Phase-contrast Flow Measurements Research Articles

MRI after implantation of a novel femoral closure device following intra-arterial catheterisation: implications for subsequent contrast-enhanced MR angiography

To assess MRI safety aspects and artefacts of a novel femoral artery closure device during contrast-enhanced MR angiography in patients following intra-arterial catheterisation. Ten consecutive patients underwent MRI within 24 h of coronary angiography and placement of a femoral artery closure device. We used a T2-weighted gradient-echo MRI sequence to measure the device-related artefact size in comparison with a phantom image, phase-contrast flow measurement proximal to, at the level of and distal to the device to quantify potential differences in flow velocity and contrast-enhanced 3D gradient-echo MR angiography to differentiate potential femoral artery stenosis from device-related artefacts. The mean size of the oval-shaped artefact was 8.4 x 6.6 mm (+/-1.0 x 0.8 mm) and was almost identical to the maximum artefact size of the phantom measurement (8.3 x 5.7 mm). Device placement did not result in an increased peak velocity (proximal 69 +/- 23 cm/s, at the level of 64 +/- 11 cm/s and distal to the device 63 +/- 12 cm/s, p = 0.67). The mean artefact penetration into the vessel lumen was 0.5 +/- 0.5 mm (percentage vessel narrowing 7.0 +/- 6%; range 0-16%). The MR conditional StarClose femoral artery closure device was used safely within 24 h of deployment at 1.5 T. Despite clip-related artefacts MR angiography will allow for easy differentiation of clip-related artefacts from high-grade atherosclerotic stenosis.

Accelerated phase‐contrast MR imaging: Comparison of k‐t BLAST with SENSE and doppler ultrasound for velocity and flow measurements in the aorta

To evaluate differences in velocity and flow measurements in the aorta between accelerated phase-contrast (PC) magnetic resonance imaging (MRI) using SENSE and k-t BLAST and in peak velocity to Doppler ultrasound. Two-dimensional PC-MRI perpendicular to the ascending and descending aorta was performed in 11 volunteers using SENSE (R = 2) and k-t BLAST (2-, 4-, 6-, and 8-fold). Peak velocity, mean velocity, and stroke volume of the accelerated PC-MRI experiments were correlated. Peak velocities were compared to Doppler ultrasound. All acceleration techniques showed significant correlations for peak velocity with Doppler ultrasound. However, k-t BLAST 6 and 8 showed a significant underestimation. Strong correlations between SENSE and k-t BLAST were found for all three parameters. Significant differences in peak velocity were found between SENSE and all k-t BLAST experiments, but not for 2-fold k-t BLAST in the ascending aorta, and 2- and 4-fold k-t BLAST in the descending aorta. For mean velocity no significant differences were found. Stroke volume showed significant differences for all k-t BLAST experiments in the ascending and for 6- and 8-fold k-t BLAST in the descending aorta. Peak velocity of accelerated PC-MRI correlated with CW Doppler measurements, but high k-t BLAST acceleration factors lead to a significant underestimation. SENSE with R = 2 and 2-fold k-t BLAST are most highly correlated in phase-contrast flow measurements.

Respiratory self‐gating for free‐breathing abdominal phase‐contrast blood flow measurements

To demonstrate the feasibility of using a free-breathing (FB) respiratory self-gated (RSG) approach for abdominal phase-contrast (PC) blood flow measurements. PC-magnetic resonance imaging (MRI) flow measurements were performed within the right renal artery, common hepatic artery, and main portal vein during breath-hold (BH) and FB with both signal averaging and RSG in eight healthy volunteers. The resultant images were qualitatively scored by two independent reviewers blinded to acquisition techniques. Blood flow volume and cross-sectional vessel size measurements were compared for three techniques. The overall efficiency for the RSG-PC sequence was 38.9% +/- 4.7%. Images acquired with RSG effectively mitigated respiratory motion artifacts, which were clearly evident within FB signal-averaged images. RSG produced similar image quality to that of BH techniques (P > 0.146) and resulted in similar vessel size measurements (P = 0.694). Flow results for both FB RSG and signal-averaged reconstructions correlated well with BH flow measurements (r = 0.97 and 0.92, P < 0.001). However, only the RSG methods demonstrated excellent absolute agreement with BH-PC flow measurements (P = 0.600), with signal-averaged methods resulting in significant overestimations. RSG methods can limit respiratory motion artifacts to reduce flow measurement inaccuracies during free-breathing PC measurements in the abdomen.

