MR flow quantification using RACE: clinical application to the carotid arteries

Accuracy of subject-specific prediction of end-systolic time in MRI across a range of RR intervals

Background

Prediction of End-Systole time is of utmost importance for cardiac MRI to correctly associate acquired k-space lines during reconstruction of cine acquisitions. This prediction is usually based on the patient’s heart rate using Weissler’s formula, which was calibrated by linear regression within a population and cannot account for individual variability.

Objective

We propose an automatic method to build a personalized model that better predicts end-systole.

Methods

A phase contrast sequence was modified to acquire only central k-space line with 6.6ms temporal resolution, in a slice passing through the aorta during 128 heartbeats in 35 subjects. Segmentation of aorta and detection of end of systolic ejection was automatic. Duration of electromechanical systole duration as function of heart rate was determined for each subject separately.

Results

In comparison with the global models, the adapted cardiac model predicted significantly better both echocardiographic end-systolic reference (bias = 0ms vs 17ms, p<0.001) and MRI measurements (bias = 6.8ms vs 17ms). Favorable impact was shown on the cine reconstruction of the 5 subjects with the higher cardiac variability (p = 0.02).

Conclusions

Personalization of cardiac model to the subject is feasible in MRI and reduces the error of prediction of systole.

Mapping of arterial transit time by intravascular signal selection

The arterial transit time (δa ) is a potentially important physiological parameter which may provide valuable information for the characterization of cerebrovascular diseases. The present study shows that δa can be measured by arterial spin labeling (ASL) applied quasi-continuously in an amplitude-modulated fashion at the human neck. Imaging was performed using short repetition times and excitation flip angles of 90°, which resulted in the selection of an ASL signal of mostly intravascular origin. Model-independent estimates of δa were obtained directly from the temporal shift of the ASL time series. An extended two-compartment perfusion model was developed in order to simulate the basic features of the proposed method and to validate the evaluation procedure. Vascular structures found in human δa maps, such as the circle of Willis or cerebral border zones, hint at the sensitivity of the method to most sizes of arterial vessels. Group-averaged values of δa measured from the carotid bifurcation to the tissue of interest in selected regions of the human brain ranged from 925 ms in the insular cortex to 2000 ms in the thalamic region.

Theoretical and experimental evaluation of continuous arterial spin labeling techniques

Continuous arterial spin labeling is known to be the most sensitive arterial spin labeling technique. To avoid magnetization transfer effects and to overcome hardware limitations, several sequences have been proposed that adiabatically label the inflowing blood. Four of these methods are examined with respect to their sensitivity both theoretically by Bloch equation simulations and experimentally. All sequences were optimized carefully by adjusting their measurement parameters based exclusively on the results of simulations. Perfusion measurements on the human brain obtained at 3 T result in excellent images from all techniques, while differences in sensitivity are similar to those expected from the simulations.

In vivo estimation of the flow-driven adiabatic inversion efficiency for continuous arterial spin labeling: a method using phase contrast magnetic resonance angiography

The accurate quantification of perfusion with arterial spin labeling (ASL) requires consideration of a number of factors, including the efficiency of the inversion and control pulses used for spin labeling. In this study the effects of spin velocity on continuous ASL efficiency when using the amplitude modulated control strategy were investigated using simulations of the Bloch equations. The inversion efficiency was determined in vivo by combining the simulations with phase-contrast velocity mapping data acquired at the level of the tagging plane. Using this novel method, an average inversion efficiency of 69% was calculated for a group of 28 subjects, in good agreement with experimental data reported previously. There was, however, a large range in inversion efficiency measured across the subject group (50-76%), indicating that the velocity dependence of the amplitude modulated control efficiency may introduce additional variability into the perfusion calculations if not properly taken into account.

Towards quantification of blood-flow changes during cognitive task activation using perfusion-based fMRI

Multi-slice perfusion-based functional magnetic resonance imaging (p-fMRI) is demonstrated with a color-word Stroop task as an established cognitive paradigm. Continuous arterial spin labeling (CASL) of the blood in the left common carotid artery was applied for all repetitions of the functional run in a quasi-continuous fashion, i.e., it was interrupted only during image acquisition. For comparison, blood oxygen level dependent (BOLD) contrast was detected using conventional gradient-recalled echo (GE) echo planar imaging (EPI). Positive activations in BOLD imaging appeared in p-fMRI as negative signal changes corresponding to an enhanced transport of inverted water spins into the region of interest, i.e., increased cerebral blood flow (CBF). Regional differences between the localization of activations and the sensitivity of p-fMRI and BOLD-fMRI were observed as, for example, in the inferior frontal sulcus and in the intraparietal sulcus. Quantification of CBF changes during cognitive task activation was performed on a multi-subject basis and yielded CBF increases of the order of 20-30%.

