Edward W. Hsu

Magnetic resonance myocardial fiber-orientation mapping with direct histological correlation

Functional properties of the myocardium are mediated by the tissue structure. Consequently, proper physiological studies and modeling necessitate a precise knowledge of the fiber orientation. Magnetic resonance (MR) diffusion tensor imaging techniques have been used as a nondestructive means to characterize tissue fiber structure; however, the descriptions so far have been mostly qualitative. This study presents a direct, quantitative comparison of high-resolution MR fiber mapping and histology measurements in a block of excised canine myocardium. Results show an excellent correspondence of the measured fiber angles not only on a point-by-point basis (average difference of -2.30 ± 0.98°, n = 239) but also in the transmural rotation of the helix angles (average correlation coefficient of 0.942 ± 0.008 with average false-positive probability of 0.004 ± 0.001, n = 24). These data strongly support the hypothesis that the eigenvector of the largest MR diffusion tensor eigenvalue coincides with the orientation of the local myocardial fibers and underscore the potential of MR imaging as a noninvasive, three-dimensional modality to characterize tissue fiber architecture.Functional properties of the myocardium are mediated by the tissue structure. Consequently, proper physiological studies and modeling necessitate a precise knowledge of the fiber orientation. Magnetic resonance (MR) diffusion tensor imaging techniques have been used as a nondestructive means to characterize tissue fiber structure; however, the descriptions so far have been mostly qualitative. This study presents a direct, quantitative comparison of high-resolution MR fiber mapping and histology measurements in a block of excised canine myocardium. Results show an excellent correspondence of the measured fiber angles not only on a point-by-point basis (average difference of -2.30 +/- 0.98 degrees, n = 239) but also in the transmural rotation of the helix angles (average correlation coefficient of 0.942 +/- 0.008 with average false-positive probability of 0.004 +/- 0.001, n = 24). These data strongly support the hypothesis that the eigenvector of the largest MR diffusion tensor eigenvalue coincides with the orientation of the local myocardial fibers and underscore the potential of MR imaging as a noninvasive, three-dimensional modality to characterize tissue fiber architecture.

Diffusion Tensor Imaging: Background, Potential, and Utility in Psychiatric Research

Diffusion tensor imaging is a variation of magnetic resonance imaging that measures the diffusion of water in tissues. This can help measure and quantify a tissues orientation and structure, making it an ideal tool for examining cerebral white matter and neural fiber tracts. It is only beginning to be utilized in psychiatric research. This article reviews the theory behind diffusion tensor imaging, its potential to map fiber tracts in the brain, and its recent use in psychiatric research.

Diffusion tensor microscopy of the intervertebral disc anulus fibrosus

Morphologically accurate biomechanical models of the intervertebral disc anulus fibrosus (AF) require precise knowledge of its lamellar architecture; however, available methods of assessment are limited by poor spatial resolution or the destructive nature of the technique. In a novel approach, diffusion tensor microscopy was used in this study to characterize the microstructure of excised porcine AF samples. Results show diffusion in the AF to be anisotropic. The orientations of anisotropy exhibit a layered morphology that agrees with light micrographs of the corresponding samples, and the behavior of the orientation angles is consistent with the known AF collagen fiber architecture. A static magnetic field‐dependent relaxation anisotropy was observed in the AF, which has methodological implications for magnetic resonance (MR) imaging of ordered collageneous tissues. These findings present MR diffusion tensor microscopy as a potentially valuable tool to assess quantitatively and nondestructively water diffusion anisotropy and lamellar structure of the intervertebral disc AF. Magn Reson Med 41:992–999, 1999.

Biomechanical factors in tissue engineered meniscal repair.

Damage to the meniscus after trauma or injury is associated with detrimental changes in joint function that can lead to pain, disability, and degenerative joint changes. Recently, tissue engineering strategies for meniscal repair have been suggested including using biocompatible grafts as a substrate for regeneration, and cellular supplementation to promote remodeling and healing. Little is known, however, about the contributions of these novel repair strategies to restoration of normal meniscal function. Biomechanical factors play a role in the design and synthesis of tissue engineered biomaterials and bioreactors, and also are important for evaluating the efficacy of these new strategies for restoring normal meniscal function. In this report, an overview is presented of biomechanical factors that are critical to meniscal function followed by a review of biomechanical considerations for the design and evaluation of tissue engineered strategies for meniscal repair. Recommendations for future study of biomechanical factors in tissue engineered meniscal repair also are provided.

