Quantification of coronary artery volume flow rate using fast velocity-encoded cine MR imaging

Non-contrast coronary magnetic resonance angiography: current frontiers and future horizons

Coronary magnetic resonance angiography (coronary MRA) is advantageous in its ability to assess coronary artery morphology and function without ionizing radiation or contrast media. However, technical limitations including reduced spatial resolution, long acquisition times, and low signal-to-noise ratios prevent it from clinical routine utilization. Nonetheless, each of these limitations can be specifically addressed by a combination of novel technologies including super-resolution imaging, compressed sensing, and deep-learning reconstruction. In this paper, we first review the current clinical use and motivations for non-contrast coronary MRA, discuss currently available coronary MRA techniques, and highlight current technical developments that hold unique potential to optimize coronary MRA image acquisition and post-processing. In the final section, we examine the various research-based coronary MRA methods and metrics that can be leveraged to assess coronary stenosis severity, physiological function, and atherosclerotic plaque characterization. We specifically discuss how such technologies may contribute to the clinical translation of coronary MRA into a robust modality for routine clinical use.

Numerical analysis of the pressure drop across highly-eccentric coronary stenoses: application to the calculation of the fractional flow reserve

Background

Fractional flow reverse (FFR) is the gold standard assessment of the hemodynamic significance of coronary stenoses. However, it requires the catheterization of the coronary artery to determine the pressure waveforms proximal and distal to the stenosis. On the contrary, computational fluid dynamics enables the calculation of the FFR value from relatively non-invasive computed tomography angiography (CTA).

Methods

We analyze the flow across idealized highly-eccentric coronary stenoses by solving the Navier–Stokes equations. We examine the influence of several aspects (approximations) of the simulation method on the calculation of the FFR value. We study the effects on the FFR value of errors made in the segmentation of clinical images. For this purpose, we compare the FFR value for the nominal geometry with that calculated for other shapes that slightly deviate from that geometry. This analysis is conducted for a range of stenosis severities and different inlet velocity and pressure waveforms.

Results and conclusions

The errors made in assuming a uniform velocity profile in front of the stenosis, as well as those due to the Newtonian and laminar approximations, are negligible for stenosis severities leading to FFR values around the threshold 0.8. The limited resolution of the stenosis geometry reconstruction is the major source of error when predicting the FFR value. Both systematic errors in the contour detection of just 1-pixel size in the CTA images and a low-quality representation of the stenosis surface (coarse faceted geometry) may yield wrong outcomes of the FFR assessment for an important set of eccentric stenoses. On the contrary, the spatial resolution of images acquired with optical coherence tomography may be sufficient to ensure accurate predictions for the FFR value.

Cardiovascular magnetic resonance phase contrast imaging

Cardiovascular magnetic resonance (CMR) phase contrast imaging has undergone a wide range of changes with the development and availability of improved calibration procedures, visualization tools, and analysis methods. This article provides a comprehensive review of the current state-of-the-art in CMR phase contrast imaging methodology, clinical applications including summaries of past clinical performance, and emerging research and clinical applications that utilize today's latest technology.

A novel CX3CR1 antagonist eluting stent reduces stenosis by targeting inflammation

We evaluated the therapeutic efficacy of a novel drug eluting stent (DES) inhibiting inflammation and smooth muscle cell (SMC) proliferation. We identified CX3CR1 as a targetable receptor for prevention of monocyte adhesion and inflammation and in-stent neointimal hyperplasia without interfering with stent re-endothelization. Efficacy of AZ12201182 (AZ1220), a CX3CR1 antagonist was evaluated in inhibition of monocyte attachment in vitro. A prototype AZ1220 eluting PLGA-based polymer coated stent developed with an optimal elution profile and dose of 1 μM/stent was tested over 4 weeks in a porcine model of coronary artery stenting. Polymer coated stents without AZ1220 and bare metal stents were used as controls. AZ1220 inhibited monocyte attachment to CX3CL1 in a dose dependent manner. AZ1220 eluted from polymer coated stents in an ex vivo flow system retained bioactivity in inhibiting monocyte attachment to CX3CL1. At 4 weeks following deployment, AZ1220 eluting stents significantly reduced (∼60%) in-stent stenosis compared to both bare metal and polymer only coated stents and markedly reduced peri-stent inflammation and monocyte/macrophage accumulation without affecting re-endothelization. Anti-CX3CR1 drug eluting stents potently inhibited in-stent stenosis and may offer an alternative to mTOR targeting by current DES, specifically inhibiting polymer-induced inflammatory response and SMC proliferation, while retaining an equivalent re-endothelization response to bare metal stents.