Effect of flow angle and flow profile on phase contrast flow measurements: overestimation at extreme angles and skewed profiles

Flow measurements derived from phase contrast velocity mapping (PC-MRI) have become an important part of pediatric cardiology, allowing accurate quantification of shunts, valve regurgitation, and cardiac output. Clinicians generally try to align the imaging plane orthogonal to flow. However, while flow velocities decrease by the cosine of the angle from orthogonal, the area should ideally increase by the same rate. There are situations in which it may be desirable to quantify flow in a vessel on an image that was not intended during the acquisition. In addition, in some flow regimes with highly skewed flow it is sometimes difficult to align the imaging plane with flow. Understanding the settings in which PC-MRI is accurate is important to the evaluation of congenital heart disease.

Noninvasive quantitative measurement of myocardial and whole-body oxygen consumption using MRI: initial results

The purpose of this study was to investigate the feasibility of a noninvasive approach that combines magnetic resonance imaging (MRI) oximetry and flow measurement to obtain the oxygen consumption in the myocardium and in the whole body. Thirteen healthy male volunteers [mean (+/-S.D.) age: 35+/-7 years] underwent this MR study, which included myocardial oxygen consumption (MVO(2)) measurements in 11 subjects and whole-body oxygen consumption (VO(2)) measurements in 8 subjects. In six subjects, both measurements were obtained. Five subjects had repeated MRI measurements of global MVO(2) in order to verify the reproducibility of this approach. The protocol included in vitro blood sample T(2)-%O(2) calibration, coronary sinus (CS) and main pulmonary artery (MPA) T(2) and phase contrast flow measurement and left ventricular (LV) mass calculation. Based on Fick's law, a global measurement of LV MVO(2) and whole-body VO(2) using MRI was feasible. The MVO(2) values were 11+/-3 ml/min per 100 g LV mass. For repeated measurements, differences in MVO(2) of 1 ml/min per 100 g LV mass appear detectable. The whole-body VO(2) values were 3.8+/-0.8 ml/min/kg body weight. MRI techniques that combine CS and MPA T(2), flow and LV mass measurements to quantify MVO(2) and whole-body VO(2) noninvasively in healthy subjects appear feasible, based on their correspondence to previously published work.

Validation of Magnetic Resonance Phase-Contrast Flow Measurements in the Main Pulmonary Artery and Aorta Using Perivascular Ultrasound in a Large Animal Model

