Coronary flow reserve: noninvasive measurement in humans with breath-hold velocity-encoded cine MR imaging

HDL: Hybrid Deep Learning for the Synthesis of Myocardial Velocity Maps in Digital Twins for Cardiac Analysis

Synthetic digital twins based on medical data accelerate the acquisition, labelling and decision making procedure in digital healthcare. A core part of digital healthcare twins is model-based data synthesis, which permits the generation of realistic medical signals without requiring to cope with the modelling complexity of anatomical and biochemical phenomena producing them in reality. Unfortunately, algorithms for cardiac data synthesis have been so far scarcely studied in the literature. An important imaging modality in the cardiac examination is three-directional CINE multi-slice myocardial velocity mapping (3Dir MVM), which provides a quantitative assessment of cardiac motion in three orthogonal directions of the left ventricle. The long acquisition time and complex acquisition produce make it more urgent to produce synthetic digital twins of this imaging modality. In this study, we propose a hybrid deep learning (HDL) network, especially for synthetic 3Dir MVM data. Our algorithm is featured by a hybrid UNet and a Generative Adversarial Network with a foreground-background generation scheme. The experimental results show that from temporally down-sampled magnitude CINE images (six times), our proposed algorithm can still successfully synthesise high temporal resolution 3Dir MVM CMR data (PSNR=42.32) with precise left ventricle segmentation (DICE=0.92). These performance scores indicate that our proposed HDL algorithm can be implemented in real-world digital twins for myocardial velocity mapping data simulation. To the best of our knowledge, this work is the first one investigating digital twins of the 3Dir MVM CMR, which has shown great potential for improving the efficiency of clinical studies via synthesised cardiac data.

Phase contrast coronary blood velocity mapping with both high temporal and spatial resolution using triggered Golden Angle rotated Spiral k-t Sparse Parallel imaging (GASSP) with shifted binning

PURPOSE

High temporal and spatial resolutions are required for coronary blood flow measures. Current spiral breath-hold phase contrast (PC) MRI at 3T focus on either high spatial or high temporal resolution. We propose a golden angle (GA) rotated Spiral k-t Sparse Parallel imaging (GASSP) sequence for both high spatial (0.8 mm) and high temporal (<21 ms) resolutions.

METHODS

GASSP PC data are acquired in left anterior descending and right coronary arteries of eight healthy subjects. Binning of GA rotated spiral data into cardiac frames may lead to large k-space gaps. To reduce those gaps, the binning window is shifted and a triggered GA scheme that resets the rotation angle every heartbeat is proposed. The gap reductions are evaluated in simulations and all subjects. Peak systolic velocity (PSV), peak diastolic velocity (PDV), coronary blood flow rate, and vessel area are validated against two reference scans, and repeatability/reproducibility are determined.

RESULTS

Shifted binning reduced the mean k-space gaps of the triggered GA scheme by 14°-22° in simulations and about 20° in vivo. The k-space gap across three cardiac frames was reduced with the triggered GA scheme compared to the standard GA scheme (35.3°± 3.6° vs. 43°± 13.7°, t-test P = .04). PSV, PDV, flow rate, and area had high intra-scan repeatability (0.92 ≤ intraclass correlation coefficient [ICC] ≤ 0.99), and inter-scan (0.78 ≤ ICC ≤ 0.91) and intra-observer (0.91 ≤ ICC ≤ 0.98) reproducibility.

CONCLUSION

GASSP enables single breath-hold coronary PC MRI with high temporal and spatial resolutions. Shifted binning and a triggered GA scheme reduce k-space gaps. Quantitative coronary flow metrics are highly reproducible, especially within the same scanning session.

Assessment of Coronary Flow Velocity Reserve in the Left Main Trunk Using Phase-contrast MR Imaging at 3T: Comparison with 15O-labeled Water Positron Emission Tomography

Purpose:

The aim of this study was to verify coronary flow velocity reserve (CFVR) on the left main trunk (LMT) in comparison with myocardial flow reserve (MFR) by ¹⁵O-labeled water positron emission tomography (PET) (MFR-PET) in both the healthy adults and the patients with coronary artery disease (CAD), and to evaluate the feasibility of CFVR to detect CAD.

Methods:

Eighteen healthy adults and 13 patients with CAD were evaluated. CFVR in LMT was estimated by 3T magnetic resonance imaging (MRI) with phase contrast technique. MFR-PET in the LMT territory including anterior descending artery and circumflex artery was calculated as the ratio of myocardial blood flow (MBF)-PET at stress to MBF-PET at rest.

Results:

There was a significant positive relationship between CFVR and MFR-PET (R = 0.45, P < 0.0001). Inter-observer calculations of CFVR showed good correlation (R² = 0.93, P < 0.0001). The CFVR in patients with CAD was significantly lower than that in healthy adults (1.90 ± 0.61 vs. 2.77 ± 1.03, respectively, P = 0.01), which were similar to the results of MFR-PET (2.23 ± 0.84 vs. 3.96 ± 1.04, respectively, P < 0.0001). For the detection of patients with CAD, the area under the curve was 0.78 (P = 0.01). The sensitivity was 0.77 and specificity was 0.72 when a cut-off of 2.15 was used.

Conclusion:

CFVR by 3T was validated with MFR-PET. CFVR could detect the patients with CAD. This method is a simple and reliable index without radiation or contrast material.

Validation of high temporal resolution spiral phase velocity mapping of temporal patterns of left and right coronary artery blood flow against Doppler guidewire

BACKGROUND

Temporal patterns of coronary blood flow velocity can provide important information on disease state and are currently assessed invasively using a Doppler guidewire. A non-invasive alternative would be beneficial as it would allow study of a wider patient population and serial scanning.

