Magnetic Resonance Velocity Research Articles

Investigation the relationship between nuclear magnetic resonance and acoustic velocity for improving the evaluation of tight gas reservoirs

ABSTRACT Nuclear magnetic resonance (NMR) and acoustic logging play different roles in formation evaluation. Establishing a relationship model between the two is helpful for joint inversion or evaluation of the physical properties of the formation content. The rock pore structure is used as a link to analyze the theoretical relationship between the transverse relaxation time (T2) spectrum of NMR and acoustic velocity. In this paper, the relationship between NMR T2 and shear-wave velocity is determined by using NMR-acoustic velocity joint experiment. Considering that both NMR T2 spectrum and Shear-wave velocity (Vs) are highly related to porosity and pore structure, a formula based on the relationship between Vs and T2gm (NMR T2 geometric mean) was established. It is found that the T2gm is a power function of the shear-wave velocity. The respective coefficients in this formula for different lithologies were derived through petrophysical measurements of core samples (both NMR and ultrasonic experiments). The relationship model displays the same power-law relations as fitting expression from the experimental data.After that, there is a relation-model application for field data further validated effectiveness and reliability by contrasting the Shear-wave velocity determined by NMR logging with the direct measurements. This model establishment also helps to predict the mutual prediction of NMR and acoustic information of rocks. Based on the sensitivity of the vertical and transverse wave ratios of the formation to the gas-bearing properties of the reservoir, a NMR-acoustic velocity joint gas-bearing identification map was established. An extended case study demonstrated that the cross-plot between NMR logging and acoustic logging is also applicable to natural gas-bearing reservoir identification. As an important supplement and perfection of the existing methods, the relation model proposed in this paper offers a new thought for the petrophysical and in-situ field studies.

Ex Vivo Pilot Study of Cardiac Magnetic Resonance Velocity Mapping for Quantification of Aortic Regurgitation in a Porcine Model in the Presence of a Transcatheter Heart Valve.

Accuracy of aortic regurgitation (AR) quantification by magnetic resonance (MR) imaging in the presence of a transcatheter heart valve (THV) remains to be established. We evaluated the accuracy of cardiac MR velocity mapping for quantification of antegrade flow (AF) and retrograde flow (RF) across a THV and the optimal slice position to use in cardiac MR imaging. In a systematic and fully controlled laboratory ex vivo setting, two THVs (Edwards SAPIEN XT, Medtronic CoreValve) were tested in a porcine model (n = 1) under steady flow conditions. Results showed a high level of accuracy and precision. For both THVs, AF was best measured at left ventricular outflow tract level, and RF at ascending aorta level. At these levels, MR had an excellent repeatability (ICC > 0.99), with a tendency to overestimate (4.6 ± 2.4% to 9.4 ± 7.0%). Quantification of AR by MR velocity mapping in the presence of a THV was accurate, precise, and repeatable in this pilot study, when corrected for the systematic error and when the best MR slice position was used. Confirmation of these results in future clinical studies would be a step forward in increasing the accuracy of the assessment of paravalvular AR severity.

Visualization of CSF Flow with Time-resolved 3D MR Velocity Mapping in Aqueductal Stenosis Before and After Endoscopic Third Ventriculostomy : AFeasibility Study.