Phase-contrast flow magnetic resonance imaging (pc-MRI) measurements are an established technique for noninvasive assessment of hemodynamics. However, in vivo validation data on the accuracy of pc-flow especially for nonphysiologic hemodynamic conditions are missing. The goal of our work was 1) to validate pc-flow with perivascular ultrasound in different hemodynamic conditions in the main pulmonary artery (MPA) and ascending aorta (AO) and 2) to investigate the relation between pc-MRI and invasive pressure measurements. Five healthy pigs with opened chest were investigated. Ultrasound measurements were performed outside the MRI unit using a detachable MR-table. Parallel to ultrasound flow measurements, invasive pressure measurements were performed. PC-MRI (1.5 T MRI) measurements were done using a FLASH 2-dimensional sequence. First the animals were examined in normotonia, followed by hypertension (infusion of Arterenol) and hypotension (infusion of Sodiumnitropussid). Flow measurements acquired in normotonia were 2.7 +/- 0.6 L/min (ultrasound) and 2.8 +/- 0.6 L/min for pc-MRI (not significantly different, P = 0.17). During hypertonia, the blood flow increased to 3.1 +/- 0.7 and 2.7 +/- 0.8 L/min, respectively (P = 0.01). During hypotension, the blood flow decreased to 1.7 +/- 0.5 and 1.7 +/- 0.5 L/min, respectively (P = 0.7). An excellent linear correlation (taking all measurements together) between the ultrasound and pc-MRI measurements was found (r = 0.89). 95% Limits of intraindividual agreement correspond to relative differences of -36 to 44%. Systolic pressure measurements in the AO were 91 mm Hg (normotonic), 111 mm Hg (hypertonic), and 64 mm Hg (hypotonic) and in the MPA 23 mm Hg (normotonic), 29 mm Hg (hypertonic), and 20 mm Hg (hypotonic). Systolic pressure measurements showed good linear correlation with pc-MRI average flow per minute and peak velocity (AO: r = 0.79, MPA: r = 0.66). MRI pc-flow measurements are a reliable tool for noninvasive assessment of blood flow. Hemodynamic parameters derived by MRI demonstrate good linear correlations with the pressure in the systemic and pulmonary arterial circulation.

Measurement of systolic and diastolic arterial wall shear stress in the ascending aorta

Purpose Wall shear stress (WSS) appears to contribute significantly in the initiation and progression of atherosclerotic disease. The purpose of this work is to present in vivo systolic and diastolic WSS calculations in the human ascending aorta by the application of three straightforward methodologies based on Poiseuille's theory of flow. Materials and methods Blood flow measurements were performed retrospectively in the ascending aorta of 20 non-atherosclerotic patients using phase-contrast MRI. WSS calculations were performed assuming Poiseuille's theory of flow based on average flow volume, average flow velocity and maximum flow velocity. Systolic and diastolic WSS values were calculated and compared with the calculated maximum and minimum values of WSS throughout the cardiac cycle. Results Systolic WSS values calculated by average flow volume, average flow velocity and maximum flow velocity were similar (0.4 ± 0.2 N/m 2, 0.4 ± 0.3 N/m 2and 0.4 ± 0.2 N/m 2, respectively). Diastolic WSS values calculated by maximum flow velocity were significantly higher (11.6 ± 7.0 × 10 −2 N/m 2) compared to values calculated by average flow volume (0.3 ± 0.9 × 10 −2 N/m 2) and average flow velocity (0.3 ± 1.0 × 10 −2 N/m 2). Comparison of systolic and diastolic WSS values with maximum and minimum WSS values showed that time instances of maximum and minimum blood flow velocities do not coincide with time instances of maximum and minimum blood flow volume. Conclusion In vivo calculation of WSS in the ascending aorta is feasible by phase-contrast MRI flow measurements and straightforward methodologies based on Poiseuille's theory of flow. However, measurements based on maximum flow velocity show larger deviations compared to measurements based on mean flow volume or mean flow velocity.

The Quantification of Cerebral Blood Flow by Phase Contrast MRA: Basics and Applications

Phase-contrast magnetic resonance (PCMRA) flow quantification can determine vascular velocities and volumetric flow rate (VFR) non-invasively for in vitro and in vivo studies. Recently, the increasing power of MR imaging units and the reduced time for data acquisition and post-processing have led to an increasing number of investigations on the use of phase-contrast flow measurements as an additional source of quantitative functional information in MR imaging. In addition, PCMRA can be added to morphologic MRI sequences, offering the option to correlate flow to morphology based on data generated during one examination. This review discusses the basics of phase-contrast imaging, describing the errors and avoiding methods associated with PCMRA, providing guidelines for flow measurement and data analysis, and presenting the current clinical cerebral applications.