Faster flow quantification using sensitivity encoding for velocity-encoded cine magnetic resonance imaging: in vitro and in vivo validation

PURPOSE

To test the agreement between conventional and sensitivity-encoded (SENSE) velocity encoded cine (VEC) MRI in a flow phantom and in subjects with congenital and acquired heart disease.

MATERIALS AND METHODS

Flow measurements were performed in a 1.5 T scanner using a segmented k-space VEC MRI sequence and then repeated with a SENSE factor of 2. The flow phantom used a piston pump to generate physiologic arterial waveforms (0.5-4.9 L/min). In the subjects, flow measurements were performed in the ascending aorta (N = 33) and/or the main pulmonary artery (N = 24).

RESULTS

Utilization of SENSE reduced the scan time by 50%. In the phantom, measurements without and with SENSE agreed closely with a mean difference of 0.01 +/- 0.08 L/min or 0.12% +/- 3.8% (P = 0.68). In the subjects, measurements without and with SENSE also agreed closely with a mean difference of 0.08 +/- 0.36 L/min or 1.3% +/- 7.2% (P = 0.08). Compared with standard imaging, the use of SENSE reduced the signal-to-noise ratio (SNR) by 28% in the phantom (N = 10) and 27% in vivo (N = 22).

CONCLUSION

VEC MRI flow measurements with a SENSE factor of 2 were twice as fast and agreed closely with the conventional technique in vitro and in vivo. VEC MRI with SENSE can be used for rapid and reliable quantification of blood flow.

Magnetic resonance imaging for ischemic heart disease

Cardiac MRI has long been recognized as an accurate and reliable means of evaluating cardiac anatomy and ventricular function. Considerable progress has been made in the field of cardiac MRI, and cardiac MRI can provide accurate evaluation of myocardial ischemia and infarction (MI). Late gadolinium (Gd)-enhanced MRI can clearly delineate subendocardial infarction, and the assessment of transmural extent of infarction on late enhanced MRI has been shown to be useful in predicting functional recovery of dysfunctional myocardium in patients after MI. Stress first-pass contrast-enhanced (CE) myocardial perfusion MRI can be used to detect subendocardial ischemia, and recent studies have demonstrated the high diagnostic accuracy of stress myocardial perfusion MRI for detecting significant coronary artery disease (CAD). Free-breathing, whole-heart coronary MR angiography (MRA) was recently introduced as a method that can provide visualization of all three major coronary arteries within a single three-dimensional (3D) acquisition. With further improvements in MRI techniques and the establishment of a standardized study protocol, cardiac MRI will play a pivotal role in managing patients with ischemic heart disease.

Zero filled partial fourier phase contrast MR imaging: in vitro and in vivo assessment

PURPOSE

To validate partial Fourier phase contrast magnetic resonance (PC MR) with full number of excitation (NEX) PC MR measurements in vitro and in vivo.

MATERIALS AND METHODS

MR flow measurements were performed using a partial Fourier and a full NEX PC MR sequence in a flow phantom and in 10 popliteal and renal arteries of 10 different healthy volunteers. Average velocity, peak velocity, and flow results were calculated and compared with regression analysis.

RESULTS

Excellent correlations in average velocities (r = 0.99, P < 0.001), peak velocities (r = 0.99, P < 0.001), and flow rates (r = 0.98, P < 0.001) were demonstrated in vitro between the two different acquisitions. For the popliteal arteries there was excellent correlation between peak velocities for both acquisitions (r = 0.98, P < 0.0001); the correlation of average velocity measurements when using all data points in the cardiac cycle for all volunteers was 0.96 (P < 0.001). For the renal arteries the same comparison resulted in a good correlation for average velocity (0.93, P < 0.001) and peak velocity measurements (r = 0.91, P = 0.002), although the correlation coefficient for flow rates was 0.88 (P = 0.004). Blurring of the vessel margins was consistently observed on magnitude images acquired with the partial Fourier method, causing overestimation of the vessel area and some error in the flow measurements.

