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(Circulation. 1996;93:1502-1508.)
© 1996 American Heart Association, Inc.

Articles

Assessment of Coronary Arterial Flow and Flow Reserve in Humans With Magnetic Resonance Imaging

W. Gregory Hundley, MD; Richard A. Lange, MD; Geoffrey D. Clarke, PhD; Benjamin M. Meshack, BS; Jerry Payne, PA; Charles Landau, MD; Roderick McColl, PhD; Dany E. Sayad, MD; DuWayne L. Willett, MD; John E. Willard, MD; L. David Hillis, MD; Ronald M. Peshock, MD

From the Departments of Internal Medicine (Cardiovascular Division) (W.G.H., R.A.L., C.L., D.E.S., D.L.W., J.E.W., L.D.H., R.M.P.) and Radiology (G.D.C., J.P., R.M., R.M.P.), The University of Texas Southwestern Medical Center, Dallas.

Correspondence to Ronald M. Peshock, MD, Mary Nell and Ralph B. Rogers Magnetic Resonance Center, The University of Texas Southwestern Medical Center, 5801 Forest Park, Dallas, TX 75235-9085. E-mail peshock@rad-rogers.swmed.edu.

	Abstract

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Background The noninvasive measurement of absolute epicardial coronary arterial flow and flow reserve would be useful in the evaluation of patients with coronary circulatory disorders. Phase-contrast magnetic resonance imaging (PC-MRI) has been used to measure coronary arterial flow in animals, but its accuracy in humans is unknown.

Methods and Results Twelve subjects (7 men, 5 women; age, 44 to 67 years) underwent PC-MRI measurements of flow in the left anterior descending coronary artery or one of its diagonal branches at rest and after administration of adenosine (140 µg·kg^-1·min^-1 IV). Immediately thereafter, intracoronary Doppler velocity (IDV) and flow measurements were made during cardiac catheterization at rest and after intravenous administration of adenosine. For the 12 patients, the correlation between MRI and invasive measurements of coronary arterial flow and coronary arterial flow reserve was excellent: coronary flow_MRI (mL/min)= 0.85xcoronary flow_IDV (mL/min)+17 (mL/min), r=.89, and coronary flow reserve_MRI=0.79xcoronary velocity reserve_IDV+0.34, r=.89. For the range of coronary arterial flows (18 to 161 mL/min) measured by MRI, the limit of agreement between MRI and catheterization measurements of flow was -13±30 mL/min; for the range of coronary reserves (0.7 to 3.7) measured by MRI, the limit of agreement between the two techniques was 0.1±0.4.

Conclusions Cine velocity-encoded PC-MRI can noninvasively measure absolute coronary arterial flow in the left anterior descending artery in humans. PC-MRI can detect pharmacologically induced changes in coronary arterial flow and can reliably distinguish between those subjects with normal and abnormal coronary artery flow reserve.

Key Words: magnetic resonance imaging • coronary disease • regional blood flow

	Introduction

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Exercise or the administration of a pharmacological vasodilator can induce coronary microcirculatory hyperemia, thereby increasing coronary blood flow to more than four times its resting value.¹ The ratio of coronary arterial flow at maximal dilation to its baseline value (called "flow reserve")² has been used to assess the physiological importance of atherosclerotic coronary arterial narrowings³ as well as the results of balloon angioplasty⁴ and bypass grafting.⁵ In addition, measurements of flow reserve provide an assessment of the coronary microcirculation in the patient with a hypertrophic^{6 7} or dilated⁸ cardiomyopathy. Unfortunately, absolute measurements of coronary blood flow and flow reserve require invasive procedures (catheterization or open heart surgery).⁹ A noninvasive, widely available technique for measuring coronary arterial flow and flow reserve would be useful in the management of patients with a variety of coronary circulatory disorders.

PC-MRI flow measurements are noninvasive and do not require intravascular injections or the use of ionizing radiation. Hydrogen nuclei in blood moving through a magnetic field gradient accumulate a phase shift proportional to their velocity,¹⁰ and flow is calculated by integration of blood velocity over the cross-sectional area of the vascular structure of interest. PC-MRI accurately quantifies flow in the aorta^{11 12} and carotid arteries¹³ in humans. Previous investigators measured diastolic coronary arterial velocity in normal volunteers during periods of breath holding using k-space segmentation or PEG.¹⁴ However, absolute coronary flow was not measured, the results were not validated, and the utility of PC-MRI to discriminate normal from abnormal velocity reserve was not assessed. Recently, Clarke et al¹⁵ used cine PC-MRI with PEG to measure coronary flow and flow reserve in closed-chest animals with partial coronary artery occlusion and to discriminate between vessels with and without significant epicardial stenoses. Because the accuracy and applicability of this technique are unproved in humans, we performed this blinded, prospective study to assess the accuracy of cine breath-hold PC-MRI for measuring coronary arterial flow and flow reserve in humans.