METHODS

A retrospectively-gated breath-hold spiral phase velocity mapping sequence (TR 19 ms) was developed at 3 Tesla. Velocity maps were acquired in 8 proximal right and 15 proximal left coronary arteries of 18 subjects who had previously had a Doppler guidewire study at the time of coronary angiography. Cardiovascular magnetic resonance (CMR) velocity-time curves were processed semi-automatically and compared with corresponding invasive Doppler data.

RESULTS

When corrected for differences in heart rate between the two studies, CMR mean velocity through the cardiac cycle, peak systolic velocity (PSV) and peak diastolic velocity (PDV) were approximately 40 % of the peak Doppler values with a moderate - good linear relationship between the two techniques (R(2): 0.57, 0.64 and 0.79 respectively). CMR values of PDV/PSV showed a strong linear relationship with Doppler values with a slope close to unity (0.89 and 0.90 for right and left arteries respectively). In individual vessels, plots of CMR velocities at all cardiac phases against corresponding Doppler velocities showed a consistent linear relationship between the two with high R(2) values (mean +/-SD: 0.79 +/-.13).

CONCLUSIONS

High temporal resolution breath-hold spiral phase velocity mapping underestimates absolute values of coronary flow velocity but allows accurate assessment of the temporal patterns of blood flow.

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.

Highly accelerated cardiac cine phase-contrast MRI using an undersampled radial acquisition and temporally constrained reconstruction

PURPOSE

To evaluate a method to enable single-slice or multiple-slice cine phase contrast (cine-PC) acquisition during a single breath-hold using a highly sparsified radial acquisition ordering and temporally constrained image reconstruction with a spatially varying temporal constraint.

MATERIALS AND METHODS

Simulated and in vivo cine-PC datasets of the proximal ascending aorta were obtained at different acceleration factors using a view projection acquisition order optimized for temporally constrained reconstruction (TCR). Reconstruction of the sparse cine-PC data performed with TCR was compared to reconstructions using zero-filled regridding and temporal interpolation.

RESULTS

TCR resulted in more accurate velocity measurements than regridding or temporal interpolation. In one dataset, TCR of undersampled in vivo data (16 views per cardiac phase) resulted in a peak systolic velocity within 3.3% of the value measured by Doppler ultrasound while shortening the scan time to 13 seconds. High temporal-resolution undersampled TCR was also compared lower temporal-resolution, more highly sampled, regridding in three normal volunteers.

CONCLUSION

TCR proved to be an effective method for reconstructing undersampled radial PC data. Although TCR utilizes a temporal constraint, temporal blurring was minimized by using appropriate constraint weights in addition to a spatially varying temporal constraint. TCR allowed for the acquisition time to be reduced to the duration of a breath-hold, while still resulting in accurate velocity measurements.

Motion in cardiovascular MR imaging

Modern rapid magnetic resonance (MR) imaging techniques have led to widespread use of the modality in cardiac imaging. Despite this progress, many MR studies suffer from image degradation due to involuntary motion during the acquisition. This review describes the type and extent of the motion of the heart due to the cardiac and respiratory cycles, which create image artifacts. Methods of eliminating or reducing the problems caused by the cardiac cycle are discussed, including electrocardiogram gating, subject-specific acquisition windows, and section tracking. Similarly, for respiratory motion of the heart, techniques such as breath holding, respiratory gating, section tracking, phase-encoding ordering, subject-specific translational models, and a range of new techniques are considered.

Noninvasive assessment of coronary vasodilation using cardiovascular magnetic resonance in patients at high risk for coronary artery disease

BACKGROUND

Impaired coronary vasodilation to both endothelial-dependent and endothelial-independent stimuli have been associated with atherosclerosis. Direct measurement of coronary vasodilation using x-ray angiography or intravascular ultrasound is invasive and, thus, not appropriate for asymptomatic patients or for serial follow-up. In this study, high-resolution coronary cardiovascular magnetic resonance (CMR) was used to investigate the vasodilatory response to nitroglycerine (NTG) of asymptomatic patients at high risk for CAD.

METHODS

A total of 46 asymptomatic subjects were studied: 13 high-risk patients [8 with diabetes mellitus (DM), 5 with end stage renal disease (ESRD)] and 33 age-matched controls. Long-axis and cross-sectional coronary artery images were acquired pre- and 5 minutes post-sublingual NTG using a sub-mm-resolution multi-slice spiral coronary CMR sequence. Coronary cross sectional area (CSA) was measured on pre- and post-NTG images and % coronary vasodilation was calculated.

RESULTS

Patients with DM and ESRD had impaired coronary vasodilation to NTG compared to age-matched controls (17.8 +/- 7.3% vs. 25.6 +/- 7.1%, p = 0.002). This remained significant for ESRD patients alone (14.8 +/- 7.7% vs. 25.6 +/- 7.1%; p = 0.003) and for DM patients alone (19.8 +/- 6.3% vs. 25.6 +/- 7.1%; p = 0.049), with a non-significant trend toward greater impairment in the ESRD vs. DM patients (14.8 +/- 7.7% vs. 19.8 +/- 6.3%; p = 0.23).

CONCLUSION

Noninvasive coronary CMR demonstrates impairment of coronary vasodilation to NTG in high-risk patients with DM and ESRD. This may provide a functional indicator of subclinical atherosclerosis and warrants clinical follow up to determine prognostic significance.

Assessment of blood flow and valvular heart disease using phase-contrast cardiovascular magnetic resonance

Measurement of blood flow is important for assessing the severity of disease processes involving the cardiovascular system. Phase-contrast cardiovascular magnetic resonance (PC-CMR) can be used to measure blood flow noninvasively without ionizing radiation or limitations imposed by body habitus. This review describes the performance of PC-CMR and its clinical utility in assessing patients with cardiovascular or valvular heart disease.