CONCLUSION

Partial Fourier PC MR is able to provide comparable average and peak velocity values when using 1 NEX PC MRI as a reference.

MR-based coronary artery blood velocity measurements in patients without coronary artery disease

To evaluate the feasibility of MR-based coronary blood velocity measurements (MRvenc) in patients without coronary artery disease (CAD). Eighty-three patients with angiographically excluded CAD received MRvenc of the proximal segments of both coronary arteries (CAs). Using a retrospectively ECG-gated breath-hold phase-contrast FLASH sequence with high temporal resolution, flow data were technically acquirable in 137/166 (83%) CAs. Quantification and analysis of blood velocities in systole and diastole of both CAs were performed. Biphasic velocity profiles were found in 83/100 CAs. Median systolic and diastolic velocities differed significantly in LCA (19 cm/s, 24 cm/s; P<0.0001) and RCAs (14 cm/s, 16 cm/s; P<0.01). The diastolic/systolic velocity ratio was calculated in LCAs and RCAs with a median of 1.3 and 1.1, respectively. The velocity profiles of the remaining CAs were monophasic (17 CAs) or revealed severe alterations of the physiologic velocity profile with reduced flow undulations and steady velocities (37 CAs). Optimized clinical MRvenc is feasible to quantify blood velocities in the CAs. Potential indications are (1) non-invasive monitoring of patients after aortic valve reconstruction as well as (2) detection of asymptomatic CAD patients.

Coronary artery fistula in a patient with pulmonary atresia and tricuspid atresia clinical and MRI findings

The MRI findings of a case of coronary artery fistula occurring in a patient with pulmonary atresia and tricuspid atresia is presented.

Magnetic resonance measurement of coronary blood flow

Magnetic resonance (MR) flow measurement in the coronary artery can be achieved with either a breath-hold acquisition or a respiration-triggered acquisition. MR measurements of cardiac output are significantly depressed during breath-holding at deep inspiration, but the advantage is that the breath-hold method requires less scan time. Blood flow in the coronary sinus reflects the global myocardial blood flow because it represents approximately 96% of the total myocardial blood flow of the left ventricle (LV). If blood flow in the coronary sinus is measured with phase-contrast cine magnetic resonance imaging (MRI) and LV myocardial mass is measured with cine MRI, both the total myocardial blood flow and the average coronary blood flow per gram of myocardial mass can be quantified. Coronary flow reserve with volumetric MR flow measurement is measured to be within 4.2-5.0-fold. The noninvasive MR measurement of coronary flow reserve has been shown to be useful in identifying the functional significance of stenoses in the left anterior descending artery. The sensitivity and specificity of MR coronary flow velocity reserve for identifying stenosis of 70% or greater in the left main or left anterior descending artery were 100% and 83%, respectively. The MR quantification of total coronary blood flow and coronary blood flow per gram of myocardial mass seems to be an ideal method for evaluating coronary hemodynamics and may be useful in evaluating endothelial dysfunction of the coronary circulation.

Blood Flow in Coronary Artery Bypass Vein Grafts: Volume versus Velocity at Cardiovascular MR Imaging

Forty-nine patients with previous bypass surgery underwent coronary angiography and cardiovascular magnetic resonance (MR) imaging of single-vein bypass grafts. Volume flow and velocity analyses were performed and compared on MR velocity maps. Bland-Altman analysis showed close agreement between the two types of analysis. Comparison of areas under the receiver operating characteristic curve revealed no significant differences between the analyses for detection of stenoses of 70% or greater. Diagnostic accuracy for volume flow and velocity parameters was 92% and 93%, respectively. Velocity analysis appears to be the preferred method, because it is less time-consuming and has a similar diagnostic accuracy to volume flow analysis.

Role of MRI in clinical cardiology

Rapid progress has been made in cardiac MRI (CMRI) over the past decade, which has firmly established it as a reliable and clinically important technique for assessment of cardiac structure, function, perfusion, and myocardial viability. Its versatility and accuracy is unmatched by any other individual imaging modality. CMRI is non-invasive and has high spatial resolution and avoids use of potentially nephrotoxic contrast agent or radiation. It has been extensively studied against other established non-invasive imaging modalities and has been shown to be superior in many scenarios, particularly with respect to assessment of cardiac and great vessel morphology and left ventricular function. Furthermore, its clinical use continues to expand with increasing experience and proliferation of CMRI centres. As worldwide prevalence of cardiovascular disease continues to rise, CMRI provides opportunity for improved and cost-effective non-invasive assessment. Continued progress in CMRI technology promises to further widen its clinical application in coronary imaging, myocardial perfusion, comprehensive assessment of valves, and plaque characterisation.