Three-dimensional diffusion tensor microscopy of fixed mouse hearts

The relative utility of 3D, microscopic resolution assessments of fixed mouse myocardial structure via diffusion tensor imaging is demonstrated in this study. Isotropic 100‐μm resolution fiber orientation mapping within 5.5° accuracy was achieved in 9.1 hr scan time. Preliminary characterization of the diffusion tensor primary eigenvector reveals a smooth and largely linear angular rotation across the left ventricular wall. Moreover, a higher level of structural hierarchy is evident from the organized secondary and tertiary eigenvector fields. These findings are consistent with the known myocardial fiber and laminar structures reported in the literature and suggest an essential role of diffusion tensor microscopy in developing quantitative atlases for studying the structure–function relationships of mouse hearts. Magn Reson Med 52:453–460, 2004.

Noise removal in magnetic resonance diffusion tensor imaging.

Although promising for visualizing the structure of ordered tissues, MR diffusion tensor imaging (DTI) has been hampered by long acquisition time and low spatial resolution associated with its inherently low signal‐to‐noise ratio (SNR). Moreover, the uncertainty in the DTI measurements has a direct impact on the accuracy of structural renderings such as fiber streamline tracking. Noise removal techniques can be used to improve the SNR of DTI without requiring additional acquisitions, albeit most low‐pass filtering methods are accompanied by undesirable image blurring. In the present study, a modified vector‐based partial‐differential‐equation (PDE) filtering formalism was implemented for smoothing DTI vector fields. Using an image residual‐energy criterion to equate the degree of smoothing and error metrics empirically derived from DTI data to quantify the relative performances, the effectiveness in denoising DTI data is compared among image‐based and vector‐based PDE and fixed and adaptive low‐pass k‐space filtering. The results demonstrate that the edge‐preservation feature of the PDE approach can be highly advantageous in enhancing DTI measurements, particularly for vector‐based PDE filtering in applications relying on DTI directional information. These findings suggest a potential role for the postprocessing enhancement technique to improve the practical utility of DTI. Magn Reson Med 54:393–407, 2005.

Myocardial Fiber Orientation Mapping Using Reduced Encoding Diffusion Tensor Imaging

A precise knowledge of the myocardial fiber architecture is essential to accurately understand and interpret cardiac electrical and mechanical functions. Diffusion tensor imaging has been used to noninvasively and quantitatively characterize myocardial fiber orientations. However, because the approach necessitates diffusion to be measured in multiple encoding directions and frequently at multiple weighting levels, the required data set size may present a limitation on its acquisition time efficiency. Applying the principles of reduced encoding imaging (REI), four basic reconstruction schemes, keyhole using direct substitution, keyhole with baseline correction, symmetrically encoded REI with generalized-series reconstruction (RIGR), and asymmetrically encoded RIGR, are evaluated in terms of their accuracy in diffusion tensorfiber orientation mapping of excised myocardial samples. Results show that the performances of all REI schemes, at approximately 50% reduced encoding, are at least comparable with that of a control experiment consisting of proportionally reduced number of full k-space images. Moreover, although performances of the symmetrically and asymmetrically encoded RIGR schemes are similar, both methods provide significant improvements over the control experiment and the direct-substitution keyhole technique. These findings demonstrate the potential of the general REI methodology for diffusion tensor imaging and pave the way for modified schemes involving rapid imaging sequences or alternative k-space sampling strategies to achieve even better data acquisition time efficiency and performance.