	Methods

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Study Population
The study was approved by the Institutional Review Board for Human Experimentation at The University of Texas Southwestern Medical Center at Dallas, and all participants gave written, informed consent. The study population consisted of 15 subjects (10 men and 5 women; age, 44 to 67 years) referred for cardiac catheterization because of chest pain. Patients were ineligible for enrollment if they had a pacemaker, intracranial clips, cochlear or intraocular implants, a history of metal fragments in the eye, claustrophobia, marked ventricular ectopy (>20 premature ventricular contractions per minute), atrial fibrillation, previous coronary artery bypass grafting, unstable angina, myocardial infarction within 4 days of the procedure, or a contraindication to receiving adenosine (heart block or reactive airways disease). All substances or medications, such as caffeine, chocolate, mint, or theophylline, that might interfere with the action or metabolism of adenosine were withheld 24 hours before the study.

Study Design
Each subject underwent MRI scanning followed immediately by cardiac catheterization, so that both measurements were separated by <2 hours. During both studies, coronary arterial velocity and cross-sectional area were measured under resting conditions and after administration of adenosine 140 µg·kg^-1·min^-1 IV. This dose is commonly used during pharmacological coronary vasodilation myocardial scintigraphy with thallium or technetium sestamibi¹⁶ and is known to cause coronary vasodilation similar in magnitude to that caused by intracoronary injection of papaverine in normal subjects.¹⁷ Heart rate and systemic arterial pressure were monitored and recorded during both studies. All data, including heart rate, systemic arterial pressure, coronary velocity, and area determinations, were compiled, analyzed, and stored without knowledge of the findings obtained during the other procedure.

MRI Technique
MRI was performed with a 1.5-T Picker Vista HPQ whole-body imaging system (Picker International, Inc). A standard quadrature spine coil (20x26 cm) was used as a radiofrequency receiver. Each subject was imaged in the supine position after placement of ECG monitoring leads, a respiratory gating belt (to monitor the breath holds), and the surface coil on the chest. Imaging parameters for coronal and long-axis scout images of the heart were the same as for previously published techniques¹⁸ incorporating breath hold, fast-field echo sequences with first moment compensation in the readout and slice-select directions, a repetition time of 20 ms, and an echo time of 9.4 ms. PEG was used to obtain multiple phase-encoding steps for each frame during a cardiac cycle.^{19 20}

After obtaining scout views, we imaged the artery of interest in short-axis, tangential, and longitudinal planes. The purpose of these scans was to visualize the coronary vessel and to obtain a cross-sectional view perpendicular to the direction of flow throughout the cardiac cycle. The extra time spent in proper slice positioning was important to ensure that through-plane motion and partial volume effects would be minimal when vessel images were analyzed.¹⁵ In subjects in whom a signal void was detected proximally in the vessel of interest (indicating a potential arterial stenosis), cross-sectional images of the vessel were obtained along a straight segment distal to the area of dropout. Images were obtained with the same number of frames and PEG sizes as those of the scout images with two modifications: the field of view was decreased to 21 to 23 cm, thereby decreasing pixel sizes to 0.82 to 1.00 mm in the phase-encoding and readout directions, and an in-plane presaturation pulse was applied at the first frame of each cardiac cycle. The presaturation pulse was used to suppress signal from tissue in the slice so that in-flowing blood appeared bright and of different contrast relative to stationary tissue; no fat saturation pulses were used.

To measure flow, cine breath-hold PEG acquisitions with velocity compensation and encoding interleaved were positioned perpendicularly across vessel segments in the optimal slice positions determined from the cine scout images. A PEG size of 3 to 7 was selected for each subject studied to yield four to five frames per cardiac cycle. Other imaging parameters included a 7-mm slice thickness with a 256x256 to 256x224 matrix, a field of view of 21 to 23 cm (yielding pixel sizes of 0.82x0.82 to 0.89x1.00 mm), a flip angle of 40°, a repetition time of 19.5 ms, and an echo time of 11 ms. The total number of views was limited to keep the duration of the breath hold at 15 to 25 seconds. Resting coronary arterial flow was measured twice with the breath-hold technique, after which adenosine 140 µg·kg^-1·min^-1 IV was infused for 6 minutes. During the last 3 minutes of the infusion, coronary flow measurements were repeated twice (at 3 and 5 minutes into the infusion), each during periods of breath holding.