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.

Mechanism of ST segment depression during exercise tests in patients with liver cirrhosis

PURPOSE

To our experience, ST segment depression is sometimes detected in an exercise electrocardiogram (ECG) test in patients with liver cirrhosis who have no significant coronary stenosis. In this study, the mechanism of ST segment depression in liver cirrhosis was examined using (99m)Tc-methoxy-isobutyl-isonitrile (MIBI) myocardial scintigraphy.

METHODS

Six patients with liver cirrhosis (LC group), and 15 normal subjects (N group) were examined. To evaluate the level of myocardial blood flow, a Bull's eye display of myocardial blood flow was performed after dividing the left ventricle into 9 segments. Exercise myocardial scintigraphy with MIBI was performed to obtain the increase in % uptake. Angiographies were performed with a CAG system by inserting a 5 French Judkins catheter via the right femoral artery.

RESULTS

No significant coronary stenosis was found in any of the LC patients. Neither a decrease in MIBI uptake nor defect was observed on Bull's eye images from the LC group. The mean % uptake increase was 61.0 +/- 5.6% in the N group. In the LC group, although neither a decrease in MIBI uptake nor a defect was visually observed on Bull's eye images obtained during exercise, the % uptake increases (mean: 52.5 +/- 5.8%) were lower than those of the N group (p<0.05).

CONCLUSION

These findings suggest that a disorder in coronary flow reserve occurs in liver cirrhosis patients, because the decreased MIBI uptake during exercise is due to the depression of flow-mediated vasodilatation controlled by the endothelium of the coronary artery and the estrogenic digitalis action of blood flow independency.

Quantification of the pulse wave velocity of the descending aorta using axial velocity profiles from phase-contrast magnetic resonance imaging

The pulse wave velocity (PWV) of aortic blood flow is considered a surrogate for aortic compliance. A new method using phase-contrast (PC)-MRI is presented whereby the spatial and temporal profiles of axial velocity along the descending aorta can be analyzed. Seventeen young healthy volunteers (the YH group), six older healthy volunteers (the OH group), and six patients with coronary artery disease (the CAD group) were studied. PC-MRI covering the whole descending aorta was acquired, with velocity gradients encoding the in-plane velocity. From the corrected axial flow velocity profiles, PWV was determined from the slope of an intersecting line between the presystolic and early systolic phases. Furthermore, the aortic elastic modulus (Ep) was derived from the ratio of the brachial pulse pressure to the strain of the aortic diameter. The PWV increased from YH to OH to CAD (541 +/- 94, 808 +/- 184, 1121 +/- 218 cm/s, respectively; P = 0.015 between YH and OH; P = 0.023 between OH and CAD). There was a high correlation between PWV and Ep (r = 0.861, P < 0.001). Multivariate analysis showed that age and CAD were independent risk factors for an increase in the PWV. Compared to existing methods, our method requires fewer assumptions and provides a more intuitive and objective way to estimate the PWV.

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.

Measurement of coronary vasomotor function: getting to the heart of the matter in cardiovascular research

Measurement of endothelial function in patients has emerged as a useful tool for cardiovascular research. Although no gold standard for the measurement of endothelial function exists, the measurement of flow-mediated dilation in the brachial artery, assessed with Doppler ultrasonography, is the most studied method. However, the assumption that endothelial dysfunction detected in brachial arteries is a manifestation of systemic endothelial dysfunction including the coronary circulation may not be entirely valid. Brachial and myocardial circulations differ in terms of the microvascular architecture, the pattern of blood flow and vascular resistance (e.g. shunt vessels occur in the hand but not in the myocardium), their metabolic regulation, type of receptors that contribute to humoral regulation and the pathways that are activated to induce hyperaemia. In this context, measuring coronary vasomotor function may be more useful than brachial artery measures to predict and assess potential myocardial damage related to limited vascular responsiveness. This review aims to provide an overview of the basic concept of coronary flow reserve and its different modalities of measurement, as well as its utility in cardiovascular research.

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.

Spiral phase velocity mapping of left and right coronary artery blood flow: Correction for through-plane motion using selective fat-only excitation

PURPOSE

To develop a method of correcting both right and left coronary artery flow velocities for the through-plane motion of the vessel, in order to allow details in the temporal flow profiles to be viewed.

MATERIALS AND METHODS

The methods developed use selective excitation and velocity mapping of the epicardial fat surrounding the artery, either in a separate acquisition (temporal resolution = 22 msec) or interleaved with the water-excitation acquisition (temporal resolution = 44 msec) used to determine coronary blood flow velocities. The two methods were compared in 10 right and 13 left coronary arteries in healthy volunteers.

RESULTS

For the right coronary arteries, correction for through-plane motion significantly reduces the mean systolic flow velocity (75.3 mm/second vs. 90.0 mm/second, P < 0.01), while the mean diastolic flow velocity is unchanged (96.8 mm/second vs. 94.5 mm/second, P = ns). The resulting profiles are biphasic, with approximately equal flow in systole and diastole. For the left arteries, correction for through-plane motion reduces the mean systolic flow velocity (25.0 mm/second vs. 72.8 mm/second, P < 0.001), resulting in the expected diastolic predominant flow profiles. For the right arteries, there were no significant differences in the mean systolic and mean diastolic velocities after correction with the separate fat-excitation acquisition, and after correction the poorer temporal resolution combined water excitation/fat excitation acquisition. However, for the left coronary arteries, the combined water excitation/fat excitation acquisition resulted in a slight reduction in the mean diastolic velocity (121.5 mm/second vs. 130.9 mm/second, P < 0.05).

