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

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

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

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

The B-11 NMR spectra in polycrystalline MgB2 were measured for several magnetic fields (1.97 and 3.15 T) as a function of temperature from 5 to 40 K. The composite spectra in the superconducting state can be understood in terms of anisotropy of the upper critical field, gamma(H), which is determined to be 5.4 at low temperature. Using Brandt's algorithm (Brandt 1997 Phys. Rev. Lett. 78 2208) the full spectrum, including satellites, was simulated for the temperature 8 K and a magnetic field of 1.97 T. The penetration depth. was determined to be 1152 +/- 50 angstrom, and the anisotropy of the penetration depth, gamma(lambda), was estimated to be close to one at low temperature. Therefore, our findings establish that there are two different anisotropies for upper critical field and penetration depth at low temperatures

The multiple-modulation-multiple-echo sequence, previously used for rapid measurement of diffusion, is extended to a method for single shot imaging. Removing the gradient switching requirement during the application of RF pulses by a constant frequency encoding gradient can shorten experiment time for ultrafast imaging. However, having the gradient on during the pulses gives rise to echo shape variations from off-resonance effects, which make the image reconstruction difficult. In this paper, we propose a simple method to deconvolve the echo shape variation from the true one-dimensional image. This method is extended to two-dimensional imaging by adding phase encoding gradients between echoes during the acquisition period to phase encode each echo separately. Slice selection is achieved by a frequency selective pulse at the beginning of the sequence. Imaging speed is mainly limited by the phase encoding gradients' switching times and echo overlap when echo spacing is very short. This technique can produce a single-shot image of sub-millimeter resolution in 5 ms

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