Phase-contrast Flow Measurements Research Articles

Correlation of flow velocity measurements by magnetic resonance phase contrast imaging and intravascular Doppler ultrasound.

The authors compare the magnetic resonance (MR) phase contrast flow velocity measurements in varying concentric stenoses with invasive measurements obtained with a Doppler guidewire. Flow velocity measurements were obtained using a calibrated 0.018 inch 12 MHz Doppler guidewire and a 1.0 T MR imaging system in a pulsatile hydraulic model with variable arterial stenoses. Velocity measurements were performed proximal, intrastenotic, and distal to the stenoses. The cross-sectional area of stenosis was calculated from the data of both methods. For MR imaging measurements, fast low-angle shot two-dimensional phase contrast sequences with different velocity encodings were used. Phase contrast flow measurements correlated well (r = 0.95, Pearson) with Doppler guidewire-based flow velocity data. Generally, flow velocities obtained with MR imaging were lower when compared with the Doppler-based data (P < or = 0.001, Wilcoxon matched pairs test). However, the ratios and the calculated cross-sectional area of stenoses showed a high correlation (r = 0.96) with the predefined area of stenoses. The assessment of flow alterations in vitro due to variable stenoses using MR phase contrast flow measurements is very well correlated with the Doppler guidewire. Consequently, these results required in vivo measurements of atherosclerotic lesions to evaluate the clinical impact.

Using cardiac phase to order reconstruction (CAPTOR): A method to improve diastolic images

A method is proposed to reconstruct multiphase images that accurately depicts the entire cardiac cycle. A segmented, gradient-recalled-echo sequence (FASTCARD) was modified to acquire data continuously. Images were reconstructed retrospectively by selecting views from each heartbeat based on cardiac phase rather than the time elapsed from the QRS complex. Cardiac phase was calculated using a model that compensates for beat-to-beat heart rate changes. Images collected using cardiac phase to order reconstruction (CAPTOR) depict the entire cardiac cycle and lack the temporal gap that is characteristic of prospectively reconstructed sequences. Time-volume curves of the left ventricle capture the contribution of atrial contraction to end-diastolic volume (EDV). Transmitral phase-contrast flow measurements show a second peak inflow (alpha wave) that is absent in the standard sequence. Because atrial contraction contributes to ventricular EDV, images using CAPTOR potentially may provide a more reliable measure of EDV, stroke volume, and ejection fraction than standard techniques.

MRA flow quantification in patients with a symptomatic internal carotid artery occlusion. The Dutch EC-IC Bypass Study Group.

Flow measurements in the collateral arteries of patients with internal carotid artery (ICA) occlusions may be important to estimate the risk of future stroke. Quantitative flow measurements in cerebropetal vessels can be reliably assessed by means of magnetic resonance angiography (MRA). Fifty-four patients with transient or minor ischemic neurological deficits and an angiographically proven ICA occlusion and 16 control subjects underwent two-dimensional phase-contrast MRA quantitative flow measurements through the common carotid arteries, basilar artery, ICAs, and middle cerebral arteries (MCA). Patients with a unilateral ICA occlusion and a 0% to 69% stenosis of the contralateral ICA had increased flow in the contralateral ICA (P < .005) and in the basilar artery (P < .005) compared with control subjects. Even patients with a 70% to 99% stenosis contralateral to the ICA occlusion had increased flow in the ICA (P < .05) as well as increased flow in the basilar artery (P < .001). Total cerebropetal flow was not significantly different between these patients and control subjects. Patients with bilateral ICA occlusions had an increased flow in the basilar artery (P < .001), while the total cerebropetal flow was less than in control subjects (P < .001). In all patients, flow was decreased in the ipsilateral MCA (P < .001) and in the contralateral MCA (P < .05). The contralateral ICA is the main supplying artery in patients with an ICA occlusion. Total cerebropetal flow decreases only when both ICAs are occluded. In patients with symptomatic ICA occlusions, an open contralateral ICA is probably important to retain the cerebral blood flow within normal limits.

Relative quantification of pulmonary edema with noncontrast-enhanced MRI.

