Biomedical subjects

Van J Wedeen

Publications and source records attributed to Van J Wedeen.

7 recordsLinked to original sources

PubMed2003

Diffusion tensor MRI of myocardial fibers and sheets: correspondence with visible cut-face texture.

PURPOSE: To test the hypothesis that the primary, secondary, and tertiary eigenvectors of the diffusion tensor (DT) measured with DT-MRI correspond to the fiber, sheet, and sheet normal directions, respectively, we compared DT-MRI data with the texture visible in the cut face of fresh bovine myocardium. MATERIALS AND METHODS: DT-MRI and optical images obtained under identical conditions were compared objectively. Ink prints were made of the cut tissue, and the local orientations within these images were defined by analysis of local autocorrelations for regions matching DT-MRI pixels. Deviation angles between the cleavage orientations and the diffusion eigenvectors were analyzed in eight specimens sliced in three orthogonal planes. RESULTS: Root-mean-square (RMS) angular disparity was 11 degrees between the first eigenvectors of the DT and the fiber direction, 14 degrees between the second eigenvector and the sheet direction, 14 degrees between the third eigenvector and the sheet normal direction, and 15 degrees between the tensor orientation in the imaging plane and the cleavage orientation of the cut face. CONCLUSION: The results support a parallel relationship between the eigenvectors of the DT and symmetry axes of the myocardial architecture. Specifically, the first, second and third eigenvectors correspond to the fiber, sheet, and sheet normal directions, respectively.

Animals↗

PubMed2003

Combined diffusion and strain MRI reveals structure and function of human myocardial laminar sheets in vivo.

The mechanism of ventricular thickening in normal humans was investigated using in vivo MRI. The hypothesis that myocardial laminar sheets contribute to ventricular thickening predominantly via sheet shear and sheet extension, as previously found invasively in canine studies at particular ventricular sites, was tested. In normal human subjects, registered images of myocardial sheet architecture and strain at the mid-left ventricle (mid-LV) at mid-systole were acquired with diffusion and strain MRI. Sheet function was analyzed by computing myocardial strain in the local fiber-sheet coordinates. In general, myocardial sheets contribute to ventricular thickening through all three cross-fiber strain components: sheet shear, sheet extension, and sheet-normal thickening (previously undocumented). Each of these components demonstrated substantial spatial heterogeneity, with sheet shear and sheet extension usually predominant in the anterior free wall, and sheet-normal thickening predominant near the right ventricular (RV) insertions. However, considerable intersubject variability was also found. In all cases, the contributions to thickening of fiber strains were small. Sheet function in normal humans was found to be heterogeneous and variable, contrasting with the uniform and symmetric ventricular patterns of fiber shortening and wall thickening. The study demonstrates that noninvasive NMR imaging is a promising tool for investigations of myocardial sheet architecture and function, and is particularly suited to the evident complexity of this field of study.

Diastole↗

PubMed2003

Automatic segmentation of thalamic nuclei from diffusion tensor magnetic resonance imaging.

The nuclei of the thalamus have traditionally been delineated by their distinct cyto/myeloarchitectural appearance on histology. Here, we show that diffusion tensor magnetic resonance imaging (DTI) can noninvasively resolve the major thalamic nuclei based on the characteristic fiber orientation of the corticothalamic/thalamocortical striations within each nucleus. Using an automatic clustering algorithm, we extracted the Talairach coordinates for the individual thalamic nuclei. The center-of-mass coordinates for the segmented nuclei were found to agree strongly with those obtained from a histological atlas. The ability to resolve thalamic nuclei with DTI will allow for morphometric analysis of specific nuclei and improved anatomical localization of functional activation in the thalamus.

Algorithms↗

PubMed2002

Phase contrast MRI of myocardial 3D strain by encoding contiguous slices in a single shot.