Comparison of spiral and FLASH phase velocity mapping, with and without breath-holding, for the assessment of left and right coronary artery blood flow velocity

PURPOSE

To develop high temporal resolution coronary artery spiral phase velocity mapping sequences and to compare the results obtained with those from FLASH sequences.

MATERIALS AND METHODS

Velocity curves were obtained in eight left and eight right coronary arteries using breath-hold interleaved spiral (BH_SP), free-breathing interleaved spiral (FB_SP), breath-hold segmented FLASH (BH_FL), and free-breathing FLASH (FB_FL) sequences. Spatial resolution, temporal resolution, and acquisition durations (cardiac cycles) were as follows-BH_SP: 0.9 mm x 0.9 mm, 30 msec, 20 cycles; FB_SP: 0.9 mm x 0.9 mm, 42 msec, 100 cycles; BH_FL: 0.9 mm x 1.8 mm, 70 msec (effective), 20 cycles; FB_FL: 0.9 mm x 1.8 mm, 30 msec, 480 cycles. Peak systolic, peak diastolic, and mean velocities were compared between sequences.

RESULTS

For left and right arteries, the FB_SP velocity profiles closely followed those from the FB_FL sequence. By comparison, the BH_FL sequence failed to resolve the sharp peaks in the temporal velocity profiles of the right coronary artery, significantly underestimating the peak systolic (88 mm/second vs. 252 mm/second, P < 0.001), peak diastolic (114 mm/second vs. 153 mm/second, P < 0.01), and mean (56 mm/second vs. 93 mm/second, P < 0.001) velocities. For the less mobile left artery, the peak systolic, peak diastolic, and mean velocities were also underestimated by the BH_FL sequence, although this only reached statistical significance for the systolic peak (80 mm/second vs. 135 mm/second, P < 0.01), 142 mm/second vs. 168 mm/second, (P = ns), and 87 mm/second vs. 101 mm/second, (P = ns) respectively.

CONCLUSION

We have shown that the FB_SP sequence developed agrees well with the FB_FL sequence, while the study duration is reduced by a factor of 10 for the same spatial resolution. By comparison, the BH_FL sequence underestimates flow velocities, particularly in the more mobile right coronary artery.

Future prospects in magnetic resonance imaging

Cardiovascular magnetic resonance (CMR) is widely regarded as capable of providing a cornucopia of detailed diagnostic information. However, of that information, very little is truly unique, and can be obtained by a combination of alternate diagnostic modalities. Given this, it is anticipated that in the short term (1-5 years) CMR will find use primarily as a modality to service patients whose diagnosis is inaccessible to established technologies such as ultrasound and radionuclide imaging. Due to the evolving emphasis on finding new and more efficient approaches to disease detection and prevention, as outlined in a policy-setting speech given by the director of the National Institutes of Health, it is anticipated that the scientific and clinical trial communities will adopt CMR at a more rapid pace due to its inherent dimensional accuracy and comprehensive nature. CMR is particularly well suited to participate in the approaching explosion of nanoparticle technologies, as they are applied to diagnostic and therapeutic approaches. In the longer term (5-10 years), as paradigms of disease detection likely expand beyond evaluation of symptoms and risk factors, the comprehensive nature of information provided by CMR will drive the increase of its use as a primary, first-tier, diagnostic modality. In summary, the use of CMR will become increasingly common, and as understanding of disease processes expand, it will emerge as a diagnostic modality that provides an abundance of unique information.

MR findings of collateral circulation are more accurate measures of hemodynamic significance than arm-leg blood pressure gradient after repair of coarctation of the aorta

PURPOSE

To determine the relationship between percent stenosis and three indicators of hemodynamic significance-arm-leg blood pressure gradient, direct visualization of collaterals, and percent increase flow from proximal to distal descending thoracic aorta-in patients with prior repair of coarctation of the aorta (CoA).