Pulmonary edema is a debilitating effect of acute respiratory distress syndrome. The ability to measure it noninvasively with high sensitivity and in three dimensions could be useful in not only detection but also in assessment and guidance of treatment. To this end, a three-dimensional MRI pulse sequence to measure the formation of edema was developed and tested. Another sequence was tested to measure blood flow in distal pulmonary arteries. Pulmonary edema was induced in nine dogs via venous injections of oleic acid. Edema was verified by wet-to-dry weight ratio (5.30 +/- .38) and extra-vascular lung water at baseline (2.03 +/- 1.12 ml/g dry lung weight) versus postinjury (3.00 +/- 1.45 ml/g) (P < .005). The signal-to-noise ratio within the lungs increased from 5.47 +/- 1.00 at baseline to 7.51 +/- 1.96 (P < .005), and the time course of edema formation was resolved. Results from MR phase-contrast blood flow measurements were variable. The authors conclude that the three-dimensional scan provides a sensitive relative quantification of pulmonary edema formation without the use of contrast agents or ionizing radiation.

Noninvasive analysis of renal artery blood flow dynamics with MR cine phase-contrast flow measurements.

It was the aim of this study to quantify the measurement of pulsatile flow in the renal artery with the noninvasive magnetic resonance cine phase-contrast (MRCPC) method and combine it with the simultaneous assessment of pulsatile flow with a transit-time ultrasound (TTUS) flowmeter. In seven foxhounds with a chronically implanted precalibrated TTUS flow probe, MRCPC flow measurements were made in the renal artery with a temporal resolution of 32 ms. Mean and pulsatile flow signal were compared by the simultaneous ipsi- or contralateral measurement of the renal blood flow signal by both methods (TTUS and MRCPC). In addition, comparative MRCPC and TTUS flow measurements were made with artificial renal artery stenosis and after the administration of angiotensin II. The mean flow data assessed by the noninvasive MRCPC flow measurements showed an excellent correlation with corresponding TTUS recording (r = 0.99). The MRCPC flow signal displayed a waveform of the renal artery flow profile that was very similar to the TTUS flow pulse. The hemodynamic changes induced by angiotensin II or due to renal artery stenosis were also reliably detected by MRCPC. MRCPC provides a reliable noninvasive method for the quantification of mean blood flow and the assessment of the pulsatile flow signal in the renal artery and proves to be sensitive to hemodynamic changes of pathophysiological importance. Alternatively, the method may be used for studies in physiology that demand a noninvasive approach.

Renal artery stenosis: grading of hemodynamic changes with cine phase-contrast MR blood flow measurements.

To analyze the blood flow dynamics in renal artery stenosis with high-temporal-resolution cine phase-contrast magnetic resonance (MR) flow measurements. Cine phase-contrast MR flow measurements were invasively validated with real-time intraoperative transit-time ultrasound (US). In 23 patients, 48 renal artery stenoses were confirmed at digital subtraction angiography. Cardiac-gated cine phase-contrast MR flow measurements were obtained in 32-msec intervals, and flow curves were calculated for the whole cardiac cycle. Hemodynamic parameters evaluated included the decrease in mean flow and the delay and reduction in the systolic velocity maximum due to decrease in or absence of the early systolic peak. Overall differentiation between renal artery stenosis (n = 31) and nonstenosed vessels (n = 17) with cine phase-contrast MR revealed a sensitivity of 90% and specificity of 94% compared with findings at digital subtraction angiography. High-grade stenoses (>50%, n = 19) were detected with cine phase-contrast MR with sensitivity of 100% and specificity of 93%. Quantitative and qualitative analysis of cardiac-gated cine phase-contrast MR flow velocity curves provided a highly accurate method to detect hemodynamic abnormalities in patients with suspected renal artery stenosis.

Quantitative blood flow measurements with cine phase-contrast MR imaging of subjects at rest and after exercise to assess peripheral vascular disease.

