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Biomedical subjects

H Mathur

Publications and source records attributed to H Mathur.

12 recordsLinked to original sources
PubMed2002

Toward shielding improvements in MRI gradients and other systems.

A new method is described for reducing the shielding-error function in the 'supershielding' approach to designing MRI systems. The method is thus shown to lead to significantly better shielding and better control of eddy current effects associated with gradient coils. To illustrate this technique, a set of results for a z-gradient coil is presented. A generalization to non-standard geometries can be made in a straightforward manner with the new method. The usefulness of the relationship of all fringe-field quantities to the shielding-error function is emphasized. The formal limit of perfect shielding in a 'least-squares' sense is shown for a simple strip-shield model along with a numerical eigenvalue study for comparison with the theoretical limit.

Algorithms↗
PubMed2001

Dynamics and critical behavior of the q model.

The q model, a random walk model rich in behavior and applications, is investigated. We introduce and motivate the q model via its application proposed by Coppersmith et al. to the flow of stress through granular matter at rest. For a special value of its parameters the q model has a critical point that we analyze. To characterize the critical point we imagine that a uniform load has been applied to the top of the granular medium and we study the evolution with depth of fluctuations in the distribution of load. Close to the critical point explicit calculation reveals that the evolution of load exhibits scaling behavior analogous to thermodynamic critical phenomena. The critical behavior is remarkably tractable: the harvest of analytic results includes scaling functions that describe the evolution of the variance of the load distribution close to the critical point and of the entire load distribution right at the critical point, values of the associated critical exponents, and determination of the upper critical dimension. These results are of intrinsic interest as a tractable example of a random critical point. Of the many applications of the q model, the critical behavior is particularly relevant to network models of river basins, as we briefly discuss. Finally we discuss circumstances under which quantum network models that describe the surface electronic states of a quantum Hall multilayer can be mapped onto the classical q model. For mesoscopic multilayers of finite circumference the mapping fails; instead a mapping to a ferromagnetic supersymmetric spin chain has proved fruitful. We discuss aspects of the superspin mapping and give an elementary derivation of it making use of operator rather than functional methods.

Journal Article↗
PubMed2000

Dephasing times in a nondegenerate two-dimensional electron Gas

Studies of weak localization by scattering from vapor atoms for electrons on a liquid helium surface are reported. There are three contributions to the dephasing time. Dephasing by the motion of vapor atoms perpendicular to the surface is studied by varying the holding field to change the characteristic width of the electron layer at the surface. A change in vapor density alters the quasielastic scattering length and the contribution to dephasing due to the motion of atoms both perpendicular and parallel to the surface. Dephasing due to the electron-electron interaction is dependent on the electron density.

Journal Article↗
PubMed1980

Thyroid hormone relationships in the fetal and newborn lamb.

Plasma thyroxine (T4) and reverse triiodothyronine (rT3) concentrations have been measured in chronically catheterised sheep fetuses during the last third of gestation. In some animals T4 and rT3 concentrations fell before delivery but there was no change in the t4/rT3 ratio. In several serially sampled fetuses a small rise in plasma triiodothyronine concentration has been demonstrated in the days preceding delivery. Infusion of cortisol to fetuses commencing at 130 days of gestation (term 147 days) with the simultaneous administration of progesterone to delay labour resulted in changes in T4 and rT3 concentrations which were similar to those normally seen at term. The nature of the changes in the peripheral metabolism of T4 around the time of birth are discussed.

Animals↗
PubMed1979

Correction for the presence of cross-reactants in saturation assays: application to thyroxine cross-reactivity in 3,3',5'-(reverse)-triiodothyronine radioimmunoassay.

Cross-reaction of anti-3,3',5'-triiodothyronine (rT3) antisera with thyroxine has proved problematical in the development of radioimmunoassays for rT3. Results of experimental work with two antisera with differing specificities are presented which illustrate certain aspects of cross-reacting assay systems. The mathematical theory of a single binding-site, two ligand assay is discussed and extended by use of a multiple binding-site computer model to a two binding-site, two ligand system. It is suggested that for the practical evaluation of the nature and extent of cross-reaction, a family of response curves for the hormone should be drawn, each curve representing the addition of a fixed mass of the cross-reactant to a set of standard incubation mixtures. Such curves will reveal whether the antiserum is (a) specific, implying that assay results require no correction, (b) behaves as a single binding site system, in which case measurement of the relative potency of the two ligands at the point on the response curve generated by the serum sample will enable an algebraic correction to be made, given that the T4 concentration is known or (c) behaves as a multiple binding-site system where correction necessitates the use of a nomogram.

Binding Sites, Antibody↗