MATERIALS AND METHODS

Magnetic resonance imaging (MRI) examinations of 19 patients with prior repair of CoA were retrospectively reviewed. Percent stenosis was compared to the arm-leg blood pressure gradient as obtained from chart review, the depiction of collaterals by gadolinium-enhanced magnetic resonance angiography (MRA), and the percent increased flow in the distal thoracic aorta as measured by velocity-encoded cine MRI. Some imaging series or blood-pressure values were not available in some patients. All of the data were available for 15 of the 19 patients.

RESULTS

The arm-leg blood pressure gradient showed no statistical association with percent stenosis (R(2) = 0.10, P = 0.22), direct visualization of collaterals (P = 0.80), or percent increase in flow (R(2) < 0.01, P = 0.85). Percent stenosis did show association with visualization of collaterals (P = 0.01) and increase flow (R(2) = 0.62, P < 0.01).

CONCLUSION

The arm-leg blood pressure gradient is not a reliable indicator of hemodynamic significance of restenosis in patients with prior repair of CoA. Direct visualization of collateral vessels by MRA and percent increase in flow from proximal to distal descending thoracic aorta are reliable indicators of hemodynamic significance.

Future prospects in cardiac magnetic resonance imaging

To appreciate the impact that key developments will have on the future of cardiovascular magnetic resonance (CMR) imaging, it is instructive to consider its present status. CMR has passed the threshold of being used primarily by innovators, and is now in the early adopter stage. To reach this threshold has taken many years, but its adoption by early majority users is expected to accelerate the growth of CMR. A number of factors govern its natural growth potential, including physician education and credentialing, scanner availability, technology, and reimbursement policies. The intrinsic dimensional accuracy of CMR, coupled with its high level of reproducibility, make it ideal for inclusion in trials, potentially with dramatic reductions in trial duration and the number of subjects required. Clinically, there are a number of applications for which CMR is widely regarded as being the diagnostic test of choice. Software and hardware developments that speed up the basic CMR procedure are being incorporated into scanners, extending the functionality of routine applications such as flow imaging and tag visualization. Exciting areas that are close to routine application include coronary artery imaging, and evaluation of myocardial perfusion and viability status.

Mr flow measurement in the internal mammary artery-to-coronary artery bypass graft: comparison with graft stenosis at radiographic angiography

PURPOSE

To evaluate the sensitivity and specificity of breath-hold magnetic resonance (MR) flow measurement for detection of significant stenosis in internal mammary artery bypass grafts.

MATERIALS AND METHODS

Twenty-six consecutive patients who had undergone coronary artery bypass surgery were examined. Breath-hold velocity-encoded cine MR images were obtained at the midpoint of the internal mammary artery between its origin from the subclavian artery and the distal anastomosis to the left anterior descending artery.

RESULTS

MR images were obtained successfully in 24 patients. At conventional angiography, no significant stenosis was observed in 17 patients (group A), and significant stenosis (diameter > 70%) was observed in seven patients (group B). The mean diastolic-to-systolic peak velocity ratio in group B (0.61 +/- 0.44 [SD]) was significantly lower than that in group A (1.88 +/- 0.96; P <.01). Evaluation of graft stenosis with the diastolic-to-systolic peak velocity ratio revealed a sensitivity of 86% and a specificity of 88%. The mean blood flow rate at baseline in group B (16.9 mL/min +/- 5.5) was significantly lower than that in group A (79.8 mL/min +/- 38.2; P <.01). The sensitivity and specificity of MR blood flow measurement in predicting significant stenosis were 86% and 94%, respectively. The mean pharmacologic flow reserve ratios were 2.00 +/- 1.43 in group A and 1.39 +/- 1.46 in group B (P >.05).

CONCLUSION

Fast MR blood flow measurement at baseline is highly useful for predicting significant stenosis in internal mammary arterial grafts.

Accuracy of segmented MR velocity mapping to measure small vessel pulsatile flow in a phantom simulating cardiac motion

The purpose of this study was to investigate the accuracy of conventional, segmented, and echo-shared MR velocity mapping sequences to measure pulsatile flow in small moving vessels using a phantom with simulated cardiac motion. The phantom moved either cyclically in-plane, through-plane, in- and through-plane, or was stationary. The mean error in average flow was -2% +/- 3% (mean +/- SD) for all sequences under all conditions, with or without background correction, as long as the region of interest (ROI) size was equal to the vessel cross-sectional size. Overestimation of flow as a result of an oversized ROI was less than 20%, and independent of field of view (FOV) and matrix, as long as the offset in angle between the imaging plane and flow direction was less than 10 degrees. Segmented velocity mapping sequences are surprisingly accurate in measuring average flow and render flow profiles in small moving vessels despite the blurring in the images due to vessel motion. J. Magn. Reson. Imaging 2001;13:722-728.