Flow Measurement by Magnetic Resonance: A Unique Asset Worth Optimising

Users and manufacturers of cardiovascular magnetic resonance (CMR) systems have, potentially, an unrivalled asset. Phase contrast mapping of velocities through planes transecting the great arteries should provide the most accurate measurements available of cardiac output, shunt flow, aortic or pulmonary regurgitation and, indirectly, of mitral regurgitation. But the reality is that phase contrast velocity mapping remains under-used, and may have become discredited in the eyes of some CMR users and referring clinicians. Even when appropriate methods of acquisition have been used, there can be inaccuracies of flow measurement on some CMR systems caused by background phase errors due to eddy currents or uncorrected concomitant gradients. Measurements of regurgitant or shunt flow can be seriously affected by these errors which should be minimised or corrected by appropriate hardware and software design. If they have not been, inaccuracies can be detected and corrected by repeating identical velocity acquisitions on a static phantom, and subtracting the corresponding apparent phantom velocities from those of the clinical acquisition. For accurate measurements of aortic regurgitation or mitral inflow, motion tracking and velocity correction with respect to the cyclic displacements of the valves are needed, but few if any commercial systems provide this facility. Measurements of jet velocity pose different challenges, mainly related to the size and placement of voxels relative to a narrow jet. Awareness of the potential problems and concerted efforts towards optimisation are needed from manufacturers and users to make appropriate use of phase contrast flow measurement - a unique strength of cardiovascular magnetic resonance.

Impact of Oxygen Inhalation on the Pulmonary Circulation

Oxygen-enhanced magnetic resonance (MR)-ventilation imaging of the lung is based on the inhalation of a high concentration of oxygen (hyperoxia). However, the effect of hyperoxia on the pulmonary circulation is not yet fully understood. In this study the impact of hyperoxia on the pulmonary circulation was evaluated. Ten healthy volunteers were examined in a 1.5 T MRI system with contrast-enhanced perfusion MRI (saturation recovery 2D turboFLASH) of the lung and phase-contrast flow measurements in the pulmonary trunk. Both measurements were performed breathing room air (RA) and, subsequently, 100% oxygen (15 L/min) (O(2)). The perfusion measurements showed a significant difference between RA and O(2) for the pulmonary blood flow (181 vs. 257 mL/min/100 mL, P = 0.04) and blood volume (14 vs. 21 mL/100 mL, P = 0.008). The mean transit time of the contrast bolus was not changed (P = 0.4) in the dorsal part of the lung, whereas it was significantly prolonged (P = 0.006) in the central part. The mean heart rate during flow measurements breathing RA (67 +/- 11 beats/min) and O(2) (61 +/- 12 beats/min) were not significantly different (P = 0.055). The average cardiac output (pulmonary trunk) was not significantly lower while breathing O(2) (RA: 5.9 vs. O(2): 5.5 L/min, P = 0.054). Hyperoxia causes a significant increase and redistribution of the pulmonary perfusion, whereas it leads to a not significant decrease in cardiac output. Thus, for MR-perfusion and MR-flow measurements oxygen inhalation should be avoided, if possible. In the context of oxygen-enhanced MR-ventilation imaging of the lung the contribution of this effect needs to be further evaluated.

Flussmessungen in der kardialen MRT

Phase-contrast flow measurements have become an established method in cardiac MRI. The quantification of intra- and extracardiac shunt volumes as well as the evaluation of valvular disease and aortic coarctation have proved their clinical usefulness. There are some rules that have to be followed when performing and analyzing phase-contrast flow measurements. With these rules in mind, quantitative phase-contrast flow measurements are a reliable and precise method for clinical use of cardiac MRI.