CONCLUSION

Selective excitation of the surrounding epicardial fat enables through-plane correction of both left and right coronary flow velocities, enabling the temporal details of flow velocity to be viewed. With a combined WE/FE acquisition, this can be performed without extending the study duration; however, the reduced temporal resolution and temporal mismatch of the excitations results in a blunting of rapidly changing flow profiles. As such, it may be less suitable for the left coronary artery, which has a greater range of through-plane motion than the right, and correction using separate WE and FE acquisitions, or the adjacent myocardium, may be preferable.

Vein graft function improvement after percutaneous intervention: evaluation with MR flow mapping

PURPOSE

To provide functional reference values in single and sequential vein grafts by using magnetic resonance (MR) flow mapping and to examine the effect of percutaneous intervention (PCI) on coronary artery bypass graft function.

MATERIALS AND METHODS

Fast MR flow mapping at baseline and during adenosine-induced stress was performed in 39 nonstenotic single vein grafts and 20 nonstenotic sequential vein grafts, as well as in 15 stenotic vein grafts before and 7.3 weeks +/- 1.5 after successful PCI. We evaluated the following parameters (in terms of mean values +/- SDs): average peak velocity (APV) at baseline, stress APV, and velocity reserve. Parameters in nonstenotic single and sequential vein grafts were compared by means of unpaired two-tailed Student t testing. To evaluate changes in velocities before and after PCI, a paired two-tailed Student t test was used. P <.05 was considered to indicate a statistically significant difference.

RESULTS

Reference values in single vein grafts for baseline APV, stress APV, and velocity reserve were 8.6 cm/sec +/- 3.4, 20.2 cm/sec +/- 9.5, and 2.4 +/- 0.8, respectively. In sequential vein grafts, significantly higher values for baseline APV (12.2 cm/sec +/- 5.0) and stress APV (27.2 cm/sec +/- 10.6) but a similar velocity reserve (2.3 +/- 0.7) were found. Significant improvements were observed after PCI in baseline APV (before PCI: 9.2 cm/sec +/- 6.6; after PCI: 12.9 cm/sec +/- 7.9; P =.008) and stress APV (before PCI: 12.9 cm/sec +/- 6.3; after PCI: 27.1 cm/sec +/- 13.9; P <.001). No improvement in velocity reserve was observed.

CONCLUSION

Significantly higher absolute velocity and flow values were observed in sequential versus single vein grafts, underscoring the need for separate functional reference values for different graft types. Graft function showed significant improvement after PCI to the point that it was restored or nearly restored to reference values.

Noninvasive determination of coronary blood flow velocity with cardiovascular magnetic resonance in patients after stent deployment

BACKGROUND

In patients with coronary artery stents, no direct noninvasive coronary artery imaging is possible with magnetic resonance (MR). A well-established method for the assessment of the functional significance of a coronary lesion is the measurement of coronary flow reserve by invasive intracoronary Doppler. The purpose of the study was to determine coronary flow velocity reserve (CFVR) with MR after stent deployment.

METHODS AND RESULTS

Thirty-eight patients after successful PTCA and stent deployment were included. CFVR was measured perpendicular to the artery distal to the stent using phase-contrast velocity quantification at rest and during adenosine-stimulated hyperemia with a 1.5T MR tomograph (ACS NT, Philips). Measurements were repeated after 3 months and compared with invasive coronary angiography. In 18 patients, additional invasive Doppler flow measurements were obtained. CFVR could be determined in 29 of 38 (76%) of the patients. After 3 months, significant differences were obtained between coronary arteries with and without restenosis. Using a threshold of 1.2, a sensitivity of 83% with a specificity of 94% was achieved for > or =75% stenoses. CFVR with CMR was similar to Doppler results (r=0.87), with a mean relative difference of 7.5%.

CONCLUSIONS

In patients with preserved coronary microcirculating vasoreactivity that are suitable for MR coronary angiography and flow assessments, CMR measures of coronary blood flow velocities reserve may be used to detect in-stent restenosis.

Value of magnetic resonance imaging for the noninvasive detection of stenosis in coronary artery bypass grafts and recipient coronary arteries

BACKGROUND

Magnetic resonance imaging (MRI) is a potential noninvasive diagnostic tool to detect coronary artery bypass graft stenosis, but its value in clinical practice remains to be established. We investigated the value of MRI in detecting stenotic grafts, including recipient vessels.

METHODS AND RESULTS

We screened for inclusion 173 consecutive patients who were scheduled for coronary angiography because of recurrent chest pain after coronary artery bypass grafting (CABG). We studied 69 eligible patients with 166 grafts (81 single vein, 44 sequential vein, and 41 arterial grafts). MRI with baseline and stress flow mapping was performed. Both scans were successful in 80% of grafts. Grafts were divided into groups with stenosis > or =50% (n=72) and > or =70% (n=48) in the graft or recipient vessels. Marginal logistic regression was used to predict the probability for the presence of stenosis per graft type using multiple MRI variables. Receiver operator characteristics (ROC) analysis was performed to assess the diagnostic value of MRI. Sensitivity (95% confidence interval)/specificity (95% confidence interval) in detecting single vein grafts with stenosis > or =50% and > or =70% were 94% (86 to 100)/63% (48 to 79) and 96% (87 to 100)/92% (84 to 100), respectively.

CONCLUSIONS

MRI with flow mapping is useful for identifying grafts and recipient vessels with flow-limiting stenosis. Flow scans could be obtained in 80% of the grafts. This proof-of-concept study suggests that noninvasive MRI detection of stenotic grafts in patients who present with recurrent chest pain after CABG may be useful in selecting those in need of an invasive procedure.