Improved MR flow mapping in coronary artery bypass grafts during adenosine-induced stress

PURPOSE

To validate a recently developed fast high-temporal-resolution magnetic resonance (MR) flow sequence and use it to assess coronary artery bypass graft function during pharmacologic stress.

MATERIALS AND METHODS

Aortic and internal mammary artery flow was measured in 11 healthy volunteers by using conventional cine gradient-echo imaging as a reference standard method and turbo-field echo-planar imaging (TFEPI). By using TFEPI, breath-hold flow mapping with a spatial and temporal resolution of 0.8 mm(2) and 23 msec, respectively, can be performed. This sequence was applied in 20 angiographically normal grafts, and total blood flow at rest and during adenosine infusion (140 microgram/kg/min) was measured.

RESULTS

Good agreement in aortic and internal mammary artery flow values between conventional fast-field echo and TFEPI techniques was found. The mean bypass graft total flow (+/- SD), as assessed with TFEPI, increased from 30.8 mL/min +/- 13.5 to 76.7 mL/min +/- 36.5 (P <.05) to yield a flow reserve of 2.7. Furthermore, this sequence revealed a difference in total flow between single and sequential grafts at rest (25.4 mL/min vs 40.9 mL/min; P <.05) and during stress (65.2 mL/min vs 98.3 mL/min; P <.05).

CONCLUSION

Breath-hold TFEPI provides fast accurate flow measurements with high temporal resolution and allows motion-compensated flow quantification in multiple coronary artery bypass grafts during one 6-minute adenosine infusion.

Coronary sinus flow measurement by means of velocity-encoded cine MR imaging: validation by using flow probes in dogs

PURPOSE

To validate coronary sinus flow measurements for quantification of global left ventricular (LV) perfusion by means of velocity-encoded cine (VEC) magnetic resonance (MR) imaging and flow probes.

MATERIALS AND METHODS

Measurements of coronary sinus flow were performed in seven dogs by using VEC MR imaging at baseline, single coronary arterial stenosis, dipyridamole stress, and reactive hyperemia. These measurements were compared with flow probe measurements of coronary blood flow (CBF) in the left anterior descending coronary (LAD) and circumflex (CFX) arteries (CBF(LAD+CFX)) and coronary sinus. LV blood perfusion was calculated in milliliters per minute per gram from coronary sinus flow, and LV mass was obtained by using VEC and cine MR imaging. LV mass was validated at autopsy.

RESULTS

CBF(LAD+CFX) and coronary sinus flow at VEC MR imaging showed close correlation (r = 0.98, P: <.001). The difference between CBF(LAD+CFX) and MR coronary sinus flow was 3.1 mL/min +/- 8.5 (SD). LV mass at cine MR imaging was not significantly different from that at autopsy (73.2 g +/- 12.8 vs 69. 4 g +/- 12.8). At baseline, myocardial perfusion was 0.40 mL/min/g +/- 0.09 at VEC MR imaging, and CBF(LAD+CFX) was 0.44 mL/min/g +/- 0. 08 (not significant). Reactive hyperemia resulted in 2.7- and 2. 3-fold increases in coronary sinus flow at VEC MR imaging and flow probe CBF(LAD+CFX), respectively.

CONCLUSION

VEC MR imaging has the potential to measure coronary sinus flow during different physiologic conditions and can serve as a noninvasive modality to quantify global LV perfusion in patients.

Assessment of coronary flow reserve using fast velocity-encoded cine MR imaging: validation study using positron emission tomography

OBJECTIVE

Previous studies using intravascular Doppler sonography and positron emission tomography (PET) have shown that the hemodynamic significance of coronary artery stenosis can be evaluated by measuring coronary flow reserve. The purpose of this study was to assess whether MR imaging measurements of coronary flow reserve in the left anterior descending artery are comparable with those obtained with PET in the corresponding territory.