The aim of this study was to evaluate timed-resolved three-dimensional (3D) magnetic resonance (MR) velocity mapping as amethod for investigation of cerebrospinal fluid (CSF) flow changes in patients with aqueductal stenosis (AS) treated by endoscopic third ventriculostomy (ETV). The MR velocity mapping was performed in 12AS patients before and after ETV and in 10 healthy volunteers by using a3-Tesla MR system. Time-resolved 3D MR velocity mapping data were acquired with astandard 3D phase contrast (PC) sequence with cardiac triggering. Values of mean (vmean) and maximum (vpeak) velocity were measured at several ventricular structures using dedicated software. Of the patients 11 showed asatisfactory clinical improvement after ETV, whereas one patient needed subsequent shunt implantation. All AS patients showed significant hypomotile CSF flow dynamics in the third ventricle when compared to healthy subjects before surgery (p< 0.05). In contrast, CSF flow velocity was increased within the Foramen of Monro in AS patients. After ETV, all AS patients showed adecrease of CSF flow dynamics within the third ventricle. Mean and peak CSF flow velocities through the ventriculostomy were 1.72± 0.59cm/s (vmean) and 3.53± 0.79cm/s (vpeak), respectively after ETV. The patient who needed shunt implantation after ETV had the lowest flow velocities through the ventriculostomy. This study demonstrates that timed-resolved 3D MR velocity mapping is auseful imaging investigation for diagnostics and follow-up in patients with AS. This new technique provides an insight into the physiological CSF flow changes related with AS and its treatment.

Magnetic resonance measurement of fluid dynamics and transport in tube flow of a near-critical fluid

An ability to predict fluid dynamics and transport in supercritical fluids is essential for optimization of applications such as carbon sequestration, enhanced oil recovery, “green” solvents, and supercritical coolant systems. While much has been done to model supercritical velocity distributions, experimental characterization is sparse, owing in part to a high sensitivity to perturbation by measurement probes. Magnetic resonance (MR) techniques, however, detect signal noninvasively from the fluid molecules and thereby overcome this obstacle to measurement. MR velocity maps and propagators (i.e., probability density functions of displacement) were acquired of a flowing fluid in several regimes about the critical point, providing quantitative data on the transport and fluid dynamics in the system. Hexafluoroethane (C2F6) was pumped at 0.5 ml/min in a cylindrical tube through an MR system, and propagators as well as velocity maps were measured at temperatures and pressures below, near, and above the critical values. It was observed that flow of C2F6 with thermodynamic properties far above or below the critical point had the Poiseuille flow distribution of an incompressible Newtonian fluid. Flows with thermodynamic properties near the critical point exhibit complex flow distributions impacted by buoyancy and viscous forces. The approach to steady state was also observed and found to take the longest near the critical point, but once it was reached, the dynamics were stable and reproducible. These data provide insight into the interplay between the critical phase transition thermodynamics and the fluid dynamics, which control transport processes.

Assessment of myocardial reactivity to controlled hypercapnia with free-breathing T2-prepared cardiac blood oxygen level-dependent MR imaging.

To examine whether controlled and tolerable levels of hypercapnia may be an alternative to adenosine, a routinely used coronary vasodilator, in healthy human subjects and animals. Human studies were approved by the institutional review board and were HIPAA compliant. Eighteen subjects had end-tidal partial pressure of carbon dioxide (PetCO2) increased by 10 mm Hg, and myocardial perfusion was monitored with myocardial blood oxygen level-dependent (BOLD) magnetic resonance (MR) imaging. Animal studies were approved by the institutional animal care and use committee. Anesthetized canines with (n = 7) and without (n = 7) induced stenosis of the left anterior descending artery (LAD) underwent vasodilator challenges with hypercapnia and adenosine. LAD coronary blood flow velocity and free-breathing myocardial BOLD MR responses were measured at each intervention. Appropriate statistical tests were performed to evaluate measured quantitative changes in all parameters of interest in response to changes in partial pressure of carbon dioxide. Changes in myocardial BOLD MR signal were equivalent to reported changes with adenosine (11.2% ± 10.6 [hypercapnia, 10 mm Hg] vs 12% ± 12.3 [adenosine]; P = .75). In intact canines, there was a sigmoidal relationship between BOLD MR response and PetCO2 with most of the response occurring over a 10 mm Hg span. BOLD MR (17% ± 14 [hypercapnia] vs 14% ± 24 [adenosine]; P = .80) and coronary blood flow velocity (21% ± 16 [hypercapnia] vs 26% ± 27 [adenosine]; P > .99) responses were similar to that of adenosine infusion. BOLD MR signal changes in canines with LAD stenosis during hypercapnia and adenosine infusion were not different (1% ± 4 [hypercapnia] vs 6% ± 4 [adenosine]; P = .12). Free-breathing T2-prepared myocardial BOLD MR imaging showed that hypercapnia of 10 mm Hg may provide a cardiac hyperemic stimulus similar to adenosine.