A vascular stent as an active component for locally enhanced magnetic resonance imaging: initial in vivo imaging results after catheter-guided placement in rabbits

RATIONALE AND OBJECTIVE

A vascular stent constructed as a high frequency resonator improves the local signal-to-noise ratio at magnetic resonance (MR) imaging. After catheter placement and intravascular expansion, the stent can be used as an inductively coupled coil for MRI. The imaging properties of this balloon-expandable active MRI stent (AMRIS) were evaluated after x-ray fluoroscopy guided placement in the abdominal aorta of five rabbits using MR angiography (MRA) and flow measurements.

METHODS

The AMRIS was implanted in the abdominal aorta of five rabbits using a balloon catheter inserted through the common carotid artery. The rabbits were examined by MRA (3D fast low-angle shot) at 1.5 tesla before and after intravenous injection of an iron-oxide-based blood pool contrast medium (dose 50 micro mol Fe/kg) and flow measurements (ECG-triggered phase contrast cine gradient-echo sequence). Signal-to-noise ratios (SNR) were calculated and flow volume curves were generated. The in-stent increase in temperature was measured in vitro using a fiberoptic thermometry system.

RESULTS

The SNR was 5.0 +/- 0.6 outside the stent and 23.2 +/- 14.1 within the stent ( < 0.0 5) in plain MRA, 19.5 +/- 5.0 outside and 30.7 +/- 8.2 within the stent ( < 0.05) in contrast enhanced MRA, and 5.8 +/- 1.6 and 13.9 +/- 5.9, respectively ( < 0.05) in the magnitude images of the flow measurements. Flow volume curves within and distal to the stent were comparable.

CONCLUSIONS

The expandable active MRI stent produces local signal enhancement in MRA and MR flow measurements after catheter placement and thus may improve assessment of the stented vessel segment by MR imaging.

Magnetic resonance imaging versus Doppler guide wire in the assessment of coronary flow reserve in patients with coronary artery disease

BACKGROUND

Coronary flow velocity reserve (CFVR), defined as the ratio of maximal hyperaemic to baseline flow velocity, has been validated as a marker of physiological significance of a coronary lesion. Clinically, this parameter is measured invasively during X-ray angiography using the Doppler guide wire. With magnetic resonance (MR) imaging it is possible to quantify CFVR non-invasively.

DESIGN

The purpose of the study was to compare CFVR, acquired with MR imaging and the Doppler guide wire in patients with coronary artery disease.

METHODS

Twenty-two patients suffering from one- or two-vessel coronary artery disease as derived from diagnostic X-ray coronary angiography were included. Coronary flow velocity reserve was measured at baseline and during maximal hyperaemia, obtained by intravenous administration of adenosine using MR phase contrast velocity quantification. Within 2 weeks CFVR was measured invasively with a Doppler guide wire.

RESULTS

In 26 coronary arteries CFVR was acquired with both techniques. Mean CFVR in the stenosed and healthy reference arteries was 1.5 +/- 0.7 and 2.7 +/- 1.0 (P < 0.01) respectively for MR measurements and 1.9 +/- 0.7 and 3.1 +/- 0.6 (P < 0.01) respectively for Doppler measurements. Bland-Altman analysis revealed a non-significant mean difference between the two techniques of 0.4 +/- 1.2.

CONCLUSION

In a selected group of stable patients with coronary artery disease MR flow velocity quantification provides non-invasive data equivalent to the invasive Doppler guide wire data. Variability in both the MR and Doppler ultrasound measurement resulted in a significant scatter of data without systematic difference.

Magnetic resonance imaging of coronary artery occlusions in the navigator technique

Non-invasive assessment of coronary arteries is possible with magnetic resonance imaging (MRI). Respiratory gated MR coronary angiography is a new imaging technique that permits reconstruction of the coronary arteries based on a three-dimensional (3D) data set obtained from the free-breathing patient. In this study, respiratory gated MR angiography (MRA) was performed to assess coronary artery occlusions. MRI was performed in 25 patients who had been referred for conventional coronary angiography because of suspected coronary artery disease. Coronary artery occlusion was evaluated in the proximal and middle vessel segments after multiplanar coronary reconstruction of the MR images. Five patients were excluded from the study; in the remaining 20 patients 120 coronary artery segments were analyzed. Good image quality could be obtained for 85% of the segments. Eighteen of the 24 occlusions were confirmed by MRI, the overall sensitivity was 75% and the specificity was 100%. The best results were found in the proximal left anterior descending (LAD) and descending parts of the right coronary artery (RCA), where all occlusions were confirmed. These results showed that coronary artery occlusions can be detected in the proximal and middle LAD and RCA using 3D respiratory gated MRA. Further technical improvements, especially in spatial resolution, are necessary before MRA can become a reliable diagnostic tool in the non-invasive evaluation of coronary arteries.

High temporal resolution phase contrast MRI with multiecho acquisitions

Velocity imaging with phase contrast (PC) MRI is a noninvasive tool for quantitative blood flow measurement in vivo. A shortcoming of conventional PC imaging is the reduction in temporal resolution as compared to the corresponding magnitude imaging. For the measurement of velocity in a single direction, the temporal resolution is halved because one must acquire two differentially flow-encoded images for every PC image frame to subtract out non-velocity-related image phase information. In this study, a high temporal resolution PC technique which retains both the spatial resolution and breath-hold length of conventional magnitude imaging is presented. Improvement by a factor of 2 in the temporal resolution was achieved by acquiring the differentially flow-encoded images in separate breath-holds rather than interleaved within a single breath-hold. Additionally, a multiecho readout was incorporated into the PC experiment to acquire more views per unit time than is possible with the single gradient-echo technique. A total improvement in temporal resolution by approximately 5 times over conventional PC imaging was achieved. A complete set of images containing velocity data in all three directions was acquired in four breath-holds, with a temporal resolution of 11.2 ms and an in-plane spatial resolution of 2 mm x 2 mm.