SUBJECTS AND METHODS

MR imaging and PET flow measurements were obtained in 10 healthy volunteers. Blood flow velocity in the left anterior descending artery was measured with breath-hold velocity-encoded cine MR imaging before and after IV administration of dipyridamole. The coronary flow velocity reserve measured by MR imaging was compared with the myocardial perfusion reserve in the anterior myocardium quantified on using PET and (15)O-labeled water.

RESULTS

The average flow velocity reserve in the left anterior descending artery measured on MR imaging was 2.44+/-1.14 in healthy volunteers, which was comparable with the myocardial perfusion reserve measured by PET (2.52+/-0.84). MR imaging and PET measurements of the coronary flow reserve showed a significant correlation (r = 0.79, p<0.01).

CONCLUSION

MR imaging measurement of the flow velocity reserve in the proximal left anterior descending artery correlates well with the myocardial perfusion reserve obtained with PET and (15)O-labeled water.

Magnetic resonance-based assessment of global coronary flow and flow reserve and its relation to left ventricular functional parameters: a comparison with positron emission tomography

BACKGROUND

Measurement of coronary sinus blood flow (CSF) by phase-contrast magnetic resonance (PC-MR) imaging at rest and during hyperemia may allow noninvasive assessment of global coronary hemodynamics.

METHODS AND RESULTS

Sixteen healthy volunteers (age, 22 to 32 years) were examined with MR and PET in random order within 1 to 2 days. At rest and during hyperemia (dipyridamole 0.56 mg/kg), CSF was measured by a cine PC-MR technique (temporal resolution, 40 ms; spatial resolution, 1.25x0.8 mm(2)), and myocardial blood flow (MBF) was measured by [(13)N]NH(3) PET. PET and MR agreed closely for coronary flow reserve (CFR; mean difference, 2.2+/-14.7%; Bland-Altman method). CSF divided by either total left ventricular mass or an estimate of drained myocardium (LVM(drain)) correlated highly with PET flow data (r=0.93 and 0.95, respectively) and with measures of oxygen demand, ie, heart rate, afterload-corrected fiber shortening, and peak systolic stress determined by MR (overall correlation coefficients, 0.81 and 0.87, respectively, multivariate analysis). CSF/LVM(drain) did not differ significantly from PET-derived MBF (difference, 3.6+/-16.6%). In orthotopic heart transplant recipients (n=9), CFR was reduced and blood supply-demand relationships at rest were shifted toward higher flows (P<0.0001).

CONCLUSIONS

This integrated MR approach allows comprehensive assessment of autoregulated and hyperemic coronary flow and is suitable for serial measurements in patients. In transplanted hearts, elevated resting flow is the major cause of reduced CFR.

Interleaved spiral cine coronary artery velocity mapping

Navigator-echo controlled interleaved spiral cine coronary-artery velocity maps were acquired in eight free-breathing volunteers. Good quality data were achieved in all subjects with phasic flow being clearly demonstrated. The improved efficiency of spiral k-space coverage compared to that of more conventional spin-warp techniques resulted in a mean scan time for 256 x 256 pixel-matrix studies of 94 sec (range 69 - 123 sec, SD = 20 sec), the navigator acceptance rate being typically 40-45%. Repeat scans in five subjects showed a high degree of reproducibility, with no significant differences occurring in the peak velocities (14.6+/-1.4 cm/sec vs. 14.5+/-1.5 cm/sec, P = ns) nor in the mean velocities measured over all cine phases (8.3+/-2.1 cm/sec vs. 8.7+/-1.6 cm/sec, P = ns). The potential benefits of using this sequence for assessing coronary blood flow and flow reserve are discussed. Magn Reson Med 43:787-792, 2000.

MR measurement of coronary blood flow

The functional significance of coronary arterial stenosis can be evaluated by measuring the pharmacological flow reserve. Magnetic resonance (MR) imaging has a unique potential for noninvasive measurement of coronary blood flow and flow reserve in the native coronary artery and bypass graft. Restenosis after coronary balloon angioplasty and stenting in the left anterior descending artery can be detected noninvasively with serial MR measurements of the coronary flow reserve. Further refinement of the MR pulse sequences to improve spatial and temporal resolutions may permit accurate quantification of blood flow volume and flow reserve in all major coronary arterial branches. MR assessments of blood flow volume and flow pattern allow noninvasive detection of significant stenosis in the coronary artery bypass graft as well. By integrating MR blood flow measurement in the coronary sinus and cine MR assessment of left ventricular myocardial mass, altered myocardial micro-circulation in patients with diffuse myocardial diseases, such as hypertrophic cardiomyopathy and cardiac transplant, has been documented. J. Magn. Reson. Imaging 1999;10:728-733.