The purpose of this study was to determine whether cine phase-contrast MR volume flow measurements can identify patients with peripheral vascular disease. We performed MR measurements of volume blood flow in the popliteal artery of subjects at rest and after 5 min of plantar flexion exercise in 10 volunteers (mean age, 28 years old), in five patients suspected of having peripheral vascular disease (mean age, 58 years old), and in five other volunteers of a similar age (mean age, 57 years old). Volume blood flow at rest was similar in volunteers and in patients. Four patients who had abnormal ankle-brachial indexes had lower flow increases after exercise (2.6-fold) compared with the five older normal volunteers (4.8-fold; p < .03, t test). These flow increases correlated well with ankle-brachial indexes: r = .97. The four patients with abnormal ankle-brachial indexes had monophasic resting waveforms, whereas all other subjects had triphasic waveforms. MR volume blood flow measurement may aid in evaluating peripheral vascular disease. Studies of larger patient groups will be necessary.

Variability of consecutive in vivo MR flow measurements in the main portal vein.

The variability of consecutive cine phase-contrast MR flow measurements could significantly affect their use for clinical decisions, especially during provocative testing. The purposes of this study were to determine the normal variability of flow and consecutive flow measurements in the main portal vein on MR images and to determine how intraobserver variability, interobserver variability, and MR imager variability affect these measurements. MR flow measurements were acquired four consecutive times at the same location in the main portal vein of 12 subjects and three consecutive times at the same location in a nonpulsatile vessel model. All acquisitions were completed within 10 min. All main portal vein MR data sets were evaluated manually in a blinded review by two independent observers during three separate sessions spaced a mean of 4.5 weeks apart. Flow model data sets were evaluated during a single session by one observer. Variabilities were subsequently calculated by a components-of-variance analysis and by the coefficient of variation (SD/mean x 100). Of the total variance, 90% was due to flow variability among subjects (intersubject), 6% to flow variability within one subject (intrasubject), 2% to intraobserver variability, and 2% to interobserver variability. The coefficient of variation of consecutive MR portal vein flow measurements within a single subject was 11% +/- 5% (range, 3-23%). Intra- and interobserver variabilities were 5% +/- 2% (range, 1-11%) and 4% +/- 4% (range, 0-17%), respectively. MR imager variability was 1% +/- 1% (range, 0-2%). The mean variability of consecutive cine phase-contrast MR flow measurements in the main portal vein is 11% +/- 5% and could affect research and clinical protocols that employ this technique.

Improved target volume definition in radiosurgery of arteriovenous malformations by stereotactic correlation of MRA, MRI, blood bolus tagging, and functional MRI.

In this methodological paper the authors report the stereotactic correlation of different magnetic resonance imaging (MRI) techniques [MR angiography (MRA), MRI, blood bolus tagging (STAR), and functional MRI] in 10 patients with cerebral arteriovenous malformations (AVM) and its application in precision radiotherapy planning. The patient's head was fixed in a stereotactic localization system that is usable at the MR and the linear accelerator installations. By phantom measurements different materials (steel, aluminium, titanium, plastic, wood, ceramics) used for the stereotactic system were tested for mechanical stability and geometrical MR image distortion. All metallic stereotactic rings (closed rings made of massive metal) led to a more or less dramatic geometrical distortion and signal cancellation in the MR images. The best properties-nearly no distortion and high mechanical stability-are provided by a ceramic ring. If necessary, the remaining geometrical MR image distortion can be "corrected" (reducing displacements to the size of a pixel) by calculations based on modeling the distortion as a fourth-order two-dimensional polynomial. Using this method multimodality matching can be performed automatically as long as all images are acquired in the same examination and the patient is sufficiently immobilized. Precise definition of the target volume could be performed by the radiotherapist either directly in MR images or in calculated projection MR angiograms obtained by a maximum-intensity projection algorithm. As a result, information about the hemodynamics of the AVM was provided by a three-dimensional (3D) phase-contrast flow measurement and a dynamic MRA with the STAR technique leading to an improved definition of the size of the nidus, the origin of the feeding arteries, and the pattern of the venous drainage. In addition, functional MRI was performed in patients with lesions close to the primary motor cortex area leading to an improved definition of structures at risk for high-dose application in radiosurgery. The different imaging techniques of MR provide a sensitive, noninvasive, 3D method for defining target volume, critical structures, and for calculating dose distributions for radiosurgery of cerebral arteriovenous malformations, because dose calculation of radiosurgery at sufficient accuracy can be based on 3D MR data of the geometrical conformation of the patient's head.

Flow quantitation with echo-planar phase-contrast velocity mapping: in vitro and in vivo evaluation.

