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At least 217 records · Page 12Linked to original sources
PubMedDate not supplied

High-speed black blood imaging of vessel stenosis in the presence of pulsatile flow.

Stenosis phantoms were created to study the ability of "black blood" methods to image a vessel stenosis in the presence of pulsatile flow. Black blood images were acquired with a modified TurboFLASH (fast low-angle shot) method that eliminates flow signal by applying a set of prepulses before segmented data acquisition. With this high-speed approach, imaging can be completed within 16 seconds. This technique was compared with conventional spin-echo black blood, gradient-echo black blood, and gradient-echo bright blood methods. Loss of flow signal, which extended beyond the site of the stenosis, was seen on the gradient-echo bright blood images. The pattern of signal loss varied with the type of stenosis. Flow voids were achieved with spin-echo black blood imaging; however, substantial ghosting artifacts were seen. With gradient-echo black blood imaging, it was difficult to eliminate all flow signal, particularly for in-plane flow. The modified TurboFLASH method produced high-quality black blood images in a fraction of the time needed for spin-echo imaging. It showed no ghosting artifacts even in the presence of pulsatile flow.

Blood Flow Velocity↗
PubMedDate not supplied

Comparison of velocity-encoded MR imaging and fluid dynamic modeling of steady and disturbed flow.

The contrast of flow-encoded magnetic resonance (MR) images obtained in vivo and the accuracy of velocity measurements are complicated by the presence of complex flow states. The effects of complex flow states on MR flow-encoded images were studied and quantitative flow information was obtained with an MR phase-subtraction technique. Regions of complex flow, including flow stagnation and separation and laminar flow, could be clearly identified on the phase images. The MR imaging velocity measurements were validated by comparison with numerical simulation results for three-dimensional velocity distributions. The velocity MR images and the profiles obtained from the simulation generally agreed well for flow rates of 660 and 1,680 mL/min. This agreement lends support to both the fluid dynamic model and the physical basis of the phase imaging technique and establishes the validity of flow-encoded phase imaging as an in vivo flow quantitation method, especially under low Reynolds number flow conditions.

Blood Flow Velocity↗
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

Fast, interactive algorithm for segmentation of a series of related images: application to volumetric analysis of MR images of the heart.

Magnetic resonance (MR) cine images of the beating heart have excellent spatial and temporal resolution. Extracting the boundaries of the heart from MR images for volumetric measurements is of considerable interest; however, since the number of images involved is large, tracing the boundaries by hand is tedious and prohibitively time consuming. The authors have developed an interactive method of boundary detection that uses the correlation between the cardiac boundaries on temporally or spatially adjacent images to increase the speed of the process and reproducibility of the measurement. A simulated cine MR study of a phantom (total of 155 images) and cardiac cine studies of two patients (192 images each) were analyzed by two independent observers. Analysis of the phantom data was completed in 5.6 minutes (2.16 seconds per image) by observer 1 and 6.3 minutes (2.4 seconds per image) by observer 2. The percent measurement errors for 31 phantom volumes (30-120 mL) were 0.96% and 0.83% for observers 1 and 2, respectively. The observers analyzed the patient studies in 14-23 minutes (4.4-7.2 seconds per image), with interobserver variabilities of 5.8% and 3.7% for the two patients, respectively. The authors conclude that their flexible, semiautomatic, interactive algorithm allows rapid and reproducible detection of structural boundaries.

Algorithms↗
PubMedDate not supplied

Computer-controlled flow simulator for MR flow studies.

A novel computer-controlled flow simulator for use in magnetic resonance (MR) flow experiments was evaluated. The accuracy in constant-flow mode was better than 1%. The accuracy in pulsatile-flow mode was found to be dependent on the interconnecting tubing. The short-term and long-term reproducibilities of pulsatile waveforms were less than or equal to 0.4 mL/sec (1 standard deviation). Increased response times due to the lengths of tubing required in MR flow experiments were surmounted by using a modified tubing configuration and precompensated waveforms. Piston reversal was found not to cause major difficulties in MR flow experiments.

Blood Circulation↗
PubMedDate not supplied

Accuracy of phase-contrast flow measurements in the presence of partial-volume effects.

The accuracy of volume flow rate measurements obtained with phase-contrast methods was assessed by means of computer simulation and in vitro experiments. Factors studied include (a) the partial-volume effect due to voxel dimensions relative to vessel dimensions and orientation and (b) intravoxel phase dispersion. It is shown that limited resolution (partial-volume effect) is the major obstacle to accurate flow measurement for both laminar and plug flow. The results show that at least 16 voxels must cover the cross section of the vessel lumen to obtain a measurement accuracy to within 10%. Measurement accuracy also greatly depends on the relative signal intensity of stationary tissue and is better for laminar flow than plug flow.