Magnetic resonance imaging of the rheology of ionic liquid colloidal suspensions

The rheology, and underpinning colloidal interactions, of ionic liquid (IL) dispersions of colloidal silica have been investigated using bulk rheological measurements with magnetic resonance (MR) velocity and relaxation measurements. Two ionic liquids were investigated: tetradecyl(trihexyl)phosphonium bistriflamide ([P6,6,6,14][NTf2]) and 1-butyl-methylimidizolium tetrafluoroborate ([C4mim][BF4]), in the absence and presence of hydrophilic silica nanoparticles (Aerosil 200). Bulk rheology was probed using measurements of shear stress and viscosity as a function of shear rate in a cone-and-plate rheometer. Local rheology was probed using MR velocity imaging of flow in Couette and cone-and-plate cells. Velocity profiles were extracted from the Couette measurements and fitted using a power-law model. Newtonian rheology was observed for both ILs in the absence of dispersed silica. For the dispersion of 15% silica in [C4mim][BF4], bulk rheology and MR velocity imaging measurements showed Newtonian behaviour at low shear rates ( 10 s−1). For the dispersion of 5% silica in [P6,6,6,14][NTf2], more complex rheology was observed in the flow curve, which was suggestive of shear-banding. This was investigated further using the MR velocity profiles in a Couette cell and velocity images in a cone-and-plate cell, which both showed the coexistence of regions of sheared and unsheared fluid. The sheared fluid was found to be highly shear-thinning and close inspection of the flow profile at the interface between sheared and unsheared fluid suggested that the behaviour was shear-banding rather than shear-localisation. This was further confirmed by the velocity images in the cone-and-plate rheometer, which showed sheared and unsheared fluid in a uniform shear stress environment.

Time-resolved Three-dimensional Magnetic Resonance Velocity Mapping of Chronic Thoracic Aortic Dissection: A Preliminary Investigation

The blood flow patterns of chronic thoracic aortic dissection are complicated, and their clinical significance remains unknown. We evaluated the technical and clinical potentials of time-resolved 3-dimensional (3D) magnetic resonance (MR) velocity mapping for assessing these patterns. We used data collected from time-resolved 3D phase-contrast MR imaging of 16 patients with chronic thoracic aortic dissection to generate time-resolved 3D MR velocity mapping that included 3D streamline and path line. We investigated blood flow patterns of this disease in the mapping and compared them with the morphological changes of the patent false lumen. Time-resolved 3D MR velocity mapping visualized rapid flow at the entry and in the true lumen immediately distal to the entry. We observed slower helical or laminar flow in the patent false lumen. In patients with disease progression, slower helical flow following rapid entry jet collided with the outer wall of the false lumen and was also observed in a growing ulcer-like projection. We showed the potential of time-resolved 3D MR velocity mapping for visualizing pathologic flow patterns related to chronic thoracic aortic dissection.

In Vivo MR Angiography and Velocity Measurement in Mice Coronary Arteries at 9.4 T: Assessment of Coronary Flow Velocity Reserve