Value of MR phase-contrast flow measurements for functional assessment of pulmonary arterial hypertension

Goals of our study were to compare the pulmonary hemodynamics between healthy volunteers and patients with pulmonary arterial hypertension (PAH) and correlate MR flow measurements with echocardiography. Twenty-five patients with PAH and 25 volunteers were examined at 1.5 T. Phase-contrast flow measurements were performed in the ascending aorta and pulmonary trunk, resulting in the following parameters: peak velocity (cm/s), average blood flow (l/min), time to peak velocity (ms), velocity rise gradient and pulmonary distensibility (cm(2)). The bronchosystemic shunt was calculated. In PAH patients transthoracic echocardiography and right-heart catheterization (RHC) served as the gold standard. In comparison to volunteers, the PAH patients showed significantly reduced pulmonary velocities (P = 0.002), blood flow (P = 0.002) and pulmonary distensibility (P = 0.008). In patients, the time to peak velocity was shorter (P<0.001), and the velocity rise gradient was steeper (P = 0.002) than in volunteers. While in volunteers the peak velocity in the aorta was reached earlier, it was the reverse in patients. Patients showed a significant bronchosystemic shunt (P = 0.01). No meaningful correlation was found between MRI measurements and echocardiography or RHC. MRI is a feasible technique for the differentiation between PAH and volunteers. Further studies have to be conducted for the absolute calculation of pressure estimates.

MR flow measurements for assessment of the pulmonary, systemic and bronchosystemic circulation: Impact of different ECG gating methods and breathing schema

Purpose Different ECG gating techniques are available for MR phase-contrast (PC) flow measurements. Until now no study has reported the impact of different ECG gating techniques on quantitative flow parameters. The goal was to evaluate the impact of the gating method and the breathing schema on the pulmonary, systemic and bronchosystemic circulation. Material and methods Twenty volunteers were examined (1.5 T) with free breathing phase-contrast flow (PC-flow) measurements with prospective ( free- prospective) and retrospective ( free- retrospective) ECG gating. Additionally, expiratory breath-hold retrospective ECG gated measurements ( bh- retrospective) were performed. Blood flow per minute; peak velocity and time to peak velocity were compared. The clinically important difference between the systemic and pulmonary circulation (bronchosystemic shunt) was calculated. Results Blood flow per minute was lowest for free- prospective (6 l/min, pulmonary trunc) and highest for bh- retrospective measurements (6.9 l/min, pulmonary trunc). No clinically significant difference in peak velocity was assessed (82–83 cm/s pulmonary trunc, 109–113 cm/s aorta). Time to peak velocity was shorter for retro-gated free- retrospective and bh- retrospective than for pro-gated free- prospective. The difference between systemic and pulmonary measurements was least for the free- retrospective technique. Conclusion The type of gating has a significant impact on flow measurements. Therefore, it is important to use the same ECG gating method, especially for follow-up examinations. Retrospective ECG gated free breathing measurements allow for the most precise assessment of the bronchosystemic blood flow and should be used in clinical routine.

Partielle k-Raum-Auslesung mit Null-Interpolation bei Phasenkontrast-Flussmessungen: In-vivo- und In-vitro-Validierung

To validate the technique of partial k-space sampling and zero filling with phase-contrast flow measurements as compared to measurements with full k-space sampling. In vitro: A laminar flow phantom was utilized to evaluate the effect of partial k-space sampling on the accuracy, precision and signal-to-noise ratio of phase-contrast flow measurements. In vivo: The effect of partial k-space sampling on the quantification of cardiac output (n = 40 patients) and the duration of the scan were evaluated in the ascending aorta (n = 37) and pulmonary trunk (n = 34) in a prospective study. Partial k-space sampling resulted in an increase in the SNR by 2 % in vitro. The precision was altered by less than 1 %. Flow volumes were systematically overestimated by 3.5 %. No significant differences were found in the in vivo measurements of cardiac output. The scan duration was reduced by 34 % by utilizing partial k-space sampling. Partial k-space sampling can be used to reduce scan time without a significant decrease in the accuracy or precision of phase-contrast flow measurements in large arteries.