Assessing coronary sinus blood flow in patients with coronary artery disease: a comparison of phase-contrast MR imaging with positron emission tomography

OBJECTIVE

This study was performed to determine whether MR imaging can be used to reliably measure global myocardial blood flow and coronary flow reserve in patients with coronary artery disease as compared with such measurements obtained by positron emission tomography (PET).

SUBJECTS AND METHODS

We measured myocardial blood flow first at baseline and then after dipyridamole-induced hyperemia in 20 patients with coronary artery disease. Myocardial blood flow as revealed by MR imaging was calculated by dividing coronary sinus flow by the left ventricular mass. Coronary flow reserve was calculated by dividing the rate of hyperemic flow by the rate of baseline flow.

RESULTS

Using MR imaging, myocardial blood flow at baseline was 0.73 +/- 0.23 mL x min(-1) x g(-1), and at hyperemia the blood flow was 1.43 +/- 0.37 mL x min(-1) x g(-1), yielding an average coronary flow reserve of 1.99 +/- 0.47. Using PET, myocardial blood flow was 0.89 +/- 0.21 mL x min(-1) x g(-1) at baseline and 1.56 +/- 0.42 mL x min(-1) x g(-1) at hyperemia, yielding an average coronary flow reserve of 1.77 +/- 0.36. The correlation of myocardial blood flow and coronary flow reserve measurements for these two methods was an r of 0.80 (p < 0.01) and an r of 0.50 (p < 0.05), respectively.

CONCLUSION

This study shows that myocardial blood flow measurements obtained using MR imaging have a good correlation with corresponding PET measurements. Coronary flow reserve measurements obtained using MR imaging had only moderate correlation with PET-obtained measurements. Our results suggest that MR imaging flow quantification could potentially be used for measuring global myocardial blood flow in patients in whom interventional treatment for coronary artery disease is being evaluated.

The active magnetic resonance imaging stent (AMRIS): initial experimental in vivo results with locally amplified MR angiography and flow measurements

RATIONALE AND OBJECTIVES

Magnetic resonance (MR) is limited by artifacts in vessels after stenting. An active MR imaging stent (AMRIS) allows for artifact-free imaging with local improvement in signal-to-noise ratio (SNR). In a rabbit model, we evaluated the imaging properties by MR angiography (MRA) and flow measurements.

METHODS

The AMRIS was placed in the abdominal aorta of five rabbits. At 1.5 T, MRA (three-dimensional fast low-angle shot) was performed before and after intravenous injection of an iron oxide-based, blood-pool contrast medium (dose, 50 micromol Fe/kg), and flow measurements were performed (electrocardiographically triggered phase-contrast cine gradient-echo sequence). Mean SNRs were calculated and flow volume curves were generated.

RESULTS

The SNR was 6.0 +/- 0.6 (outside the stent) versus 12.3 +/- 1.1 (inside the stent, P < 0.05) for plain MRA, 21.2 +/- 0.6 versus 40.6 +/- 5.2 (P < 0.05) for contrast-enhanced MRA, and 5.4 +/- 0.4 versus 13.7 +/- 2.1 (P < 0.05) for the magnitude images of flow measurements. Flow volume curves within and distal to the stent were comparable.

CONCLUSIONS

By using the AMRIS as a vascular stent, the stented vessel segment can be examined with enhanced signal intensity on MRI.

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.

Global myocardial blood flow and global flow reserve measurements by MRI and PET are comparable

Coronary flow reserve (CFR) measurements have been widely used in assessing the functional significance of coronary artery stenosis because they are more sensitive in predicting major cardiac events than angiographically detected reductions of coronary arteries. Myocardial blood flow can be determined by measuring coronary sinus (CS) flow with velocity-encoded cine magnetic resonance imaging (VEC-MRI). The purpose of this study was to compare global myocardial blood flow (MBF) and CFR measured using VEC-MRI with MBF and CFR measured using positron emission tomography (PET). We measured MBF at baseline and after dipyridamole-induced hyperemia in 12 male volunteers with VEC-MRI and PET. With VEC-MRI, MBF was 0.64 +/- 0.09 (ml/min/g) at baseline and 1.59 +/- 0.79 (ml/min/g) at hyperemia, which yielded an average CFR of 2.51 +/- 1.29. With PET, MBF was 0.65 +/- 0.20 (ml/min/g) at baseline and 1.78 +/- 0.72 (ml/min/g) at hyperemia, which yielded an average CFR of 2.79 +/- 0.97. The correlation of MBFs between these two methods was good (r = 0.82, P < 0.001). The CFRs measured by MRI correlated well with those measured using PET (r = 0.76, P < 0.004). These results suggest that MRI is a useful and accurate method to measure global MBF and CFR. Therefore, it would be suitable for studying risk factor modifications of vascular function at an early stage in healthy volunteers.

Effect of breath holding on blood flow measurement using fast velocity encoded cine MRI

Breath-hold MR measurement of cardiac output was compared with results from respiratory triggered MR acquisitions, since flow measurement during breath-holding may be different from physiological blood flow. Cardiac output during large lung volume breath-holding (4.47 +/- 0.63 l/min in the aorta and 4.53 +/- 0.59 l/min in the pulmonary artery) was significantly lower than that measured during normal breathing (6.09 +/- 0.49 l/min and 6.48 +/- 0.67 l/min, P < 0.01). In contrast, no significant difference was found between measurements conducted with small lung volume breath-holding (5.87 +/- 0.53 l/min and 6.41 +/- 0.75 l/min) and normal breathing. In conclusion, breath-hold MR flow measurement using small lung volume by shallow inspiration can provide a blood flow quantification that is close to physiological blood flow. Magn Reson Med 45:346-348, 2001.