To demonstrate the feasibility of coronary magnetic resonance (MR) angiography in living mice and to evaluate a dynamic MR angiographic method for coronary flow measurement at 9.4-T field strength. This study was conducted according to European law and was in full compliance with National Institutes of Health recommendations for animal care and a local institutional animal care committee. Mice were anesthetized by using isoflurane. First, time-of-flight MR angiography was performed in 10 mice to measure coronary diameters at 80-mum isotropic resolution. Second, left coronary artery (LCA) velocity measurements were performed at seven cardiac phases in nine other mice to assess the velocity curve profile. Third, coronary velocities were measured at the middiastolic phase in 13 mice at rest and during adenosine-induced hyperemia to calculate coronary flow velocity reserve (CFVR). The Pearson coefficient compared the correlation between isoflurane dose and CFVR. Paired t tests compared R-R intervals and respiratory rates between rest and hyperemia. Proximal diameters were, respectively, 404 mum +/- 34 [standard deviation] and 259 mum +/- 22 for the LCAs and the right coronary arteries, which were in accordance with reported values. The velocity curve profile throughout the cardiac cycle was similar to values from the literature. Baseline and hyperemic velocities were, respectively, 19.0 cm/sec +/- 4.4 and 33.7 cm/sec +/- 4.7 (P<.001), resulting in a CFVR of 1.77 +/- 0.19. CFVR did not correlate with isoflurane dose (r = 0.05, P = .88). R-R intervals shortened by 2.5% during hyperemia (P = .04). Respiratory rates showed no difference between rest and hyperemia (P = .39). High-spatial-resolution three-dimensional coronary MR angiography is feasible in living mice. Dynamic MR angiography depicts coronary velocity changes throughout the cardiac cycle and between rest and maximum hyperemia, providing a tool for CFVR assessment.

The effect of dynamic vessel motion on haemodynamic parameters in the right coronary artery: a combined MR and CFD study

Human right coronary artery (RCA) haemodynamics is investigated using computational fluid dynamics (CFD) based on subject-specific information from magnetic resonance (MR) acquisitions. The dynamically varying vascular geometry is reconstructed from MR images, incorporated in CFD in conjunction with pulsatile flow conditions obtained from MR velocity mapping performed on the same subject. The effects of dynamic vessel motion on instantaneous and cycle-averaged haemodynamic parameters, such as wall shear stress (WSS), time-averaged WSS (TAWSS) and oscillatory shear index (OSI), are examined by comparing an RCA model with a time-varying geometry and those with a static geometry, corresponding to nine different time-points in the cardiac cycle. The results show that the TAWSS is similar for the dynamic and static wall models, both qualitatively and quantitatively (correlation coefficient 0.89-0.95). Conversely, the OSI shows much poorer correlations (correlation coefficient 0.38-0.60), with the best correspondence being observed with the static models constructed from images acquired in late diastole (at t = 0 and 800 ms, the cardiac cycle is 900 ms). These findings suggest that neglecting dynamic motion of the RCA is acceptable if TAWSS is the primary focus but may result in underestimation of haemodynamic parameters related to the oscillatory nature of the blood flow.

Choice of In Vivo Versus Idealized Velocity Boundary Conditions Influences Physiologically Relevant Flow Patterns in a Subject-Specific Simulation of Flow in the Human Carotid Bifurcation

Accurate fluid mechanics models are important tools for predicting the flow field in the carotid artery bifurcation and for understanding the relationship between hemodynamics and the initiation and progression of atherosclerosis. Clinical imaging modalities can be used to obtain geometry and blood flow data for developing subject-specific human carotid artery bifurcation models. We developed subject-specific computational fluid dynamics models of the human carotid bifurcation from magnetic resonance (MR) geometry data and phase contrast MR velocity data measured in vivo. Two simulations were conducted with identical geometry, flow rates, and fluid parameters: (1) Simulation 1 used in vivo measured velocity distributions as time-varying boundary conditions and (2) Simulation 2 used idealized fully-developed velocity profiles as boundary conditions. The position and extent of negative axial velocity regions (NAVRs) vary between the two simulations at any given point in time, and these regions vary temporally within each simulation. The combination of inlet velocity boundary conditions, geometry, and flow waveforms influences NAVRs. In particular, the combination of flow division and the location of the velocity peak with respect to individual carotid geometry landmarks (bifurcation apex position and the departure angle of the internal carotid) influences the size and location of these reversed flow zones. Average axial wall shear stress (WSS) distributions are qualitatively similar for the two simulations; however, instantaneous WSS values vary with the choice of velocity boundary conditions. By developing subject-specific simulations from in vivo measured geometry and flow data and varying the velocity boundary conditions in otherwise identical models, we isolated the effects of measured versus idealized velocity distributions on blood flow patterns. Choice of velocity distributions at boundary conditions is shown to influence pathophysiologically relevant flow patterns in the human carotid bifurcation. Although mean WSS distributions are qualitatively similar for measured and idealized inlet boundary conditions, instantaneous NAVRs differ and warrant imposing in vivo velocity boundary conditions in computational simulations. A simulation based on in vivo measured velocity distributions is preferred for modeling hemodynamics in subject-specific carotid artery bifurcation models when studying atherosclerosis initiation and development.