Limited Flow Reserve in Non-Obstructed Bypass Grafts Supplying Infarcted Myocardium: Implications for Cardiovascular Magnetic Resonance Imaging Protocols

We evaluated flow reserve in non-obstructed bypass grafts supplying infarcted and noninfarcted myocardium. Bypass grafts were examined by phase-contrast flow measurements and myocardial viability was assessed with late enhancement technique. Flow reserve was higher in bypasses supplying normal myocardium compared to those supplying infarcted myocardium (2.9 vs. 1.5, p<.0001). This difference remained significant after adjusting for co-variables. Bypass grafts supplying infarcted myocardium were more likely to have lower flow reserve than those supplying normal myocardium (flow reserve < or =2, 84% vs 18%, p = .0003). Flow reserve is reduced in non-stenosed bypasses supplying infarcted myocardium, likely due to altered microcirculation. Thus, cardiovascular magnetic resonance based bypass assessment must include myocardial viability testing.

Current Awareness in NMR in Biomedicine

Current Awareness in NMR in Biomedicine

MR-Angiographie und Flussreservenbestimmung nach minimalinvasiver direkter Koronararterien-Bypass(MIDCAB)-Operation der linken Arteria mammaria interna im Vergleich zur Mehrzeilen-CT

To evaluate graft patency, flow and flow reserve in patients with minimal invasive direct coronary artery bypass (MIDCAB) of internal mammary artery (IMA) grafts using a combined MR protocol with phase-contrast technique and MR angiography. At a 1.5T Magnetom Sonata (SIEMENS), 19 symptomatic (angina CCS I-III, intermittent thoracic discomfort, scar disorders) patients (59.9 +/- 7.9 years old) with 19 left internal mammary artery (LIMA) grafts implanted in minimal invasive technique were examined 6.9 +/- 1.5 years post surgery. Contrast enhanced MR angiography (TR 2.5 ms, TE 1 ms, flip angle 20 (o), spatial resolution 1.4 x 0.9 x 1.0 mm(3), breath hold technique, no ECG-triggering, 25 ml Gd-DTPA) was performed to assess bypass patency. Phase-contrast flow measurements with retrospective gating (TR 41 msec, TE 3.2 msec, flip angle 30 degrees , spatial resolution 1.1 x 1.1 x 5 mm(3), temporal resolution 42 msec, venc 90 cm/sec) were applied in the IMA grafts at rest and after stress induction with dipyridamole (0.56 mg/kg/BW). For comparison, graft patency was evaluated by multidetector-row computed tomography (16-row CT). In 9 patients a selective catheter angiography was performed. MIDCAB grafts were occluded in 4/19 patients. In 4 patients the anastomosis to LAD was highly stenotic (> 70 %) at MDCT (2 experienced investigators in consensus reading). In MRA 9 grafts could be delineated completely including the distal anastomosis to LAD (47 %). In 9 patients the distal part could not be evaluated. In patients with patent grafts (MDCT), a significant improvement of graft flow (at rest 75.4 +/- 33.3 ml/min; after stress 202.7 +/- 49.6; P < 0.002) and flow reserve (patent grafts 3.0 +/- 1.1; stenotic grafts 1.5 +/- 0.2, P < 0.02; occluded grafts 0.9 +/- 0.2, P < 0.01) after stress induction was detected. Diastolic-to-systolic peak velocity ratios (D/S-PVR) at baseline were not significant between patent and stenotic grafts. Mean flow at baseline and after stress induction and flow reserve show a high sensitivity (91/92 /83 %) and specificity (86 /100/83 %) for detection of graft stenosis. MR angiography combined with flow reserve measurements could distinguish between occluded/stenotic and patent grafts in all MIDCAB grafts. MR imaging allows combined assessment of bypass patency and flow with flow reserve in patients after MIDCAB. The protocol of this study is applicable for the evaluation of graft patency in patients after revascularization.