Quantitative measurements of inherently three-dimensional (3D) cardiac strain and strain rate require 3D data; MRI provides uniquely high sensitivity to material strain by combining phase contrast with single-shot acquisition methods, such as echo-planar imaging (EPI). Previous MRI methods applied to 3D strain used multiple two-dimensional (2D) acquisitions and suffered loss of sensitivity due to magnification within the strain calculation of physiologic noise related to cardiac beat-to-beat variability. In the present work, each single-shot acquisition generates 3D image data by acquiring two contiguous 2D Fourier transform (FT) images in a single echo train of an EPI readout. Although strain encoding divides across multiple EPI shots, each strain component is computed only within single-shot data, avoiding noise magnification. Strain tensor maps are displayed using iconic 3D graphics or a simple color code of tensor shape. In a deforming gel phantom, gradient-recalled echo (GRE) MRI movies of 3D strain rates match expected strain fields. In normal human subjects, 3D strain rate tensor movies of heart and brain comprising seven slices in each of seven cardiac phases were completed in 56 heartbeats. Stimulated echo (STE) MRI of net systolic 3D strain was also demonstrated. Two-slices-in-one-shot spatial encoding permits a complete quantitative survey of ventricular 3D strain in under a minute, with routine patient supervision and turnkey image processing.

Brain↗

PubMed2002

Cardiac diffusion MRI without motion effects.

We present a method for diffusion tensor MRI in the beating heart that is insensitive to cardiac motion and strain. Using a stimulated echo pulse sequence with two electrocardiogram (ECG) triggers, diffusion-encoding bipolar gradient pulses are applied at identical phases in consecutive cardiac cycles. In this experiment, diffusion is encoded at a single phase in the cardiac cycle of less than 30 ms in duration. This encoding produces no phase shifts for periodic motion and is independent of intervening strains. Studies in a gel phantom with cyclic deformation confirm that by using this sequence we can map the diffusion tensor free of effects of cyclic motion. In normal human subjects, myocardial diffusion eigenvalues measured with the present method showed no significant change between acquisitions encoded at maximum contractile velocity (peak) vs. at myocardial standstill (end-systole), demonstrating motion independence of in vivo diffusion measurements. Diffusion tensor images acquired with the present method agree with registered data acquired with a previous cardiac diffusion MRI method that was shown to be valid in the normal heart, strongly supporting the validity of MRI diffusion measurement in the beating heart. Myocardial sheet and fiber dynamics measured during systole showed that normal human myocardial sheet orientations tilt toward the radial during systole, and fiber orientations tilt toward the longitudinal, in qualitative agreement with previous invasive studies in canines. These results demonstrate the technique's ability to measure myocardial diffusion accurately at any point in the cardiac cycle free of measurable motion effect, as if the heart were frozen at the point of acquisition.

Diffusion↗

PubMed2002

Simultaneous echo refocusing in EPI.

A method to encode multiple two-dimensional Fourier transform (2D FT) images within a single echo train is presented. This new method, simultaneous echo refocusing (SER), is a departure from prior echo planar image (EPI) sequences which use repeated single-shot echo trains for multislice imaging. SER simultaneously acquires multiple slices in a single-shot echo train utilizing a shared refocusing process. The SER technique acquires data faster than conventional multislice EPI since it uses fewer gradient switchings and fewer preparation pulses such as diffusion gradients. SER introduces a new capability to simultaneously record multiple spatially separated sources of physiologic information in subsecond image acquisitions, which enables several applications that are dependent on temporal coherence in MRI data including velocity vector field mapping and brain activation mapping.

Brain↗

PubMed2002

High angular resolution diffusion imaging reveals intravoxel white matter fiber heterogeneity.

Magnetic resonance (MR) diffusion tensor imaging (DTI) can resolve the white matter fiber orientation within a voxel provided that the fibers are strongly aligned. However, a given voxel may contain a distribution of fiber orientations due to, for example, intravoxel fiber crossing. The present study sought to test whether a geodesic, high b-value diffusion gradient sampling scheme could resolve multiple fiber orientations within a single voxel. In regions of fiber crossing the diffusion signal exhibited multiple local maxima/minima as a function of diffusion gradient orientation, indicating the presence of multiple intravoxel fiber orientations. The multimodality of the observed diffusion signal precluded the standard tensor reconstruction, so instead the diffusion signal was modeled as arising from a discrete mixture of Gaussian diffusion processes in slow exchange, and the underlying mixture of tensors was solved for using a gradient descent scheme. The multitensor reconstruction resolved multiple intravoxel fiber populations corresponding to known fiber anatomy. Ma

Anisotropy↗