Biophysical Phenomena↗
PubMedDate not supplied

Analysis of MR phase-contrast measurements of pulsatile velocity waveforms.

Errors in the measurement of the mean velocity of pulsatile velocity waveforms with ungated phase-contrast techniques were studied theoretically and experimentally. Waveforms consisting of a constant and two sinusoidal components were analyzed. Variations in magnitude and phase of the vascular magnetic resonance (MR) signal resulted in errors, the severity of which increased when either factor increased. Magnitude variations always resulted in overestimation. The general shape of the waveform greatly influenced the error, with certain waveforms producing greater inherent error than others. Experimental measurements were performed, validating the predicted sensitivity of these errors to changes in imaging parameters, including TR and flow-encoding sensitivity. Errors generally became more severe with increased flow-encoding sensitivity. The theoretical and experimental results suggest that accurate mean velocity measurements in many vessels of the body--with acquisition times of less than 15 seconds--should be attainable with ungated imaging techniques and with careful selection of relevant imaging parameters.

Aorta, Abdominal↗
PubMedDate not supplied

Noise reduction in three-dimensional phase-contrast MR velocity measurements.

The authors have developed a method to reduce noise in three-dimensional (3D) phase-contrast magnetic resonance (MR) velocity measurements by exploiting the property that blood is incompressible and, therefore, the velocity field describing its flow must be divergence-free. The divergence-free condition is incorporated by a projection operation in Hilbert space. The velocity field obtained with 3D phase-contrast MR imaging is projected onto the space of divergence-free velocity fields. The reduction of noise is achieved because the projection operation eliminates the noise component that is not divergence-free. Signal-to-noise ratio (S/N) gains on the order of 15%-25% were observed. The immediate effect of this noise reduction manifests itself in higher-quality phase-contrast MR angiograms. Alternatively, the S/N gain can be traded for a reduction in imaging time and/or improved spatial resolution.

Algorithms↗
PubMedDate not supplied

Pulsatile flow artifacts in two-dimensional time-of-flight MR angiography: initial studies in elastic models of human carotid arteries.

Initial experimental and numerical analysis of artifacts due to pulsatile flow in two-dimensional time-of-flight (2D-TOF) magnetic resonance (MR) angiography are presented. The experimental studies used elastic models of the carotid artery bifurcation cast from fresh cadavers and accurately reproducing the twisting and tapering of the human blood vessels, allowing direct comparison of images with and without flow. Prominent image artifacts, including periodic ghosts and signal loss, were produced by pulsatile flow even though flow-compensated gradient waveforms were used. The dependence of artifacts due to partial saturation on pulse sequence parameters (TR and flip angle) was investigated theoretically for a simple pulsatile velocity profile and compared with experimental results from a model of a normal carotid artery. Signal reduction was observed proximal and distal to the stenosis in a model with a 70% internal carotid artery (ICA) stenosis and a model with 90% stenoses in both the ICA and the external carotid artery. Although this study deals exclusively with 2D-TOF imaging, the methods can also be applied to evaluate other MR angiography techniques.

Artifacts↗
PubMedDate not supplied

Time-resolved MR imaging by automatic data segmentation.

A method for time-resolved imaging that provides a flexible trade-off between imaging time and temporal resolution is presented. It is based on a view order selection technique that automatically segments the acquired raw data into appropriate temporal frames. When used with cardiac monitoring and phase-contrast imaging, data similar to that obtained with a conventional gated phase-contrast sequence are acquired rapidly. For many applications, the temporal resolution can be reduced enough to permit imaging within a breath-hold interval, while still allowing accurate time-averaged flow quantitation. This is a general technique that can be implemented within a variety of pulse sequences and can resolve other motion cycles, including the respiratory cycle.

Algorithms↗
PubMedDate not supplied

In vivo quantification of the unidirectional influx constant for Gd-DTPA diffusion across the myocardial capillaries with MR imaging.

The authors present an in vivo method for measuring the unidirectional influx constant (Ki) for gadolinium diethylenetriaminepentaacetic acid (DTPA) diffusion across the capillary membrane in the human myocardium with magnetic resonance imaging. Ki is related to the extraction fraction (E) and the perfusion (F) by the equation Ki = E.F.Ki was obtained by using the longitudinal relaxation rate (R1) as a measure of the myocardial concentration of Gd-DTPA in the mathematical model for transcapillary transport across capillary membranes. Myocardial enhancement after Gd-DTPA injection was followed by using inversion-recovery Turbo-FLASH (fast low-angle shot) images obtained in real time. The results were comparable to those obtained from studies with positron emission tomography in humans and invasive studies in animals. A method for obtaining the input function noninvasively is also presented. Comparison with direct arterial blood sampling showed that the noninvasive input function may be even more accurate with regard to timing and curve shape than the invasive input function. The procedure may therefore prove useful in clinical studies.