Anteil des frühen systolischen Flussanstiegs am antegrad fließenden Gesamtvolumen bei Phasenkontrast-Flussmessungen in Atemanhaltetechnik

Purpose: To evaluate the contribution of early systole for the assessment of antegrade aortic flow volume by breath-hold velocity-encoded magnetic resonance (MR) flow measurements. Materials and Methods: Expiratory breath-hold fast low-angle shot (FLASH) phase-contrast flow measurements (temporal resolution tRes 61 msec, shared phases) perpendicular to the proximal ascending aorta and short axis true fast imaging with steady-state precession (TrueFISP) cine MR ventriculometry (tRes 34.5 msec) were performed in ten healthy male volunteers on a 1.5 T MR system (Sonata, Siemens Medical Solutions). Antegrade aortic flow volume (AFV) and left ventricular stroke volume (LV-SV) were evaluated using Argus Ventricular Function and Argus Flow Software, version MR 2002B (Siemens Medical Solutions). A beta release of Argus Flow MR 2004A allowed interpolation of the flow up-slope during early systole to the preceding R-wave trigger. The respective intraindividual median differences between the AFV of each flow evaluation and LV-SV as well as between both AFV measurements were calculated and compared using the sign test for paired samples. Results: Non-interpolated AFV significantly deviated from LV-SV (p=0.006), underestimating the latter by 13.1 mL (13%). Interpolating aortic flow during early systole significantly increased AFV by 10.8 mL (13 %) compared to the flow evaluation which did not take early systole into account (p = 0.006). AFV with interpolation of early systolic flow agreed well with LV-SV (median difference -3.0mL or -3%, respectively), and no significant difference between these measurements was found (p = 1.0). Conclusion: Flow during early systole contributes substantially to total forward flow volume in the ascending aorta. Interpolation of the early systolic up-slope is therefore recommended for the evaluation of breath-hold phase-contrast flow measurements.

In vitro validation of phase‐contrast flow measurements at 3 T in comparison to 1.5 T: Precision, accuracy, and signal‐to‐noise ratios

To evaluate the signal-to-noise ratio (SNR), precision, and accuracy of phase-contrast flow measurements at 3 T with the help of an in vitro model and to compare the results with data from two 1.5-T scanners. Using an identical setup of a laminar flow model and sequence parameters, measurements were done at one 3-T and at two 1.5-T systems. Precision, accuracy, and SNR were obtained for velocity encodings ranging from 55 up to 550 cm(-1). SNRs were calculated from the magnitude as well as the flow encoded images. Precision and accuracy for the in vitro flow model were similarly high in all scanners with no significant difference. For velocity encodings from 55 cm(-1) up to 550 cm(-1), the SNR in magnitude as well as phase encoded images of the 3-T measurements was approximately 2.5 times higher than the SNR obtained from the two 1.5-T systems. Even without optimization for the 3-T environment, flow measurements show the same high accuracy and precision as is known from clinical 1.5-T scanners. The superior SNR at 3 T will allow further improvements in temporal and spatial resolution. This will be of interest for small-size vessels like coronary arteries or for slow diastolic flow patterns.

Perioperative monitoring of flow and patency in native and grafted internal mammary arteries using a combined MR protocol

The objective of this study was to evaluate graft flow (f) and patency (p) in patients with internal mammary artery (f,p) and venous (p only) grafts using a combined MR protocol with phase-contrast technique and MR angiography. 42 patients with 42 left internal mammary artery (LIMA) and 63 venous grafts were examined pre and 6 months post coronary artery bypass graft (CABG) surgery. Phase-contrast flow measurements were applied to the IMA. Post-operatively, a contrast enhanced MR angiogram was performed to assess bypass patency. LIMA/venous grafts were occluded in 3/42 and 13/63, respectively. Flow in LIMA decreased from 19.4+/-10.4 ml min(-1) m(-2) pre-operatively to 13.4+/-9.7 ml min(-1) m(-2) post-operatively (p<0.002). In contrast, flow in the native right IMA increased from 17.6+/-8.7 ml min(-1) m(-2) pre-operatively to 24.8+/-9.0 ml min(-1) m(-2) post-operatively (p<0.001). MRI allows a combined assessment of bypass patency and flow. This study protocol may be applicable to perioperative follow-up studies in patients after CABG surgery.