Faculty Bibliography

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

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

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

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

In this article, we demonstrate a single-scan method to measure an average flow velocity vector along an arbitrary direction. This method is based on the MMME sequence and utilizes static and pulsed magnetic field gradients along multiple directions for the optimal determination of flow velocity components in three-dimensional space. Experimentally measured average flow velocities from the flow induced phase shift with a single-scan MMME sequence show excellent agreements with the known flow rate, and the signal decay of each echo due to a velocity distribution is also quantitatively verified with known laminar flow patterns

In this article, the authors demonstrate a rapid NMR method to measure a full three-dimensional diffusion tensor. This method is based on a multiple modulation multiple echo sequence and utilizes static and pulsed magnetic field gradients to measure diffusion along multiple directions simultaneously. The pulse sequence was optimized using a well-known linear inversion metric (condition number) and successfully tested on both isotropic (water) and anisotropic (asparagus) diffusion systems

Spatially resolved distributions of T-2 relaxation times in carbonate rocks are measured with slice-selective multiple spin echo magnetic resonance imaging to study the length scales of heterogeneity in these samples. Single-voxel Carr-Pureell-Mciboom-Gill decays are fit to double exponential functions, and the results of those fits are combined into a histogram. We describe a novel qualitative method of assessing the importance of different length scales of heterogeneity, involving comparing various aspects of these histograms to the full-core T2 distributions. Using this technique, it is found that almost all individual voxels relax not only with more than one time constant but indeed with a range of relaxation times that approximates the full breadth of relaxation times for the entire core, indicating significant subvoxel heterogeneity. In addition, different voxcls are found to exhibit relaxation times that differ by orders of magnitude, indicating significant heterogeneity between the scale of a voxel (1 min) and that of the entire core (several centimeters). These results reflect the importance of a broad range of length scales of heterogeneity in these carbonate rocks

We describe several applications of a versatile pulse sequence family employing multiple spin echoes within one acquisition to accelerate multidimensional experiments. The core sequence, called multiple modulation multiple echo, measures the maximal set of spin echoes generated from a set of RF pulses with unequal time spacings. These echoes can be modulated individually to exhibit different encoding. As a result, one scan of the sequence determines a series of data points as a function of the specific encoding mechanism. The variety of realizations of this sequence includes single shot measurements of diffusion or flow in ID, 2D, and single-scan 2D slice-selective imaging. The essential spin dynamics of such sequences is introduced and their applications are reviewed. (C) 2007 Wiley Periodicals, Inc

The diffusion of glycerol molecules decreases with decreasing temperature as its viscosity increases in a manner simply described by the Stokes-Einstein relation. Approaching the glass transition, this relation breaks down as it does with a number of other pure liquid glass formers. We have measured the diffusion coefficient for binary mixtures of glycerol and water and find that the Stokes-Einstein relation is restored with increasing water concentration. Our comparison with theory suggests that adding water postpones the formation of frustration domains

The standard method of diffusion tensor imaging (DTI) involves one diffusion-sensitizing gradient direction per acquired signal. This paper describes an alternative method in which the entire direction set required for calculating the diffusion tensor is captured in a few scans. In this method, a series of radiofrequency (RF) pulses are applied, resulting in a train of spin echoes. A pattern of applied magnetic field gradients between the RF pulses generates a different diffusion weighting in both magnitude and direction for each echo, resulting in a dataset sufficient to determine the tensor. This significantly reduces the time required for a full DTI scan and potentially allows a tradeoff of this time for image quality. In the present work, this method is demonstrated in an anisotropic diffusion phantom (asparagus)