We present a multishot echo-planar imaging (EPI) phase-contrast implementation for flow quantitation. The measurement accuracy of this technique was evaluated in vitro and in vivo. A gated eight-shot EPI phase-contrast sequence (TR/TE = 16/7.4, 45 degrees flip angle), with a flow-phase interval of 32 msec and an in-plane resolution of 2 x 2 mm was initially evaluated in a pulsatile flow phantom. Subsequently, EPI phase-contrast flow measurements of the ascending and descending aorta, obtained in 10 volunteers, were compared with flow volume data acquired with a conventional cine phase-contrast sequence (TR/TE = 24/7, 45 degrees flip angle, 48-msec flow-phase interval, 2 x 1 mm in-plane resolution). Comparisons between flow measurements were made using data obtained with the flow probe and cine phase contrast as the standard of reference for in vitro and in vivo measurements, respectively. EPI phase-contrast sequences reduced data acquisition times tenfold compared with cine phase-contrast sequences. EPI phase-contrast flow measurements correlated well with phantom flow (r = .98, slope = 1.1) as well as with aortic cine phase-contrast flow volume determinations (r = .98). A 95% confidence interval of measurement differences between echo-planar and cine phase-contrast imaging, ranging from 2.0 to -1058 mL/min was computed. Ultrafast phase-contrast flow measurements are possible. Multishot EPI phase-contrast imaging provides high measurement accuracy in pulsatile vessels while keeping the image acquisition interval short enough to be accomplished in a comfortable breath-hold.

Error in MR volumetric flow measurements due to ordered phase encoding in the presence of flow varying with respiration.

Respiratory ordered phase encoding is often employed in MRI studies to reduce image artifacts due to breathing motion. The purpose of this work was to evaluate error caused by the use of respiratory ordering of phase encoding in MR cine phase-contrast (CPC) volumetric flow measurements when the flow rate is sensitive to respiration. It was hypothesized that this effect is due to the systematic biasing of a respiratory-induced phase modulation function in k-space. A theoretical model for the effects of respiration was developed and then tested in flow phantom studies and in normal volunteer studies. In phantom experiments, the use of respiratory ordering induced an error of as much as 13% in CPC volumetric flow measurements. In preliminary volunteer studies, error was as high as 26% in superior vena cava flow measurements versus less than 1% error in the ascending aorta. It is concluded that a potential for error exists in CPC volumetric flow measurements obtained with the use of respiratory ordering schemes. Volunteer studies with larger numbers are warranted. Clinical applications in which this effect may be important include flow measurements in vessels subject to variations in flow due to respiration, such as the venae cavae, pulmonary vasculature, and portal vein.

MR measurements of mesenteric venous flow: prospective evaluation in healthy volunteers and patients with suspected chronic mesenteric ischemia.

To quantify portal vein (PV) and superior mesenteric vein (SMV) flow before and after a standardized meal in healthy volunteers and to prospectively evaluate patients with a clinical suspicion of chronic mesenteric ischemia on the basis of magnetic resonance (MR) measurement of flow in the mesenteric venous system in volunteers. Cine phase-contrast flow measurements were acquired in 10 asymptomatic volunteers and in 10 patients. In volunteers, the difference between the fasting and post-prandial flows in the SMV and PV was significant (P < .001), with a peak flow augmentation of 245% +/- 74 and 70% +/- 29, respectively. Postprandial augmentation of peak flow in the SMV was significantly less in patients with mesenteric ischemia compared with volunteers (64% +/- 28; P = .02). SMV flow augmentation in patients without mesenteric ischemia did not differ significantly from that in volunteers (206% +/- 36; P = .31). Measurement of postprandial flow augmentation in the SMV with MR imaging shows promise as a noninvasive screening test for chronic mesenteric ischemia.

Correction of partial-volume effects in phase-contrast flow measurements.

Because of the relatively small size of vessels and limited magnetic resonance (MR) imaging resolution, the accuracy of volume flow rate measurements is limited. A technique that corrects the partial-volume effect in volume flow rate measurements is presented. The technique uses small-phase-shift approximation, with the assumption that blood flow in the voxels at the boundary of the vessel is slow. With the proposed correction technique, the volume flow rate in partially occupied voxels is corrected on a voxel-by-voxel basis and the accuracy of flow measurements increases. Results are shown analytically and for MR phantom data.