Water distribution and permeability of zebrafish embryos, Brachydanio rerio

Teleost embryos have not been successfully cryopreserved. To formulate successful cryopreservation protocols, the distribution and cellular permeability to water must be understood. In this paper, the zebrafish (Brachydanio rerio) was used as a model for basic studies of the distribution to permeability to water. These embryos are a complex multi-compartmental system composed of two membrane-limited compartments, a large yolk (surrounded by the yolk syncytial layer) and differentiating blastoderm cells (each surrounded by a plasma membrane). Due to the complexity of this system, a variety of techniques, including magnetic resonance microscopy and electron spin resonance, was used to measure the water in these compartments. Cellular water was distributed unequally in each compartment. At the 6-somite stage, the percent water (V/V) was distributed as follows: total in embryo = 74%, total in yolk = 42%, and total in blastoderm = 82%. A one-compartment model was used to analyze kinetic, osmotic shrinkage data and determine a phenomenological water permeability parameter, Lp, assuming intracellular isosmotic compartments of either 40 or 300 mosm. This analysis revealed that the membrane permeability changed (P < 0.05) during development. During the 75% epiboly to 3-somite stage, the mean membrane permeability remained constant (Lp = 0.022 +/- 0.002 micron x min-1atm-1 [mean +/- S.E.M.] assuming isosmotic is 40 mosm or Lp = 0.049 +/- 0.008 micron x min-1atm-1 assuming isosmotic is 300 mosm). However, at the 6-somite stage, Lp increased twofold (Lp = 0.040 +/- 0.004 micron x min-1atm-1 assuming isosmotic is 40 mosm or Lp = 0.100 +/- 0.017 micron x min-1atm-1 assuming isosmotic is 300 mosm). Therefore, the low permeability of the zebrafish embryo coupled with its large size (and consequent low area to volume ratio) led to a very slow osmotic response that should be considered before formulating cryopreservation protocols.

Putamen coactivation during motor task execution

Models of corticostriatal motor circuitry have focused on the role of the circuit in the hemisphere of the motor cortex providing primary control (contralateral to the movement). We used functional magnetic resonance imaging and functional connectivity analyses to study circuit function in both the controlling and noncontrolling hemispheres. During the completion of a unilateral motor task with either hand, each putamen nucleus demonstrated strong coactivation with structures in both hemispheres. The putamen in the noncontrolling hemisphere (ipsilateral to the movement) coactivated more strongly with the controlling motor cortex than with the noncontrolling cortex. These findings suggest that the two corticostriatal circuits are functionally integrated. New circuit models based on functional connectivity may need to be developed.

Magnetic resonance imaging-based finite element stress analysis after linear repair of left ventricular aneurysm

OBJECTIVE Linear repair of left ventricular aneurysm has been performed with mixed clinical results. By using finite element analysis, this study evaluated the effect of this procedure on end-systolic stress. METHODS Nine sheep underwent myocardial infarction and aneurysm repair with a linear repair (13.4 +/- 2.3 weeks postmyocardial infarction). Satisfactory magnetic resonance imaging examinations were obtained in 6 sheep (6.6 +/- 0.5 weeks postrepair). Finite element models were constructed from in vivo magnetic resonance imaging-based cardiac geometry and postmortem measurement of myofiber helix angles using diffusion tensor magnetic resonance imaging. Material properties were iteratively determined by comparing the finite element model output with systolic tagged magnetic resonance imaging strain measurements. RESULTS At the mid-wall, fiber stress in the border zone decreased by 39% (sham = 32.5 +/- 2.5 kPa, repair = 19.7 +/- 3.6 kPa, P = .001) to the level of remote regions after repair. In the septum, however, border zone fiber stress remained high (sham = 31.3 +/- 5.4 kPa, repair = 23.8 +/- 5.8 kPa, P = .29). Cross-fiber stress at the mid-wall decreased by 41% (sham = 13.0 +/- 1.5 kPa, repair = 7.7 +/- 2.1 kPa, P = .01), but cross-fiber stress in the un-excluded septal infarct was 75% higher in the border zone than remote regions (remote = 5.9 +/- 1.9 kPa, border zone = 10.3 +/- 3.6 kPa, P < .01). However, end-diastolic fiber and cross-fiber stress were not reduced in the remote myocardium after plication. CONCLUSION With the exception of the retained septal infarct, end-systolic stress is reduced in all areas of the left ventricle after infarct plication. Consequently, we expect the primary positive effect of infarct plication to be in the infarct border zone. However, the amount of stress reduction necessary to halt or reverse nonischemic infarct extension in the infarct border zone and eccentric hypertrophy in the remote myocardium is unknown.