Continuous arterial spin labeling at the human common carotid artery: the influence of transit times

In evaluating the sensitivity of arterial spin labeling (CASL) and for quantification of perfusion, knowledge of the transit time from the labeling plane to the imaging slice is crucial. The purpose of the current study was to obtain estimates of transit times relevant under the specific experimental conditions of CASL in human subjects using a separate local labeling coil at the neck. Specifically, the post-label delay (PLD), i.e. the time between the end of the labeling period and the image acquisition, was varied either with or without additional application of crusher gradients to suppress intravascular signal contributions. The overall sensitivity change for varying the PLD between 1000 and 1700 ms was low. A tissue transit time from the neck to an axial supraventricular section through Broca's knee was obtained by fitting the PLD dependence to a two-compartment model. Averaging over subjects yielded 1930 +/- 110 ms for the tissue transit time, and 73 +/- 5 ml min(-1) 100 g(-1) for the cerebral blood flow. Small areas that exhibited a very high signal change upon labeling were indicative of regional variation in cerebral blood flow related to vascular anatomy.

Efficiency of flow-driven adiabatic spin inversion under realistic experimental conditions: a computer simulation

Continuous arterial spin labeling (CASL) using adiabatic inversion is a widely used approach for perfusion imaging. For the quantification of perfusion, a reliable determination of the labeling efficiency is required. A numerical method for predicting the labeling efficiency in CASL experiments under various experimental conditions, including spin relaxation, is demonstrated. The approach is especially useful in the case of labeling at the carotid artery with a surface coil, as consideration of the experimental or theoretical profile of the B(1) field is straightforward. Other effects that are also accounted for include deviations from a constant labeling gradient, and variations in the blood flow velocity due to the cardiac cycle. Assuming relevant experimental and physiological conditions, maximum inversion efficiencies of about 85% can be obtained.

Cardiovascular flow measurement with phase-contrast MR imaging: basic facts and implementation

Phase-contrast magnetic resonance (MR) imaging is a well-known but undervalued method of obtaining quantitative information on blood flow. Applications of this technique in cardiovascular MR imaging are expanding. According to the sequences available, phase-contrast measurement can be performed in a breath hold or during normal respiration. Prospective as well as retrospective gating techniques can be used. Common errors in phase-contrast imaging include mismatched encoding velocity, deviation of the imaging plane, inadequate temporal resolution, inadequate spatial resolution, accelerated flow and spatial misregistration, and phase offset errors. Flow measurements are most precise if the imaging plane is perpendicular to the vessel of interest and flow encoding is set to through-plane flow. The sequence should be repeated at least once, with a high encoding velocity used initially. If peak velocity has to be estimated, flow measurement is repeated with an adapted encoding velocity. The overall error of a phase-contrast flow measurement comprises errors during prescription as well as errors that occur during image analysis of the flow data. With phase-contrast imaging, the overall error in flow measurement can be reduced to less than 10%, an acceptable level of error for routine clinical use.

Quantification of blood flow in the carotid arteries: comparison of Doppler ultrasound and three different phase-contrast magnetic resonance imaging sequences

RATIONALE AND OBJECTIVES

To compare blood flow velocities in the carotid arteries measured with three different magnetic resonance (MR) phase-contrast imaging techniques and with percutaneous Doppler ultrasound.

METHODS

Fourteen healthy male volunteers with a mean age of 33 +/- 3.8 years were studied. Ultrasound and MR phase velocity mapping of both common carotid arteries (n = 28) was performed within 5 hours. A two-dimensional fast low-angle shot sequence with retrospective cardiac gating, a sequence with prospective cardiac triggering, and a breath-hold sequence with prospective cardiac triggering were used. Resistance indexes and pulsatility indexes were calculated for all modalities.

RESULTS

The comparison of flow velocities obtained with ultrasound and the different MR techniques led to a moderate correlation of the retrospective gated and prospective triggered MR techniques (eg, r = 0.73 for maximum systolic velocity). The worst correlation was found between the breath-hold technique and retrospective cardiac gating (eg, r = 0.004 for pulsatility index). There was a weak correlation of all three MR sequences compared with ultrasound (r = 0.19-0.60)

CONCLUSIONS

A moderate correlation was found between velocities and indexes measured with the prospective cardiac-triggered phase-contrast MR technique and the retrospective cardiac-gated phase-contrast MR technique. A weak correlation was found between the three different MR techniques and ultrasound, as well as between the breath-hold prospective cardiac-triggered MR sequence and both of the other MR sequences. The influence of temporal and spatial resolution on MR phase-contrast velocity mapping was confirmed.