A complex-difference phase-contrast technique for measurement of volume flow rates.

Magnetic resonance (MR) phase-difference methods work well for measuring volumetric flow rates when the vessel diameter is large compared with the in-plane voxel dimensions. For small vessels (eg, coronary arteries), partial-volume effects introduce substantial errors in the measured volume flow rate. To correctly measure flow rates through a voxel, both the fraction of the voxel containing moving spins and the phase shift imparted to those spins must be known. The authors propose a flow measurement method that combines information obtained with both the complex-difference and phase-difference processing techniques and thereby provides the fractional volume occupied by the moving spins and the phase of those spins. The complex-difference flow map method proposed results in improved accuracy of MR phase-contrast flow measurements in the presence of partial-volume effects.

Phase contrast MRI assessment of pedal blood flow

This study attempts to evaluate the reliability of cine phase contrast (PC) flow measurements in the assessment of normal pedal blood flow and quantitation of revascularisation-induced flow changes in patients with end-stage peripheral vascular occlusive disease (PVOD). Oblique axial cine-PC acquisitions were obtained on a 1.5 T MRI system at the level of the talotibial joints in 8 normal subjects on four separate occasions. Subsequently 8 patients with end-stage PVOD were examined before and after surgical revascularisation (bilateral, n = 2; unilateral, n = 6). Measured flow in the trifurcation vessels was highly variable among normal subjects. Total pedal flow ranged from 32 to 183 ml/min (mean 91 ml/min) and was significantly different between the subjects evaluated (P < 0.0001). Measurements in the same subject over time were considerably less variable (P < 0.005). Normal arterial flow patterns were consistently triphasic; those in patients with PVOD were either mono- or biphasic. Pedal flow measured by cine-PC in patients was reduced compared with normal subjects (mean 38.3 ml/min). Flow was slower in symptomatic limbs (26.7 ml/min) compared with asymptomatic ones (48.9 ml/min). Flow increases in revascularised limbs (mean 315%) were significantly different from those observed in non-affected limbs (P < 0.005). The ability to quantitate pedal blood flow and subsequent revascularisation-induced flow increases appears promising for the identification of optimal treatment options and monitoring of treatment results.

Cine phase-contrast MR flow measurements: improved precision using an automated method of vessel detection.

The purpose of this study was to construct a method of vessel edge detection that correctly identifies vessel pixels and to compare the interuser variability of cine phase contrast MR volumetric flow rates obtained with the conventional manual method and an automated method. The automated method was developed based on a magnitude image threshold value and compared with the manual method in a flow phantom (three users) and in velocity images of the portal vein (five users). The threshold value determined from the magnitude image was applied to a region of interest surrounding the vessel of interest on the magnitude image to construct a vessel edge detection mask m(x,y). The velocity images were then multiplied by the mask m(x,y) and volumetric flow rates determined using the identified vessel pixels. In the flow phantom, flow measurements with the magnitude threshold method had significantly less interuser variability compared with the manual method (p < 0.01) and were within 10% (mean 6.0%) of the actual flow versus 35% (mean 18.6%) with the manual method. Regarding flow measurements in the portal vein of six volunteers, the magnitude threshold method was significantly more precise (p < 0.01) than the manual method with a mean standard deviation between the five users of 40.4 +/- 12.9 ml/min (range 22-60 ml/min) and 110.4 +/- 32.7 ml/min (range 70-155 ml/min), respectively. The magnitude threshold method of vessel edge detection developed in this study yields flow measurements that are accurate in the model system and have significantly less interuser variability than the manual method. This method shows promise for improving the precision of cine phase contrast flow measurements.

Evaluation of portal venous hypertension with cine phase-contrast MR flow measurements: high association of hyperdynamic portal flow with variceal hemorrhage.