Coronary flow reserve: measurement with transthoracic Doppler echocardiography is reproducible and comparable with positron emission tomography

Detection of early vascular changes indicated by lowered coronary flow reserve (CFR) would allow early treatment and prevention of atherosclerosis. The purpose of this study was to test whether it is possible to reproducibly measure CFR with transthoracic Doppler echocardiography (TTE) in healthy volunteers. We measured CFR using dipyridamole infusion in ten healthy male volunteers with two methods: TTE and positron emission tomography (PET) with oxygen-15-labelled water (group A). However, CFR was assessed twice with TTE in eight healthy male volunteers (group B) to study the reproducibility of this method. We compared CFRs obtained using TTE flow measurements in the left anterior descending coronary artery (LAD) and PET flow measurements in the corresponding myocardial area. Coronary flow in LAD could be measured in all subjects using TTE. By TTE, an average CFR based on peak diastolic flow velocity (PDV) was 2.72 +/- 1.16, mean diastolic flow velocity (MDV) 2.56 +/- 1.06 and velocity time integral (VTI) 1.87 +/- 0.49. The results were reproducible in two repeated TTE studies (coefficient of variation in MDV 6.1 +/- 4.3%, n=8). By PET, CFR was 2.52 +/- 0.84. CFR assessed by TTE correlated closely with that measured by PET (MDV r=0.942, P<0.001; PDV r=0.912, P<0.002 and VTI r=0.888, P<0.006) and intraclass correlation was 0.929 (MDV) and tolerance limits for differences of CFRs was -0.78 to 0.72. We show that CFR measured by TTE has an excellent correlation with CFR measured by PET. We also found that TTE measurements of CFR were highly reproducible.

Peak blood flow measurement of coronary sinus by cine phase contrast MR imagings: comparison between breath-hold and respiratory compensation techniques

The aim of this study was to assess the feasibility of cine phase contrast (PC) magnetic resonance (MR) imaging for the peak blood flow measurement of the coronary sinus. Conventional PC imaging demonstrated the coronary sinus clearly and the significantly higher peak flow compared with the corresponding values measured with breath-hold fast cine PC imaging techniques at end-inspiration and end-expiration. This study showed the feasibility of conventional cine PC imaging with respiratory compensation in measurement of coronary sinus blood flow.

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.

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.

Magnetic resonance coronary angiography in patients with ischemic heart disease: analysis of coronary arterial blood flow velocity pattern

Only a few reports evaluating coronary arterial blood flow velocity patterns using magnetic resonance (MR) coronary angiography have appeared to date. This study reports an evaluation of coronary arterial blood flow velocity patterns in patients with ischemic heart disease and in healthy subjects using MR coronary angiography. The subjects consisted of 20 patients with ischemic heart disease (IHD group) and 20 normal healthy subjects (N group). Using the fCARD PC method, ECG-gated MR coronary angiography was performed using an anteroposterior opposing phased array coil. Regions of interest were placed on bilateral coronary arteries to measure coronary arterial blood flow velocity patterns. The IHD group was divided into two subgroups, based on the presence (MI group) or absence (AP group) of infarcted myocardium using 99m Tc-methoxyisobutylisonitrile (MIBI) myocardial scintigraphy. Average diastolic peak velocity (ADPV) was lower in the IHD group than in the N group. In addition, the diastolic / systolic velocity ratio (DSVR) was significantly lower in the MI group. Moreover, in the AP group, both the ADPV and DSVR values were significantly increased in those who had undergone percutaneous transluminal coronary angioplasty postoperatively. Different from the Doppler guidewire method, MR coronary angiography facilitates noninvasive evaluation of coronary arterial blood flow velocity. Therefore, these results indicate that MR coronary angiography represents a potentially useful technique for diagnosing lesions of coronary arteries and evaluating their functions. This noninvasive method can be expected to replace the invasive Doppler guidewire method in the near future with development of MR coronary angiography technology.

Three-dimensional cardiac cine magnetic resonance imaging with an ultrasmall superparamagnetic iron oxide blood pool agent (NC100150)

The purpose of this study was to assess image quality of three-dimensional (3D) cardiac cine magnetic resonance (MR) imaging before and after administration of a T1-shortening ultrasmall superparamagnetic iron oxide blood pool agent (NC100150). 3D cardiac cine MR imaging was performed in 13 volunteers using a radiofrequency-spoiled cardiac-gated 3D cine gradient-echo sequence with short repetition and echo times. Compared with precontrast images, postcontrast images showed no enhancement in fat and skeletal muscle, moderate enhancement in myocardium, and significant enhancement in ventricular cavity. After contrast injection, the signal ratio of the ventricular chamber to the myocardium significantly increased, and dramatic improvements were seen in the quality of the cineangiographic images and the depiction of cardiac valves. This quantitative study has shown that 3D cardiac cine MR imaging using a blood pool agent provided MR ventriculography and cineangiography with excellent image quality.

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.

Magnetic resonance imaging of valvular heart disease

The optimum management of patients with valvular heart diseases requires accurate and reproducible assessment of the valvular lesion and its hemodynamic consequences. Magnetic resonance imaging (MRI) techniques, such as volume measurements, signal-void phenomena, and velocity mapping, can be used in an integrated approach to gain qualitative and quantitative information on valvular heart disease as well as ventricular dimensions and functions. Thus, MRI may be advantageous to the established diagnostic tools in assessing the severity of valvular heart disease as well as monitoring the lesion and predicting the optimal timing for valvular surgery. This paper reviews the validation of these MRI techniques in assessing valvular heart disease and discusses some typical pitfalls of the techniques, including suggestions for solutions.J. Magn. Reson. Imaging 1999;10:627-638.