1021 Relation between the assessment of microvascular injury by cardiovascular magnetic resonance and coronary Doppler flow velocity measurements in patients with acute anterior wall myocardial infarction

1021 Relation between the assessment of microvascular injury by cardiovascular magnetic resonance and coronary Doppler flow velocity measurements in patients with acute anterior wall myocardial infarction

Numerical Simulations of Flow in Cerebral Aneurysms: Comparison of CFD Results and In Vivo MRI Measurements

Computational fluid dynamics (CFD) methods can be used to compute the velocity field in patient-specific vascular geometries for pulsatile physiological flow. Those simulations require geometric and hemodynamic boundary values. The purpose of this study is to demonstrate that CFD models constructed from patient-specific magnetic resonance (MR) angiography and velocimetry data predict flow fields that are in good agreement with in vivo measurements and therefore can provide valuable information for clinicians. The effect of the inlet flow rate conditions on calculated velocity fields was investigated. We assessed the internal consistency of our approach by comparing CFD predictions of the in-plane velocity field to the corresponding in vivo MR velocimetry measurements. Patient-specific surface models of four basilar artery aneurysms were constructed from contrast-enhanced MR angiography data. CFD simulations were carried out in those models using patient-specific flow conditions extracted from MR velocity measurements of flow in the inlet vessels. The simulation results computed for slices through the vasculature of interest were compared with in-plane velocity measurements acquired with phase-contrast MR imaging in vivo. The sensitivity of the flow fields to inlet flow ratio variations was assessed by simulating five different inlet flow scenarios for each of the basilar aneurysm models. In the majority of cases, altering the inlet flow ratio caused major changes in the flow fields predicted in the aneurysm. A good agreement was found between the flow fields measured in vivo using the in-plane MR velocimetry technique and those predicted with CFD simulations. The study serves to demonstrate the consistency and reliability of both MR imaging and numerical modeling methods. The results demonstrate the clinical relevance of computational models and suggest that realistic patient-specific flow conditions are required for numerical simulations of the flow in aneurysmal blood vessels.

Evaluation of a resistance‐based model for the quantification of pulmonary arterial hypertension using MR flow measurements

To establish an estimate for the mean pulmonary arterial pressure (mPAP) derived from noninvasive data acquired with magnetic resonance (MR) velocity-encoded sequences. In seven sedated pigs synchronous catheter-based invasive pressure measurements (IPM) and noninvasive MR were acquired in the main pulmonary artery (MPA) at different severities of pulmonary arterial hypertension (PAH) that were caused by infusion of thromboxane A2 (TxA2). The invasively measured mPAP was correlated with the noninvasive MR velocity data and linear combination equations (LCE) were computed. Intravenously applied TxA2 induced a dose dependent level of severity of PAH with an mPAP of up to 54 mmHg without systemic effects. The acceleration time (AT) measured with MR demonstrated the best correlation with the mPAP (r(2) = 0.75). The LCE with the highest correlation (R = 0.945, alpha < 0.01) between IPM and MR revealed a mean difference of 0, a SD of s = 4.66 and a maximal difference of 12.2 mmHg using the Bland-Altman analysis. Applying the identified LCE allowed the estimation of the mPAP in an acute and resistance-based model of PAH with high accuracy using noninvasive MR velocity-encoded sequences.