Adult↗
PubMedDate not supplied

Effects of intravoxel velocity distributions on the accuracy of the phase-mapping method in phase-contrast MR angiography.

The phase-mapping method of phase-contrast magnetic resonance angiography is shown to be based on an implicit assumption that the intravoxel velocity distribution is symmetric about its mean velocity. The effect of asymmetric distributions on the accuracy of quantitative average velocity measurements is determined analytically and verified experimentally. An explicit formulation is developed for the estimated average velocity in a voxel as a function of the true average velocity and the asymmetry of the distribution about the true average velocity. Worst-case distributions are determined for unidirectional and bidirectional flow, and the special case of laminar flow is also investigated. Computer simulations and phantom imaging experiments demonstrate the accuracy of the analysis. For voxels with unidirectional flow, the phase-mapping method produces accurate estimates of average velocity, while results for bidirectional flow indicate possible large errors unless the aliasing velocity is increased, which decreases the signal-to-noise ratio in the resultant velocity map image.

Algorithms↗
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

Effects of physiologic waveform variability in triggered MR imaging: theoretical analysis.

One of the assumptions inherent in most forms of triggered magnetic resonance (MR) imaging is that the pulsatile waveform (be it cardiac, respiratory, or some other) is purely periodic. In reality, the periodicity condition is rarely met. Physiologic waveform variability may lead to image artifacts and errors in velocity or volume flow rate estimates. The authors analyze the effects of physiologic waveform variability in triggered MR imaging. They propose that this variability be treated as a modulation of the underlying motion waveform. This report concentrates on amplitude modulation of the velocity waveform, which results in amplitude and phase modulation of the transverse magnetization. Established Fourier and modulation theory and the recently described principles of (k,t)-space were used to derive the appearance of physiologic waveform variability artifacts in triggered MR images and to predict errors in time-averaged and instantaneous velocity estimates that may result from such motion effects, including effects such as ghost overlap. Simulations and experimental results are provided to confirm the theory.

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

Signal-to-noise analysis of cerebral blood volume maps from dynamic NMR imaging studies.

The use of cerebral blood volume (CBV) maps generated from dynamic MRI studies tracking the bolus passage of paramagnetic contrast agents strongly depends on the signal-to-noise ratio (SNR) of the maps. The authors present a semianalytic model for the noise in CBV maps and introduce analytic and Monte Carlo techniques for determining the effect of experimental parameters and processing strategies upon CBV-SNR. CBV-SNR increases as more points are used to estimate the baseline signal level. For typical injections, maps made with 10 baseline points have 34% more noise than those made with 50 baseline points. For a given peak percentage signal drop, an optimum TE can be chosen that, in general, is less than the baseline T2. However, because CBV-SNR is relatively insensitive to TE around this optimum value, choosing TE approximately equal to T2 does not sacrifice much SNR for typical doses of contrast agent. The TR that maximizes spin-echo CBV-SNR satisfies TR/T1 approximately equal to 1.26, whereas as short a TR as possible should be used to maximize gradient-echo CBV-SNR. In general, CBV-SNR is maximized for a given dose of contrast agent by selecting as short an input bolus duration as possible. For image SNR exceeding 20-30, the gamma-fitting procedure adds little extra noise compared with simple numeric integration. However, for noisier input images, can be the case for high resolution echo-planar images, the covarying parameters of the gamma-variate fit broaden the distribution of the CBV estimate and thereby decrease CBV-SNR. The authors compared the analytic noise predicted by their model with that of actual patient data and found that the analytic model accounts for roughly 70% of the measured variability of CBV within white matter regions of interest.

Blood Volume↗
PubMed2002

Photothermal properties of shape memory polymer micro-actuators for treating stroke.

BACKGROUND AND OBJECTIVES: In this paper the photothermal engineering issues of novel shape memory polymer (SMP) microactuators for treating stroke are presented. The engineering issues for using lasers to heat and subsequently actuate these SMP devices are presented in order to provide design criteria and guidelines for intravascular, laser activated SMP devices. MATERIALS AND METHODS: A total of three devices will be presented: two interventional ischemic stroke devices (coil and umbrella) and one device for releasing embolic coils (microgripper). The optical properties of SMP, methods for coupling laser light into SMP, heating distributions in the SMP devices, and the impact of operating the thermally activated material in a blood vessel are presented. RESULTS: Actuating the devices requires device temperatures in the range of 65-85 degrees C. Attaining these temperatures under flow conditions requires critical engineering of the SMP optical properties, optical coupling into the SMP, and device geometries. CONCLUSION: Laser-activated SMP devices are a unique combination of laser-tissue and biomaterial technologies. Successful deployment of the microactuator requires well-engineered coupling of the light from the diffusing fiber through the blood into the SMP.

Chemical Phenomena↗