Biomedical subjects

C R Ethier

Publications and source records attributed to C R Ethier.

44 records · Page 3Linked to original sources

PubMed1986

The hydrodynamic resistance of hyaluronic acid: estimates from sedimentation studies.

The problem of estimating the permeability of hyaluronic acid as a function of concentration has been examined. A previously known method of obtaining permeability from sedimentation studies has been employed, and the results have been compared to those obtained from convection studies, in which solvent is forced through the hyaluronic acid. The two sets of results were seen to be at variance, which could be explained by a flow-induced polarization of the hyaluronic acid in the convection studies. The polarization phenomenon was described in terms of a convection-associated compaction balancing the hyaluronic acid's intrinsic resistance to compression. Based on these results and other arguments, it was suggested that data from sedimentation studies provide a more accurate estimate of hyaluronic acid permeability than do convection experiments. The implications of this finding in a physiological context were briefly discussed.

Gels↗

PubMed1986

Calculations of flow resistance in the juxtacanalicular meshwork.

The structure of the juxtacanalicular meshwork (JCM) was analyzed morphometrically, and the resulting data were used to calculate the resistance to flow through this tissue. Two models of the JCM were presented and compared. In the first (Model A), aqueous humor was assumed to flow via open channels within a solid framework, while, in the second (Model B), these open spaces were assumed to be filled with extracellular matrix gel. An expression giving the resistance of such a gel as a function of gel concentration was presented and tested on corneal and scleral stroma. Morphometry of normal and glaucomatous human eyes showed that Model A underpredicted the resistance of the JCM by factors of 10-100, suggesting that a GAG or proteoglycan gel may control the flow resistance of this tissue. This was supported by Model B, which showed that measured bulk concentrations of GAGs were consistent with gel concentrations needed to account for the estimated resistance of the JCM in vivo. Some limitations and implications of Model B were discussed.

Aqueous Humor↗

PubMed1986

The flow of aqueous humor through micro-porous filters.

Flow resistance was measured as bovine and primate aqueous humor was passed through Nuclepore polycarbonate filters having flow dimensions similar to those found within the juxtacanalicular meshwork of the aqueous outflow network. The results indicate that aqueous humor has a greater flow resistance than isotonic saline; this greater resistance is attributable to proteins or glycoproteins in aqueous humor that obstruct the filters. If the same phenomenon is operative in the aqueous outflow network, it would help to explain discrepancies between calculated and measured aqueous outflow resistance.

Animals↗

PubMedDate not supplied

Accuracy of MR phase contrast velocity measurements for unsteady flow.

The accuracy of MR phase contrast (PC) velocity measurements for unsteady flow has been quantitatively assessed. Spatially resolved velocity fields were measured in a long straight tube using a gated PC technique, and the resulting MR PC velocity data were compared with velocities derived from the analytic Womersley solution to the Navier-Stokes equations governing fluid flow. The overall root-mean-square (rms) difference between the measured and analytic velocities was 1.6 cm s-1 for nominally sinusoidal flow waveforms with peak velocities ranging from 51.6 cm s-1 to 59.8 cm s-1. This rms difference corresponded to 7.5% of the mean fluid velocity, which is similar to the cited accuracy of approximately 5% for MR PC velocimetry for steady flows. Linear regression between the PC velocity measurements and the velocities obtained using the analytic expression was highly significant (r2 = 0.997) and yielded a slope of 0.998, close to the expected value of 1. We conclude that the gated MR PC velocity measurements in unsteady flow are accurate.

Blood Flow Velocity↗

PubMedDate not supplied

Combined analysis of spatial and velocity displacement artifacts in phase contrast measurements of complex flows.

MR phase contrast (PC) velocity imaging is a promising tool for quantifying blood flow velocity in vivo. PC velocity imaging is, however, susceptible to artifacts that result from the displacement of spins during the finite duration pulse sequences. Such displacement artifacts can lead to errors in velocity measurements, especially in the presence of oblique and accelerating flows, which are common throughout the cardiovascular system. By tracking particles (representing spins) through a computed velocity field, and assuming that spatial and velocity encodings occur at discrete times during the pulse sequence, we simulate the separate and combined effects of oblique and acceleration artifacts on PC velocity images. We demonstrate, both by simulation and MR measurement, the errors associated with such artifacts in PC velocity measurements in a representative flow geometry. Using example particle trajectories, we provide a fluid dynamic basis for characteristic phase-velocity image distortions that can arise when imaging complex, physiologically relevant flows.