Time-of-flight variant to image mixing of granular media in a 3D fluidized bed

This paper describes a variant of time-of-flight magnetic resonance (MR) imaging that provides a method of measuring the inherent mixing in a fluidized bed without the introduction of tracer particles. The modifications to conventional time-of-flight imaging enable the measurement of the axial mixing of a precisely controlled initial particle distribution, thereby providing measurements suitable for a direct comparison with models of solids mixing in granular systems. The imaging sequence is applied to characterize mixing, over time scales of 25–1000 ms, in a gas-fluidized bed of Myosotis seed particles; mixing over short timescales, inaccessible using conventional tracer techniques, is studied using this technique. The mixing pattern determined by this pulse sequence is used in conjunction with MR velocity images of the motion of the particles to provide new insight into the mechanism of solids mixing in granular systems.

Velocity encoding versus acceleration encoding for pressure gradient estimation in MR haemodynamic studies

Many methods have been proposed to extract pressure gradient maps from magnetic resonance (MR) images. They were based on the resolution of the haemodynamic model of Navier–Stokes and needed the flow acceleration to be known. Most used velocity data acquisition and computed acceleration from temporal and spatial derivatives of the velocity field. However, MR sequences have been developed in order to acquire the acceleration field directly. Here we compared direct MR measurements of acceleration field components with those calculated from MR velocity acquisitions. Two experimental phantoms were used to separately evaluate the inertial and convective components of the acceleration. Mathematical simulation of the convective phantom further explained the origin of the noise generated by the spatial and temporal derivatives of the velocity data, and the misregistration artefacts due to MR sequences. We found that direct measurement of the acceleration field generates less noise and fewer artefacts than calculation from velocity derivatives.

Reliable in-plane velocity measurements with magnetic resonance velocity imaging

Magnetic resonance (MR) imaging is a well-known diagnostic imaging modality. In addition to its high-quality imaging capabilities, hydrogen-based MR can also provide non-invasively the velocity of water-based fluids in all three spatial directions (through-plane and in-plane) in an image. Many previous studies showed that MR velocity imaging can accurately measure the through-plane velocity. The aim of this study was to evaluate how reliable are the in-plane velocity measurements in an image. The axial velocity of water in horizontal tubes (inner diameter: 14.7–26.2 mm) was measured with segmented (fast) and non-segmented (slow) k-space MR velocity imaging using: (a) an imaging slice placed perpendicular to the tube axis with through-plane velocity-encoding; and (b) an imaging slice placed parallel to the tube axis with in-plane velocity-encoding. The two planes intersected along the vertical tube-centerline. The flow rate was accurately quantified (mean error <10%) and the in-plane velocity profiles were not significantly different from the through-plane profiles (mean difference =6%, correlation coefficients >0.98). There was no significant difference between the velocity profiles from the segmented and the non-segmented sequences (mean difference <1%, correlation coefficients >0.95). The results of this study suggest that fast MR velocity imaging can measure the in-plane velocity in an image with reliability.

A First-Order Lagrangian-based variational approach for 3D flow vector field restoration

The analysis of blood flow patterns provides important insight into the health status of the heart. With the ability to acquire multi-dimensional cine flow data, Magnetic Resonance (MR) velocity imaging is widely used to acquire in vivo flow details. The technique, however, is generally subject to a certain amount of noise and this poses a major problem for automatic quantitative analysis of flow features. To tackle this problem, we propose a First-Order Lagrangian-based method for the restoration of flow vector fields. The restoration scheme is formulated as a constrained optimisation problem and a computational algorithm based on the First-Order Lagrangian method is employed to solve the optimisation problem. The proposed method has been proven to be effective for the restoration of both 2D and 3D datasets. To demonstrate the importance of a restoration step prior to the analysis of blood flow pattern, the proposed method is applied to MR velocity maps acquired from patients with sequential MR examination following myocardial infarction and the result shows that critical points are more readily detected in the restored flow field.