Thirty-two patients with portal venous hypertension and endoscopically proved esophageal varices who were being evaluated for possible liver transplantation were studied with cine phase-contrast magnetic resonance (MR) imaging. Flow measurements in the main portal vein were obtained and associated with the presence of variceal hemorrhage within 2 years before the MR examination. Low (hypodynamic) flow was present in 22 patients, while high (hyperdynamic) flow was present in 10 patients. The presence of variceal hemorrhage was significantly associated with a high portal venous flow rate (P = .001), high variceal grade (P = .030), and Child class A or B (P = .003); however, only portal venous flow (P = .006) and variceal grade (P = .044) were found to be associated with variceal hemorrhage in a multiple logistic regression analysis. The portal venous flow rate was significantly higher among patients with Child class A or B disease compared with those with class C disease (median, 24.6 vs 8.0 mL/min.kg; P = .004).

Volumetric flow rates in the portal venous system: measurement with cine phase-contrast MR imaging.

The purposes of this study were to (1) validate the accuracy of cine phase-contrast MR flow measurements within the portal vein, (2) develop a suitable protocol for using this method to measure volumetric flow rate in the portal venous system, and (3) use this protocol, with Doppler sonography as a reference, to measure portal venous flow in healthy volunteers and in patients with protal venous hypertension. Flow determinations were obtained in a model of fluid movement approximating blood-flow conditions in the portal venous system. A suitable protocol was based on consideration of the theoretical effects of (1) spatial resolution, (2) obliquity of the imaging plane to the direction of flow, and (3) signal-to-noise ratio of the signed quantitative velocity images (in three volunteers) on the accuracy and precision of flow measurements. This protocol was used to obtain cine phase-contrast MR images of the portal venous system in five volunteers and six patients. Values obtained with a flow phantom showed good accuracy of cine phase-contrast-measured vs actual volumetric flow rate (r = .995; p = .0001; MR rate = [0.94 x actual rate] + 65.6 ml/min; standard error of the y estimate = 67.3 ml/min). Velocity encoding and section thickness substantially influenced the signal-to-noise ratio of the velocity images, whereas flip angle and matrix size had only minimal effect. In volunteers and patients, portal volumetric flow rates determined by using MR images and Doppler sonography showed good correlation (r = .94; p = .0003). Our results indicate that cine phase-contrast MR imaging is a practical noninvasive method for measuring volumetric flow rates in the portal venous system.

5093620 Encoding for NMR phase contrast flow measurement

The magnetization response of hyperpolarized 3He gas to a steady-state free precession (SSFP) sequence was simulated using matrix product operators. The simulations included the effects of flip angle (α), sequence timings, resonant frequency, gas diffusion coefficient, imaging gradients, T1 and T2. Experiments performed at 1.5 T, on gas phantoms and with healthy human subjects, confirm the predicted theory, and indicate increased SNR with SSFP through use of higher flip angles when compared to optimized spoiled gradient echo (SPGR). Simulations and experiments show some compromise to the SNR and some point spread function broadening at high α due to the incomplete refocusing of transverse magnetization, caused by diffusion dephasing from the readout gradient. Mixing of gas polarization levels by diffusion between slices is also identified as a source of signal loss in SSFP at higher α through incomplete refocusing. Nevertheless, in the sample experiments, a SSFP sequence with an optimized flip angle of α = 20°, and 128 sequential phase encoding views, showed a higher SNR when compared to SPGR (α = 7.2°) with the same bandwidth. Some of the gas sample experiments demonstrated a transient signal response that deviates from theory in the initial phase. This was identified as being caused by radiation damping interactions between the large initial transverse magnetization and the high quality factor (Q = 250) birdcage resonator. In 3He NMR experiments, performed without imaging gradients, diffusion dephasing can be mitigated, and the effective T2 is relatively long (⩾1 s). Under these circumstances the SSFP sequence behaves like a CPMG sequence with sin(α/2) weighting of SNR. Experiments and simulations were also performed to characterize the off-resonance behaviour of the SSFP HP 3He signal. Characteristic banding artifacts due to off-resonance harmonic beating were observed in some of the in vivo SSFP images, for instance in axial slices close to the diaphragm where B0 inhomogeneity is highest. Despite these artifacts, a higher SNR was observed with SSFP in vivo when compared to the SPGR sequence. The trends predicted by theory of increasing SSFP SNR with increasing flip angle were observed in the range α = 10–20° without compromise to image quality through blurring caused by excessive k-space filtering.