Evaluation of coronary artery bypass grafts by magnetic resonance imaging

Magnetic resonance (MR) angiography and flow mapping have the potential to become a major noninvasive diagnostic tool for the assessment of coronary artery bypass graft morphology and function. Several MR sequences, such as conventional non-respiratory compensated methods, and phase contrast cine flow sequences have been reported for the evaluation of bypass graft patency. However the visualization of different graft segments and the detection of graft stenosis remains difficult. Recent advances in MR coronary angiography and flow mapping are volume coronary angiongraphy with targeted scans, navigator gated angiography, contrast-enhanced angiography, and breath-hold or navigator gated flow sequences. Future approaches, such as navigator gated fast MR techniques resulting in high-resolution angiography in combination with breath-hold MR flow mapping with high temporal resolution, might allow a comprehensive evaluation of bypass graft stenosis and function. This review article will address the major issues concerning the MR evaluation of bypass grafts.

Hypertrophic cardiomyopathy: MR measurement of coronary blood flow and vasodilator flow reserve in patients and healthy subjects

PURPOSE

To evaluate coronary blood flow per gram of myocardial mass and vasodilator flow reserve in patients with hypertrophic cardiomyopathy (HCM) and in healthy subjects by using breath-hold velocity-encoded cine (VEC) magnetic resonance (MR) imaging.

MATERIALS AND METHODS

Twenty-nine patients with HCM and nine healthy volunteers were examined. Fast VEC MR images were obtained in an oblique imaging plane perpendicular to the coronary sinus before and after intravenous injection of dipyridamole (0.56 mg/kg). The products of mean velocity and cross-sectional area of the vessel were integrated to measure blood flow. Breath-hold cine MR images encompassing the entire left ventricle were acquired to quantify the left ventricular mass.

RESULTS

In the basal state, the coronary blood flow per gram of myocardial mass was 0.74 mL/min/g +/- 0.23 in healthy subjects and 0.62 mL/min/g +/- 0.27 in patients with HCM. After administration of dipyridamole, coronary blood flow in patients with HCM increased to a level significantly less than that in healthy subjects (1.03 mL/min/g +/- 0.40 vs 2.14 mL/min/g +/- 0.51; P < .01), resulting in a severely depressed flow reserve ratio in patients with HCM compared with that in healthy subjects (1.72 +/- 0.49 vs 3.01 +/- 0.75; P < .01).

CONCLUSION

Breath-hold VEC MR imaging is a noninvasive technique for evaluating coronary flow per gram of myocardial mass and coronary flow reserve.

Coronary flow and flow reserve in canines using MR phase difference and complex difference processing

Coronary artery disease continues to be the leading cause of death for adults in the United States. Magnetic resonance imaging (MR) has the potential to dramatically impact the diagnosis of heart disease by noninvasively providing a wide range of anatomic and physiologic information. Previous research has shown that coronary flow, one component of a complete examination, can be accurately measured in the left anterior descending artery in vivo. The current work validates MR flow measurements in canine circumflex arteries using transit time ultrasound as a standard. The circumflex artery experiences greater in-plane motion and is a more stringent test for flow measurement accuracy. This work also compares two methods of processing MR velocity data, phase difference and complex difference techniques, and examines the sources of error present in the animal validation model. Phase difference processing with a 30% magnitude threshold best matched the mean ultrasound flow values (30% PD = 1.04 x US + 1.49, r = 0.94), but it was very sensitive to vessel boundary identification. The complex difference process was less sensitive to vessel boundary identification and correlated well with the transit time ultrasound despite systematic underestimations. The reasons for the discrepancies are shown to stem from a number of possible sources including variability of the ultrasound standard, low signal-to-noise ratios in the MR images, sensitivity of the MR technique to vessel boundary identification, and motion artifacts in the images.

Coronary MR angiography

Coronary MR angiography is a new noninvasive diagnostic method in rapid evolution. It has the potential to combine structural information with functional assessment of coronary blood flow. Advances in technology will undoubtedly lead to enhanced resolution, improved accuracy, and shorter scan times. It is certain that coronary MR angiography will be a prominent diagnostic clinical tool in the years to come.