and cystic portions of complex cystic RCCs were significantly different ([1.37 +/- 0.55] x 10(-3) mm(2)/sec vs [2.45 +/- 0.63] x 10(-3) mm(2)/sec, P < .0001). When data from both observers were pooled, area under the ROC curve, sensitivity, and specificity were 0.846, 71%, and 91%, respectively, for DW imaging; 0.865, 65%, and 96%, respectively, for enhancement ratio (at the excretory phase); and 0.861, 83%, and 89%, respectively, for subtraction (P = .48 and P = .85, respectively). The combination of DW imaging and subtraction resulted in area under the ROC curve, sensitivity, and specificity of 0.893, 87%, and 92%, respectively, with significantly improved reader confidence compared with subtraction alone (P = .041). CONCLUSION: The performance of DW imaging was equivalent to that of enhancement ratio in the characterization of T1 hyperintense renal lesions, with both methods having lower sensitivity than image subtraction without reaching significance

Trabecular bone structure is known to play a crucial role in the overall strength, and thus fracture risk, of such areas of the skeleton as the vertebrae, spine, femur, tibiae, or radius. Several MR methods devoted to probing this structure depend upon the susceptibility difference between the solid bone matrix and the intervening fluid/marrow/fat, usually in the context of a linewidth (1/T(2)') measurement or mapping technique. A recently demonstrated new approach to this system involves using internal gradients to encode diffusion weighting, and extracting structural information (e.g., surface-to-volume ratio) from the resulting signal decay. This contrast method has been demonstrated in bulk measurements on cleaned, water-saturated bovine trabecular bone samples. In the present work, microscopic imaging (0.156 mm in-plane resolution) is performed in order to spatially resolve this contrast on the trabecular level, and confirm its interpretation for the bulk measurements. It is found that the local rate of decay due to diffusion in the internal field (DDIF) is maximal close to the trabecular surfaces. The overall decay rate in a lower resolution scan probes the abundance of these surfaces, and provides contrast beyond that found in conventional proton density weighted or T(1)-weighted imaging. Furthermore, a microscopic calculation of internal field distributions shows a qualitative distinction between the structural sensitivities of DDIF and T(2)'. DDIF contrast is highly localized around trabecular walls than is the internal field itself, making it a less sensitive but more specific measure of such important properties as trabecular number

There is currently a growing interest in applications of diffusion-weighted imaging (DWI) in the abdomen and pelvis. DWI provides original functional information where the signal and contrast are determined by the microscopic mobility of water. DWI can provide additional information over conventional MR sequences, and could potentially be used as an alternative to contrast-enhanced sequences in patients with chronic renal insufficiency at risk of nephrogenic systemic fibrosis. We provide an overview on basic physics background on DWI applied to the kidneys, and we summarize the current available data, including our recent experience

A new approach to MR trabecular bone characterization is presented. This method probes the diffusion of spins through internal magnetic field gradients due to the susceptibility contrast between the bone and water (or marrow) phases. The resulting spin magnetization decay encodes properties of the underlying structure. This method, termed decay due to diffusion in the internal field (DDIF), is well established as a probe of pore size and structure. In the present work its application is shown for in vitro experiments on excised bovine tibiae samples. A comparison with pulsed field gradient (PFG) measurement of restricted diffusion shows a strong correlation of DDIF with the surface-to-volume ratio (SVR) of bones. Calculation of the internal magnetic field within the bone structure also supports this interpretation. These NMR measurements compare well with the image analysis from microscopic computed tomography (muCT). The SVR is not accessible in the clinically standard densitometry measurements, and provides vital information on bone strength and therefore on its fracture risk. The DDIF and PFG methods derive this information from a straightforward pulse sequence that does not employ either high applied field gradients or microimaging, and thus may have clinical potential. Magn Reson Med 59:28-39, 2008. (c) 2007 Wiley-Liss, Inc