Virtual tagging: numerical considerations and phantom validation

This paper presents a virtual tagging framework for measuring, as well as visualising, myocardial deformation using magnetic resonance (MR) velocity imaging. Tagging grids are allocated artificially according to the deformation gradient with varying shapes and densities. The control points are then deformed such that the difference between the induced deformation velocity and that of actually measured MR data is minimum. A full three-dimensional implementation of the technique combined with the mass conservation constraint is provided. Numerical considerations of applying the proposed framework and different optimization strategies have been investigated with both simulated and phantom experiments. The accuracy of the technique in terms of following material deformation is compared with that of conventional tagging technique.

Magnetic resonance phase-shift velocity mapping in pediatric patients with pulmonary venous obstruction

OBJECTIVESThis study evaluated the accuracy, advantages and clinical efficacy of magnetic resonance (MR) phase-shift velocity mapping, in delineating the site and the hemodynamic severity of pulmonary venous (PV) obstruction in patients with congenital heart disease (CHD).BACKGROUNDMagnetic resonance phase-shift velocity mapping of normal pulmonary veins and of obstructed PV pathways have been previously reported in a mainly adult population.METHODSThe study population (33 pts) underwent MR phase-shift velocity mapping of their PV pathways. These results were compared with cardiac catheterization and Doppler echocardiography data.RESULTSThe study population (0.4 to 19.5 years) consisted of a study group (PV pathway obstruction, n = 7) and a control group (no PV obstruction, n = 26). No patients had any left-to-right shunt lesions. The MR imaging displayed precise anatomical detail of the pulmonary veins. Phase velocities in the control group ranged from 20 to 71 cm/s, whereas velocities in the study group ranged from 100 to 250 cm/s (p = 0.002). The MR phase velocities (154 ± 0.53 cm/s) compared favorably with Doppler echocardiography (147 ± 0.54 cm/s), (r = 0.76; p = 0.05). The MR velocity mapping was 100% specific and 100% sensitive in detecting PV obstruction, although the absolute gradient measurements among MR phase mapping, echocardiographic Doppler and catheterization did not show statistically significant correlation.CONCLUSIONSIn the absence of any associated left-to-right shunt lesions, PV velocities of 100 cm/s and greater indicated significant obstruction. The MR phase-shift velocity mapping, together with MR spin echocardiography and MR angiography, provides comprehensive anatomic and physiologic data that may obviate the need for further invasive studies.

Precision of magnetic resonance velocity and acceleration measurements: theoretical issues and phantom experiments.

Magnetic resonance (MR) sequences have been developed for acquiring multiple components of velocity and/or acceleration in a reasonable time and with a single acquisition. They have many parameters that influence the precision of measurements: NS, the number of flow-encoding steps; NEX, the number of signal accumulations; and ND, the number of dimensions. Our aims were to establish a general relationship revealing the precision of these measurements as a function of NS, ND, and NEX and to validate it by experiments using phantoms. Previous work on precision has been restricted to two-step (NS = 2) or 1D (ND = 1) MR velocity measurements. We describe a comprehensive approach that encompasses both multistep and multidimensional strategies. Our theoretical formula gives the precision of velocity and acceleration measurements. It was validated experimentally with measurements on a rotating disk phantom. This phantom was much easier to handle than fluid-based phantoms. It could be used to assess both velocity and acceleration sequences and provided accurate and precise assessments over a wide, adjustable range of values within a single experiment. Increasing each of the three parameters, NS, ND, and NEX, improves the precision but makes the acquisition time longer. However, if only one parameter is to be assessed, maximizing the number of steps (NS) is the most efficient way of improving the precision of measurements; if several parameters are of interest, they should be measured simultaneously. By contrast, increasing the number of signals accumulated (NEX) is the least efficient strategy.