Velocity maps were generated by pixel-to-pixel subtraction of the velocity-sensitized and velocity-compensated phase images and the application of a correction algorithm designed to remove background phase error.^{11 21}

Flow was calculated by summing the flow per frame over the cardiac cycle and multiplying by the mean heart rate during the measurement:

(1)

where HR is heart rate (cardiac cycles per minute), n is number of frames in the cycle, F_i is flow in frame i of the cardiac cycle (cubic centimeters per frame), which is equal to the mean velocity over the vessel area (centimeters per second)xvessel area (square centimeters)x(2xPEG sizexrepetition time of the sequence) (seconds per frame). Vessel area was determined from

(2)

x Number of Pixels Within the Lumen of the Coronary Artery on the Magnitude Image

where FOV is the field of view.

With prospective gating, images were not acquired during the last 30 to 80 ms of diastole. For this terminal portion of the cardiac cycle, we estimated flow to be equivalent to flow in the last imaged diastolic frame.

MRI data were stored on optical disks for subsequent recall and analysis. To determine the interobserver variability of analyzing the PC-MRI images, images were analyzed by two investigators blinded to the results of the other investigator and catheterization. To evaluate the reproducibility of PC-MRI flow measurements, they were performed twice at baseline resting conditions and twice after the administration of adenosine. After completing the MRI scanning procedure, subjects were transferred immediately to the catheterization laboratory. The location within the coronary artery in which the PC-MRI flow measurements were made (with landmarks such as the branch points of septal and diagonal branches, the approximate distance in centimeters from the measurement point in the artery to the ostium of the left main coronary artery, and the distance distal to a potential stenosis)¹⁸ was given to the catheterization laboratory investigators. Other than the MRI measurement location in the coronary arterial segment, no results from the MRI study were given to the investigators in the catheterization laboratory.

Cardiac Catheterization
In each subject, a 7F or an 8F sheath was inserted percutaneously into the femoral artery. Intra-arterial and brachial cuff pressures, along with heart rate and rhythm, were monitored and recorded. A 7F diagnostic catheter was positioned in the left coronary ostium, and a single arteriogram was performed with nonionic contrast to exclude disease of the left main coronary artery. A 0.014-in Doppler velocity wire (FloWire, Cardiometrics Inc) was advanced into the vessel of interest to the same area of the PC-MRI flow measurements and positioned to obtain a high-quality phasic velocity and an APV during normal respiration and a 25-second breath hold.^{22 23} In subjects with coronary stenoses, the wire was positioned distal to the stenosis at a location derived from the landmarks supplied by the MRI investigators. After these baseline measurements were obtained, a cineangiogram was obtained with nonionic contrast to determine coronary arterial diameter. Coronary velocities were allowed to return to baseline; then the subject was given adenosine 140 µg·kg^-1·min^-1 IV over 6 minutes. Repeated coronary APV measurements were made 3 and 5 minutes into the infusion during breath hold and normal respiration. During the last minute of the infusion, coronary arterial diameter was reassessed angiographically.

Coronary velocity reserve was defined as the ratio of peak to resting APV after maximal dilation of the vascular territory with adenosine.²⁴ Time-averaged coronary flow was determined with the following equation:

(3)

where Flow_D is Doppler-derived time-averaged flow (milliliters per minute) and d is vessel diameter.²⁴ Mean velocity was estimated by assuming a parabolic velocity profile across the vessel, and vessel area was estimated by assuming a circular lumen. Cross-sectional diameter of the vessel 5 mm distal to the steel silhouette of the velocity wire seen on the cineangiograms (location of the sample volume reported by the manufacturer) was determined by use of computer-assisted quantitative coronary angiography (CAAS system, PIE Medical) according to previously published techniques.²⁵ Coronary flow reserve was defined as the ratio of peak to resting flow velocity.