Animals↗

PubMedDate not supplied

Accuracy of computational hemodynamics in complex arterial geometries reconstructed from magnetic resonance imaging.

PURPOSE: Combining computational blood flow modeling with three-dimensional medical imaging provides a new approach for studying links between hemodynamic factors and arterial disease. Although this provides patient-specific hemodynamic information, it is subject to several potential errors. This study quantifies some of these errors and identifies optimal reconstruction methodologies. METHODS: A carotid artery bifurcation phantom of known geometry was imaged using a commercial magnetic resonance (MR) imager. Three-dimensional models were reconstructed from the images using several reconstruction techniques, and steady and unsteady blood flow simulations were performed. The carotid bifurcation from a healthy, human volunteer was then imaged in vivo, and geometric models were reconstructed. RESULTS: Reconstructed models of the phantom showed good agreement with the gold standard geometry, with a mean error of approximately 15% between the computed wall shear stress fields. Reconstructed models of the in vivo carotid bifurcation were unacceptably noisy, unless lumenal profile smoothing and approximating surface splines were used. CONCLUSIONS: All reconstruction methods gave acceptable results for the phantom model, but in vivo models appear to require smoothing. If proper attention is paid to smoothing and geometric fidelity issues, models reconstructed from MR images appear to be suitable for use in computational studies of in vivo hemodynamics.

Carotid Arteries↗

PubMedDate not supplied

Computational blood flow modeling based on in vivo measurements.

Study of the relationship between hemodynamics and atherogenesis requires accurate three-dimensional descriptions of in vivo arterial geometries. Common methods for obtaining such geometries include in vivo medical imaging and postmortem preparations (vessel casts, pressure-fixed vessels). We sought to determine the relative accuracy of these methods. The aorto-iliac (A/I) region of six rabbits was imaged in vivo using contrast-enhanced magnetic resonance imaging (MRI). After sacrifice, the geometry of the A/I region was preserved via vascular casts in four animals, and ex situ pressure fixation (while preserving dimensions) in the remaining two animals. The MR images and postmortem preparations were used to build computer representations of the A/I bifurcations, which were then used as input for computational blood flow analyses. Substantial differences were seen between MRI-based models and postmortem preparations. Bifurcation angles were consistently larger in postmortem specimens, and vessel dimensions were consistently smaller in pressure-fixed specimens. In vivo MRI-based models underpredicted aortic dimensions immediately proximal to the bifurcation, causing appreciable variation in the aorto-iliac parent/child area ratio. This had an important effect on wall shear stress and separation patterns on the "hips" of the bifurcation, with mean wall shear stress differences ranging from 15% to 35%, depending on the model. The above results, as well as consideration of known and probable sources of error, suggests that in vivo MRI best replicates overall vessel geometry (vessel paths and bifurcation angle). However, vascular casting seems to better capture detailed vessel cross-sectional dimensions and shape. It is important to accurately characterize the local aorto-iliac area ratio when studying in vivo bifurcation hemodynamics.

Animals↗

PubMedDate not supplied

Simulation of non-Newtonian blood flow in an end-to-side anastomosis.

In order to quantify the importance of non-Newtonian blood rheology on anastomotic flow patterns, the characteristics of Newtonian and non-Newtonian blood flows were compared in a 2-D, 45 degree end-to-side anastomosis model under both steady and unsteady flow conditions. All flows were assumed to be two-dimensional, and were simulated numerically using parameters consistent with blood flow in the femoral artery. A novel, purely viscous constitutive relation, based on a generalized form of the power law relation, was developed to model the non-Newtonian rheology of blood. The resulting wall shear stress patterns indicate that for the parameter values under consideration, non-Newtonian blood rheology has a significant effect on steady flow wall shear stresses, but no significant effect on unsteady flow wall shear stresses. Based on these and other simulations, a parameter is formulated that gives an indication of the importance of non-Newtonian blood rheology under a given set of flow conditions. In addition, an argument is presented for allowing the conclusions from this two-dimensional study to be extended to three-dimensional blood flow.

Anastomosis, Surgical↗