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C A Shiffman

Publications and source records attributed to C A Shiffman.

8 recordsLinked to original sources
PubMed2001

Spatial dependence of the phase in localized bioelectrical impedance analysis.

The variety of phase functions, theta(z) = arctan X(z)/R(z), observed earlier on the thighs of healthy and seriously ill subjects via localized bioelectrical impedance analysis, can be represented by a model which combines realistic thigh shapes with homogeneous, axially symmetric conductivity tensors. While quantitative results depend sensitively on the way current is injected, it appears to be generally true that d theta/d(z) < 0 whenever phi(r) > phi(z) (and vice versa), where phi(r) and phi(z) are the phases of the radial and longitudinal conductivity components.

Adult↗
PubMed2000

Using localized impedance measurements to study muscle changes in injury and disease.

We measured localized impedance on the surface of the thigh to determine the properties of underlying muscle; these include rho 1, the resistivity for current flowing parallel to the fibers, and theta avg, the average phase along the thigh (normalized to a standard length). The results for a modest sampling of nominally healthy subjects show that the theta avg values are substantially higher than the whole body phases encountered in standard bioimpedance analysis. When the sample is augmented to include subjects undergoing hemodialysis and/or recovering from serious illness or leg injury, the behavior of the position dependence of the phase theta (z) and the rho 1 vs. theta avg average distribution both strongly indicate a correlation between very low phase angles and injury or disease. Furthermore, measurements on a subject in a weight training program after injury provide evidence of a monotone increase in rho 1 with increasing strength. Measurements on dialysis patients during treatment show a nonlinear response of thigh muscle to the degree of fluid removal and wide disparities between individuals.

Ankle Injuries↗
PubMed1999

Resistivity and phase in localized BIA.

We describe a system for highly reproducible non-invasive rf impedance measurements as a function of position along body segments such as the thigh. Results are reported for mainly healthy male and female subjects ranging in age from 19 to 65 and in body-mass index from 15 to 40. A principal conclusion is that the phase of the impedance falls monotonically with increasing distance from the knee, with average values substantially above what is found using standard, whole-body bioelectrical impedance analysis (BIA). To compensate for thigh shape, the data are further analysed using an anatomical model based on reasonable approximations for the distributions of muscle, fat and bone, yielding values of the effective resistivity for current flow parallel to the muscle fibres. The phase and resistivity results are discussed with reference to the whole-body BIA study of maintenance haemodialysis patients by Chertow et al, and in regard to possible physiological correlations observed in this work.

Adult↗
PubMed1997

Anisotropy of human muscle via non-invasive impedance measurements.

Combining non-invasive 50 kHz impedance measurements with a mathematical model for the underlying structure, we obtain in vivo values for the transverse and longitudinal conductivities of the muscles of the human thigh and for the (isotropic) conductivity of the covering skin-fat layer. Results for a healthy male subject are in acceptably good agreement with those obtained elsewhere on surgically exposed or freshly excised animal tissue and with 'global' measurements on humans. Also, measurements using rotatable probes reveal orientations of underlying muscle fibres via minima in resistance versus angle curves. The results suggest potentially useful methods for studying muscle properties in clinical and physiological research.

Adipose Tissue↗
PubMed1997

Maximum entropy method for magnetoencephalography.

Simulations show that the maximum entropy method is a promising technique for image reconstruction in magnetoencephalography. An algorithm based on the work of Skilling and Bryan [1] and an appropriately modified expression for the "entropy" is shown to provide high-quality reconstructions of both isolated and dense distributions of neural current "dipoles", neglecting return currents. In particular the results are substantially superior to those obtained with the well-known minimum norm procedure.

Artifacts↗
PubMedDate not supplied

Line and slice selection for moving spins.

The Bloch equations are solved numerically for spins moving at constant velocity for the case of 90 degrees slice selection, and for subsequent 180 degrees spin-echo line selection. The calculations use the comoving, rotating reference frame which is shown to be valid for general accelerated motions. Magnetization profiles and phase shifts are presented for slice selection for both plug and laminar flow. Line selection is studied for the case of plug flow, where a "local vector sum" is introduced to demonstrate effective localization of activated spins. In addition to local magnetization densities, the spatially integrated signals seen by a nonselective antenna are calculated. The relation between net phase shift and flow rate is examined and deviations from the predictions of simple analytic models are discussed.

Magnetic Resonance Imaging↗