Quantification in cardiac MRI

Magnetic resonance imaging (MRI) offers several acquisition techniques for precise and highly reproducible assessment of global and regional ventricular function, flow, and perfusion at rest and under pharmacological or physical stress conditions. Recent advances in hardware and software have resulted in strong improvement of image quality and in a significant decrease in the required imaging time for each of these acquisitions. Several aspects of heart disease can be studied by combining multiple MRI techniques in a single examination. Such a comprehensive examination could replace a number of other imaging procedures, such as diagnostic X-ray angiography, echocardiography, and scintigraphy, which would be beneficial for the patient and cost effective. Despite the advances in MRI, quantitative image analysis often still relies on manual tracing of contours in the images, which is a time-consuming and tedious procedure that limits the clinical applicability of cardiovascular MRI. Reliable automated or semi-automated image analysis software would be very helpful to overcome the limitations associated with manual image processing. In this paper the developments directed toward automated quantitative image analysis and semi-automated contour detection for cardiovascular MR imaging are reviewed. J. Magn. Reson. Imaging 1999; 10:602-608.

Coronary MR angiography-a clinical experience in Japan

Magnetic resonance angiography (MRA) has been expected to provide a useful noninvasive means of assessing coronary artery disease as this disease continues to increase due to westernization of life style. Several Japanese investigators have assessed the diagnostic value of two-dimensional (2D) and 3D coronary MRA in clinical patients evaluated for ischemic heart disease. Almost all reports indicate a high correlation between findings on 2D MRA and findings on conventional coronary angiography (CAG) in patients with severe stenosis of proximal arteries. However, in our study involving 153 patients with ischemic heart disease, 2D MRA tended to underestimate lesions in patients with moderate stenotic lesions. Furthermore, this method could not be applied successfully in approximately 15% of our patients due to difficulty with breath-holding. These findings indicate some of the limitations associated with breath-holding in the 2D method. Recently, several reports have described high diagnostic accuracy using respiratory-gated 3D MRA with navigator echo. Effective use of a suitable contrast agent with better spatial and time resolution and better image reconstruction methods will enable 3D MRA to serve a useful role, even in screening for coronary artery disease. J. Magn. Reson. Imaging 1999;10:709-712.

Error assessment due to coronary stents in flow-encoded phase contrast MR angiography: a phantom study

Phase contrast magnetic resonance imaging (PC MRI) is a promising method for assessing coronary flow. MR angiography images in the presence of coronary stents display artifacts because of the metal present in the stent. Using a flow phantom, the goal of this in vitro study was to assess quantitatively the effects of flow dephasing caused by magnetic susceptibility in velocity measurements in a region where the artifact is not visualized in a magnitude image. The results showed that for high velocities, significant errors in measurements exist around the stent, outside the susceptibility artifact visible on a magnitude image. J. Magn. Reson. Imaging 1999;10:899-902.

Assessment of coronary flow reserve with fast cine phase contrast magnetic resonance imaging: comparison with measurement by Doppler guide wire

Fast cine phase contrast magnetic resonance imaging (fast cine phase contrast MRI) can measure phasic coronary flow velocity in humans. The purpose of this study was to compare the coronary flow velocity reserves measured by MR IMAGING with those obtained by Doppler guide wire. Nineteen patients with ischemic or valvular heart disease were studied. Fast cine phase contrast MR images of the left anterior descending (LAD) artery were acquired during breath-hold time in the basal state and after administration of dipyridamole. Flow velocity in the LAD artery was also measured with Doppler guide wire before and after venous injection of dipyridamole in all subjects. Flow velocity in the coronary artery measured with MR IMAGING in the basal state (12.5 +/- 4.9 cm/sec) was significantly lower than that obtained with Doppler guide wire (32.4 +/- 12.1 cm/sec, P < 0.01). However, MR assessments of coronary flow velocity reserve showed a good linear correlation with those measured by Doppler guide wire (r = 0.91). In conclusion, fast cine phase contrast MR imaging is a useful technique, which can provide a noninvasive assessment of flow reserve ratios in patients with coronary artery disease. J. Magn. Reson. Imaging 1999;10:563-568.

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.

Noninvasive assessment of the infarct-related coronary artery blood flow velocity using phase-contrast magnetic resonance imaging after coronary angioplasty

This study assesses infarct-related coronary artery blood flow velocity using phase-contrast magnetic resonance imaging (MRI) in patients with reperfused acute myocardial infarction (AMI) and compares these results with flow measurements obtained nonsimultaneously by intracoronary Doppler ultrasound. MRI examination was performed in 17 patients with AMI within 1 to 4 days (mean 2.5 days) after direct or rescue coronary angioplasty using a 0.014-in Doppler guidewire. MRI was performed on a 1.5-T clinical imager. The fast gradient echo segmented k-space phase-contrast pulse sequence was employed during breath-hold. The MRI and Doppler parameters of average peak velocity and maximum peak velocity were measured. Mean phase contrast MRI average peak velocity was 13.3+/-10.7 cm/s, and mean phase-contrast MRI maximum peak velocity was 27+/-16.6 cm/s. Mean Doppler average peak velocity was 17.1+/-5.1 cm/s, and mean Doppler maximum peak velocity was 35.5+/-10.1 cm/s. At the same anatomic levels, phase-contrast MRI average peak velocity correlated significantly to Doppler average peak velocity (r = 0.52; p<0.006) and Doppler maximum peak velocity (r = 0.42; p<0.03). Phase-contrast MRI velocity measurements were correlated with the same heterogeneity of Thrombolysis In Myocardial Infarction 3 flow velocity observed during Doppler examination. Thus, by comparing phase-contrast MRI with invasive intracoronary Doppler flow measurements, the measured MRI values showed significant correlation with Doppler data. Phase-contrast MRI has the potential to noninvasively quantify coronary flow velocity and to evaluate quality of reperfusion in patients with AMI after reperfused therapy.