Biomedical subjects

S N Urchuk

Publications and source records attributed to S N Urchuk.

7 recordsLinked to original sources

PubMed1997

In vivo validation of MR pulse pressure measurement in an aortic flow model: preliminary results.

MR imaging experiments were conducted to investigate the feasibility of estimating vascular pulse pressure waveforms from measurements of blood flow rates and vessel cross-sectional area. Blood flow waveforms were measured in the aorta's of three 25-30-kg pigs at multiple imaging sections using phase-contrast velocity imaging. Estimates of pulse pressure were derived from these data by evaluating a model characterizing the relationship between pressure, flow, and the cross-sectional area of a vessel segment. Comparisons between the MR-derived estimates of pressure and those obtained from a micromanometer pressure catheter indicate that accurate measurements (mean error +/- SD = 8.2 +/- 3.4, n = 6) can be obtained using conventional velocity imaging techniques. Optimization of the method will require the application of rapid imaging techniques and the development of strategies for obtaining a more localized measurement. With these improvements, our results suggest that MR-based measurement of pulse pressure and related elastic parameters is feasible.

Animals↗

PubMed1995

An MR compatible flow simulator for intravascular pressure simulation.

An MR compatible flow simulator is described which generates physiologically realistic pressure and flow waveforms. The simulator is based on a servomotor-driven gear pump which produces pulsatile flow by modulation of the servomotor rotation rate. Operation of the simulator is under the control of a personal computer, which executes an iterative feedback loop to minimize errors between measured and desired pressure waveforms. The simulator is totally automatic, requiring only a few minutes of iteration to generate the desired pressure waveform. Accurate sinusoidal waveforms with frequencies up to 10 Hz have been generated using the simulator, with high-frequency contamination of the measured waveform at least 80 dB below the fundamental frequency. Aortic waveforms have been produced with realistic flow rates and pressure variations. The pump assembly is mechanically straightforward and can operate at an 8-m distance from the flow phantom to allow the device to be isolated from the MR magnet room.

Biophysical Phenomena↗

PubMedDate not supplied

Mechanisms of flow-induced signal loss in MR angiography.

Mechanisms of signal loss in magnetic resonance angiography were studied with a stenotic flow phantom. The results indicate that while signal loss induced by mean fluid motions is localized about the stenosis, the fluctuating component of fluid motion induces signal loss over a much larger region, primarily distal to the stenosis. For both motion components, use of gradient moment nulling (GMN) above first order was found to be an ineffective means of reducing signal loss. In contrast, shortened gradient durations were found to reduce signal loss substantially. However, though a zeroth-order gradient is generally of the shortest duration, use of a slightly longer, first-order gradient was found to be the most robust means of reducing signal loss.

Blood Flow Velocity↗

PubMedDate not supplied

MR measurements of pulsatile pressure gradients.

A magnetic resonance (MR) imaging method for evaluating pulsatile pressure gradients in laminar blood flow is presented. The technique is based on an evaluation of fluid shear and inertial forces from cardiac-gated phase-contrast velocity measurements. The technique was experimentally validated by comparing MR and manometer pressure gradient measurements performed in a pulsatile flow phantom. Analyses of random noise propagation and sampling error were performed to determine the precision and accuracy of the method. The results indicate that a precision of 0.01-0.03 mmHg/cm and an accuracy of better than 8% can be achieved by using standard clinical pulse sequences in tubes exceeding 6 mm in diameter. The authors conclude that MR measurement of pressure gradients is feasible and that additional hemodynamic information may be derived from conventional phase-contrast imaging studies.

Algorithms↗

PubMedDate not supplied

MR measurement of time-dependent blood pressure variations.

An MR imaging method for measuring intravascular pressure variations is introduced. The technique is based on estimates of vascular compliance and vessel distension, which are obtained from a correlation of spatial and temporal velocity derivatives and measurements of the velocity gradient in the direction of flow, respectively. The accuracy of the technique was determined in vitro through a comparison of MR and transducer pressure measurements obtained in distensible vessel phantoms undergoing pulsatile flow. Results indicated that a root-mean-square error of 4-12% can be expected in phantoms covering a physiological range of compliance. In vivo feasibility was demonstrated by thoracic aorta pressure measurements, which produced pressure waveforms with an expected shape and magnitude.

Adult↗

PubMedDate not supplied

A velocity correlation method for measuring vascular compliance using MR imaging.

A method for estimating vascular compliance using MR velocity imaging is presented. The technique combines an analysis of pulse propagation, based on spatially averaged equations of continuity and momentum, together with phase-contrast velocity measurements to estimate the compliance from a correlation of second-order spatial and temporal velocity derivatives. The technique can be applied in the presence of reflected flow waves and uses velocity data acquired throughout the entire cardiac cycle. The accuracy of the technique was assessed in distensible vessel phantoms spanning a physiological range of compliance through a comparison with compliance estimates obtained using high-resolution MR imaging and pressure transducers. The mean error of all measurements was found to be 0.04 +/- 0.02% per mm Hg, with the relative errors ranging from 1.2% to 46%. Error was found to decrease as the temporal sampling rate and/or image plane separation were increased. This suggests that an accurate hemodynamic evaluation of a vessel's elastic properties is feasible with MR velocity imaging techniques.

Blood Flow Velocity↗

PubMedDate not supplied

Visualizing tissue compliance with MR imaging.

We propose a method for visualizing the mechanical properties of tissue based on the use of periodic mechanical compression in conjunction with phase-contrast MR imaging. A specialized mechanical transducer was used to provide programmable compression pulses to the surface of compliant phantoms. These compression pulses were synchronized to a spin-echo sequence with motion-sensitizing gradients to generate phase information reflecting spin displacement throughout the phantom. This sequence was tested with two agarose gel phantoms. The first was a cylinder containing three parallel layers of varying compliance and the second was composed of a semirigid sphere suspended in a uniform layer of decreased elastic modulus. Images showed complex patterns of motion throughout the phantom, which correlated with expected motion behavior of the phantom structures. This indicates that the biomechanical properties of tissues may be elucidated through the use of motion-sensitized MR imaging and suggests that a form of image contrast relating to tissue elasticity may be feasible.

Compliance↗