In vitro validation of MR measurements of arterial pulse-wave velocity in the presence of reflected waves

A magnetic resonance imaging projective velocity encoding sequence was used to determine the pulse-wave velocity in an artery model. To this end, a well-defined flow phantom simulating flow propagation in large arteries was used. In order to validate the measurement method in the presence of large reflected waves, these were deliberately created in the phantom. The projective sequence was applied to two measurement sites and the wave velocity was determined from the spatial and temporal separations of the foot of the velocity waveform. A theoretical model describing reflection and attenuation phenomena was compared with experimental velocity waveforms. The model showed that reflections and attenuation can explain the important changes in velocity waveforms. The model also confirmed that in the presence of reflecting waves, the foot of the waveform can be used as a characteristic point for measurements through changes in the waveform.

Comparison between magnetic resonance phase contrast imaging and transcranial Doppler ultrasound with regard to blood flow velocity in intracranial arteries: work in progress

OBJECTIVE

The authors evaluate blood flow velocities in the medial cerebral artery (MCA) and the basilar artery using magnetic resonance (MR) phase contrast technique in comparison with transcranial Doppler ultrasound (TCD). Eleven healthy male volunteers were studied. TCD of the MCA (n = 22) and basilar artery (n = 11) was performed. MR phase velocity mapping was done in each vessel at the same location where the TCD signal had been acquired. A 2-dimensional FLASH sequence with retrospective cardiac gating and an average temporal resolution of 45 ms was used. Resistance indices (RIs) and pulsatility indices (PIs) were calculated for both modalities. The TCD insonation angle was measured retrospectively with MR, and TCD velocities were corrected based on these measurements. The comparison of flow velocities obtained with TCD and MR led to a low correlation coefficient with regard to the basilar artery and the MCA: maximum systolic velocity, r = 0.02 and r = 0.50, respectively; enddiastolic velocity, r = 0.47 and r = 0.65, respectively; mean velocity, r = 0.52 and r = 0.66, respectively. The average PIs in the basilar artery and the MCA were 0.80 and 0.81 with MR and 0.65 and 0.85 with TCD, respectively. The average RIs in the basilar artery and the MCA were 0.52 and 0.54 with MR and 0.52 and 0.55 with TCD, respectively. The TCD insonation angle differed significantly from the ideal value in the basilar artery (mean value = 32.6 degrees) and the MCA (mean value = 26.5 degrees). The authors find a low correlation between velocities measured with MRI and TCD but similar results with regard to the PIs and RIs. Several sources of error, such as a nonideal TCD insonation angle, were identified.

Nonsubtractive spiral phase contrast velocity imaging

Phase contrast velocity imaging is a standard method for accurate in vivo flow measurement. One drawback, however, is that it lengthens the scan time (or reduces the achievable temporal resolution) because one has to acquire two or more images with different flow sensitivities and subtract their phases to produce the final velocity image. Without this step, non-flow-related phase variations will give rise to an erroneous, spatially varying background velocity. In this paper, we introduce a novel phase contrast velocity imaging technique that requires the acquisition of only a single image. The idea is to estimate the background phase variation from the flow-encoded image itself and then have it removed, leaving only the flow-related phase to generate a corrected flow image. This technique is sensitive to flow in one direction and requires 50% less scan time than conventional phase contrast velocity imaging. Phantom and in vivo results were obtained and compared with those of the conventional method, demonstrating the new method's effectiveness in measuring flow in various vessels of the body. Magn Reson Med 42:704-713, 1999.

Automatic accurate non-invasive quantitation of blood flow, cross-sectional vessel area, and wall shear stress by modelling of magnetic resonance velocity data

OBJECTIVES

To apply a new, automatic and non-invasive method for quantification of blood flow, dynamic cross-sectional vessel area, and wall shear stress (WSS) by in vivo magnetic resonance velocity mapping of normal subjects.

DESIGN

Prospective, open study.

MATERIALS

Six young volunteers.

METHODS

A three-dimensional paraboloid model enabling automatic determination of blood flow, vessel distensibility and WSS was applied to blood velocity determinations in the common carotid artery. Blood flow was also determined by a manual edge detection method.

RESULTS

Using the new method, the common carotid mean blood flow was 7.28 (5.61-9.63) (mean (range)) ml/s. By the manual-method blood flow was 7.21 (5.55-9.60) ml/s. Mean luminal vessel area was 26% larger in peak systole than in diastole. Mean/peak WSS was 0.82/2.28 N/m2. Manually and automatically determined flows correlated (r2 = 0.998, p < 0.0001). WSS and peak centre velocity were associated (r2 = 0.805, p < 0.0001).

CONCLUSIONS

Blood flow, luminal vessel area dilatation, and WSS can be determined by the automatic three-dimensional paraboloid method. The hypothesis of association between peak centre velocity and WSS was not contradicted by the results of the present study.