Biomedical subjects

K M Hasan

Publications and source records attributed to K M Hasan.

4 recordsLinked to original sources

PubMed2001

Comparison of gradient encoding schemes for diffusion-tensor MRI.

The accuracy of single diffusion tensor MRI (DT-MRI) measurements depends upon the encoding scheme used. In this study, the diffusion tensor accuracy of several strategies for DT-MRI encoding are compared. The encoding strategies are based upon heuristic, numerically optimized, and regular polyhedra schemes. The criteria for numerical optimization include the minimum tensor variance (MV), minimum force (MF), minimum potential energy (ME), and minimum condition number. The regular polyhedra scheme includes variations of the icosahedron. Analytical comparisons and Monte Carlo simulations show that the icosahedron scheme is optimum for six encoding directions. The MV, MF, and ME solutions for six directions are functionally equivalent to the icosahedron scheme. Two commonly used heuristic DT-MRI encoding schemes with six directions, which are based upon the geometric landmarks of a cube (vertices, edge centers, and face centers), are found to be suboptimal. For more than six encoding directions, many methods are able to generate a set of equivalent optimum encoding directions including the regular polyhedra, and the ME, MF and MV numerical optimization solutions. For seven directions, a previously described heuristic encoding scheme (tetrahedral plus x, y, z) was also found to be optimum. This study indicates that there is no significant advantage to using more than six encoding directions as long as an optimum encoding is used for six directions. Future DT-MRI studies are necessary to validate these observations. J. Magn. Reson. Imaging 2001;13:769-780.

Artifacts↗

PubMed2001

Analysis of partial volume effects in diffusion-tensor MRI.

The diffusion tensor is currently the accepted model of diffusion in biological tissues. The measured diffusion behavior may be more complex when two or more distinct tissues with different diffusion tensors occupy the same voxel. In this study, a partial volume model of MRI signal behavior for two diffusion-tensor compartments is presented. Simulations using this model demonstrate that the conventional single diffusion tensor model could lead to highly variable and inaccurate measurements of diffusion behavior. The differences between the single and two-tensor models depend on the orientations, fractions, and exchange between the two diffusion tensor compartments, as well as the diffusion-tensor encoding technique and diffusion-weighting that is used in the measurements. The current single compartment model's inaccuracies could cause diffusion-based characterization of cerebral ischemia and white matter connectivity to be incorrect. A diffusion-tensor MRI imaging experiment on a normal human brain revealed significant partial volume effects between oblique white matter regions when using very large voxels and large diffusion-weighting (b approximately 2.69 x 10(3) sec/mm(2)). However, the apparent partial volume effects in white matter decreased significantly when smaller voxel dimensions were used. For diffusion tensor studies obtained using typical diffusion-weighting values (b approximately 1 x 10(3) sec/mm(2)) partial volume effects are much more difficult to detect and resolve. More accurate measurements of multiple diffusion compartments may lead to improved confidence in diffusion measurements for clinical applications.

Anisotropy↗

PubMed2001

Analytical computation of the eigenvalues and eigenvectors in DT-MRI.

In this paper a noniterative algorithm to be used for the analytical determination of the sorted eigenvalues and corresponding orthonormalized eigenvectors obtained by diffusion tensor magnetic resonance imaging (DT-MRI) is described. The algorithm uses the three invariants of the raw water spin self-diffusion tensor represented by a 3 x 3 positive definite matrix and certain math functions that do not require iteration. The implementation requires a positive definite mask to preserve the physical meaning of the eigenvalues. This algorithm can increase the speed of eigenvalue/eigenvector calculations by a factor of 5-40 over standard iterative Jacobi or singular-value decomposition techniques. This approach may accelerate the computation of eigenvalues, eigenvalue-dependent metrics, and eigenvectors especially when having high-resolution measurements with large numbers of slices and large fields of view.

Algorithms↗

PubMed2001

Rarefaction of skin capillaries in patients with anginal chest pain and normal coronary arteriograms.

AIMS: Patients with arterial hypertension often have a reduction in capillary density (rarefaction) and a reduction in coronary flow reserve because of functional and structural alterations of the coronary microcirculation. Patients with chest pain and normal coronary arteriograms may have coronary microvascular dysfunction, but it is not known whether capillary rarefaction plays a role in the pathogenesis of this syndrome. The aim of this study was to compare capillary density in hypertensive and normotensive subjects with anginal chest pain and normal coronary arteriograms vs asymptomatic hypertensives and healthy controls. METHODS AND RESULTS: We studied 49 patients with typical anginal chest pain, positive exercise testing and normal coronary arteriograms; 22 were hypertensive and 27 were normotensive. We used intra-vital video-microscopy to examine the skin of the dorsum of the middle finger of the non-dominant hand before and after maximization of perfused capillaries with venous congestion. Mean capillary density was significantly lower in patients with chest pain and normal coronary arteriograms independent of their blood pressure level, compared to normotensive healthy controls. Differences were found both at baseline [51+/-2 (hypertensive) and 52+/-2 (normotensive) vs 65+/-2 (controls) per 0.56 mm(2) respectively], (P<0.0001) and after maximization [57+/-3 (hypertensive) and 59+/-2 (normotensive) versus 75+/-3 (controls) respectively] (P<0.0001). CONCLUSIONS: Skin capillary density is significantly lower in patients with chest pain and normal coronary arteriograms compared to normotensive controls. The pathophysiological importance of capillary rarefaction in patients with chest pain and normal coronary arteriograms remains unknown. Further studies are needed to determine whether the abnormality is associated with myocardial flow disturbances such that the findings can be extended to the heart.

Capillaries↗