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R Aaron

Publications and source records attributed to R Aaron.

13 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

LSP1 modulates the locomotion of monocyte-differentiated U937 cells.

To examine the effect of lymphocyte specific protein 1 (LSP1) on phagocytic cell motility, stable transfection of LSP1-null U937 cell line with an episomal expression vector carrying the LSP1 complementary DNA created lines expressing varied LSP1 levels. Mock transfectants without LSP1 (U937(-)) and cell lines with LSP1 levels similar to those of monocytes (U937(+)) or 4-fold those of monocytes (U937(+)) express LSP1 as indicated and express other actin-binding proteins at normal levels before or after monocytic induction (MI) with dibutyryl cyclic adenosine monophosphate. The cell lines were compared for rate of growth and cell division and, after monocytic differentiation, were video-tracked to measure locomotion as distance moved in 2 hours and examined for morphologic changes. Rates of cell division and growth were similar for different U937 cell lines at all LSP1 levels. In contrast, mean rate of locomotion (micrometers moved in 2 hours) was slower in MI-U937( +) (7.78 + 1.11 microm, n = 3) and MI-U937(-) (23.89 + 2.78 microm, n = 3) than in MI-U937(+) cells (50.77 + 4.11 microm, n = 3). Compared with MI-U937(-), the locomotive histogram (n = 150 cells) of MI-U937(+) or MI-U937( +) cells shows all cells move respectively faster or slower as an entire cell population. In LSP1(+) U937 phagocytes, high LSP1 levels inhibit some (locomotion) but not all (cytokinesis) cell motile behaviors and cause the formation of surface projections. In contrast, normal LSP1 levels in U937 phagocytes enhance some (locomotion) but not all (cytokinesis) cellular motile behaviors and have no effect on cell morphology. Therefore, LSP1 level has a unique biphasic effect on cellular locomotion. The data suggest LSP1 is an important regulator of phagocyte locomotion.

Cell Differentiation↗
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↗
PubMed1998

A pilot randomized trial comparing symptomatic vs. hemoglobin-level-driven red blood cell transfusions following hip fracture.

BACKGROUND: The indications for transfusion have never been evaluated in an adequately sized clinical trial. A pilot study was conducted to plan larger clinical trials. STUDY DESIGN AND METHODS: Hip fracture patients undergoing surgical repair who had postoperative hemoglobin levels less than 10 g per dL were randomly assigned to receive 1) symptomatic transfusion: that is, transfusion for symptoms of anemia or for a hemoglobin level that dropped below 8 g per dL or 2) threshold transfusion: that is, patients receive 1 unit of packed RBCs at the time of random assignment and as much blood as necessary to keep the hemoglobin level above 10 g per dL. Outcomes were 60-day mortality, morbidity, functional status, and place of residence. RESULTS: Among 84 eligible patients enrolled, mean (+/- SD) prerandomization hemoglobin was 9.1 (+/- 0.6) g/ dL. The median number of units transfused in the threshold transfusion group was 2 (interquartile range, = 1-2), and that in the symptomatic transfusion group was 0 (6; interquartile range, = 0-2) (p < 0.001). Mean hemoglobin levels were approximately 1 g per dL higher in the threshold group than in the symptomatic group: for example, on Day 2, 10.3 (+/- 0.9) g per dL versus 9.3 (+/- 1.2) g per dL, respectively (p < 0.001). At 60 days, death or inability to walk across the room without assistance occurred in 16 (39.0%) of the symptomatic transfusion group and 19 (45.2%) of the threshold transfusion group. Death occurred by 60 days in 5 (11.9%) of the symptomatic transfusion group and 2 (4.8%) in the threshold transfusion group (relative risk = 2.5; 95% CI, 0.5-12.2). Other outcomes were similar for the two groups. CONCLUSIONS: Symptomatic transfusion may be an effective blood-sparing protocol associated with the transfusion of appreciably fewer units of RBCs and lower mean hemoglobin levels than are associated with the threshold transfusion policy. However, it is unknown whether these two clinical strategies have comparable mortality, morbidity, or functional status. A definitive trial is needed.

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↗
PubMed1995

Evaluation of boundary element methods for the EEG forward problem: effect of linear interpolation.

We implement the approach for solving the boundary integral equation for the electroencephalography (EEG) forward problem proposed by de Munck [1], in which the electric potential varies linearly across each plane triangle of the mesh. Previous solutions have assumed the potential is constant across an element. We calculate the electric potential and systematically investigate the effect of different mesh choices and dipole locations by using a three concentric sphere head model for which there is an analytic solution. Implementing the linear interpolation approximation results in errors that are approximately half those of the same mesh when the potential is assumed to be constant, and provides a reliable method for solving the problem.

Action Potentials↗
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↗