Data Analysis
To be included in the analysis, each subject was required to complete both the MRI and catheterization portions of the study with heart rates that did not vary by >15% and mean systemic arterial pressures that did not vary by >20%. All data are expressed as mean±SD. The values for coronary arterial flow and coronary reserve obtained invasively were compared with those measured by MRI with a two-variable linear regression analysis. An analysis of the limits of agreement between the catheterization and MRI measurement methods as described by Bland and Altman²⁶ was performed.

	Results

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MRI studies were well tolerated in all subjects. Three subjects were excluded from further analysis. In 2, adequate velocity signals were not obtainable during catheterization; in 1, adequate signal-to-noise ratio for imaging the LAD was not obtained during MRI owing to the subject's large body habitus and large distance between the surface coil and the anterior surface of the heart. For the remaining 12 subjects, the subjects' mean height was 5 ft, 8 in (range, 5 ft, 2 in to 6 ft, 0 in), and the mean weight was 171 lb (range, 111 to 215 lb). All subjects were in sinus rhythm. During PC-MRI and catheterization, heart rates and systemic arterial pressures varied by 1±6% (range, 0 to 11%) and 7±6% (range, 0 to 17%), respectively. The duration of the MRI procedure (actual time spent in the magnet) ranged from 52 to 90 minutes (mean, 69 minutes). The Table gives detailed data on the 12 subjects. In subject 1, views for calibrating the diameter measurements of the LAD during catheterization were not available, so only catheterization velocity measurements are included. Fig 1 shows MRI magnitude images and velocity maps, along with Doppler velocity tracings, from subject 2.

View this table: [in this window] [in a new window]   Table 1. Summary of Patient Data

View larger version (135K): [in this window] [in a new window]   Figure 1. Cross-sectional images of LAD obtained in subject 2 during breath-hold velocity-encoded PC-MRI and corresponding Doppler flow wire recordings obtained in the catheterization laboratory. Magnitude images at rest (A) and during adenosine infusion (D) and the corresponding velocity images at rest (B) and during adenosine infusion (E) are shown. The white arrows on the magnitude images demarcate the LAD in cross section. From these magnitude images, a region of interest is selected for calculating the cross-sectional area of the vessel. This region of interest is then transferred directly to the velocity map, in which the intensity of the gray scale of each pixel within the region of interest encodes for a specific velocity. Within the central portion of the black circles on the velocity maps are the gray pixels that encode for the velocity within the LAD. The darker pixels in the velocity map (E) demonstrate the higher velocity within the artery after adenosine infusion. C (rest) and F (adenosine infusion) show the phasic (top) and mean (bottom) coronary flow velocities (centimeters per second) recorded with the Doppler flow wire during catheterization. Results from this subject are included in the Table.

Coronary arterial diameters by quantitative coronary arteriography ranged from 2.0 to 4.0 mm. The pixel sizes used to calculate coronary area by MRI ranged from 0.82x0.82 to 0.89x1.0 mm², and the number of pixels seen within the vessel lumen on all the frames sampled in this study ranged from 4 to 18. The difference between catheterization measurements of coronary flow and flow reserve in subjects with normal respiration and during 20 to 25 seconds of breath holding was -5±12 mL/min (-9±12%) and 0.1±0.2 (1±6%), respectively.

Fig 2 shows the correlation between PC-MRI and catheterization measurements of coronary arterial flow. As Fig 3 shows, there was good correlation between catheterization and PC-MRI coronary reserve measurements. The limits of agreement (defined as ±2 SD from the mean difference) between the invasive and PC-MRI measurements of arterial flow and coronary reserve are shown in Figs 4 and 5. For the range of coronary arterial flows (18 to 161 mL/min) measured by PC-MRI, the interobserver variability was -5±19 mL/min, and the reproducibility for repeated measurements was 5±13 mL/min. For the range of coronary flow reserves (0.7 to 3.7) measured by PC-MRI, the interobserver variability was 0±0.4 (5±14%).

View larger version (18K): [in this window] [in a new window]   Figure 2. Catheterization (x axis) and PC-MRI (y axis) measurements of coronary arterial flow for the 12 subjects. For each subject, two data points are displayed: baseline and after intravenous infusion of adenosine. The regression line (solid line) and equation are shown.

View larger version (15K): [in this window] [in a new window]   Figure 3. Catheterization (x axis) and PC-MRI (y axis) measurements of coronary reserve for the 12 subjects. Each symbol represents the data from one subject. The regression line (solid line) and equation are shown.

View larger version (15K): [in this window] [in a new window]   Figure 4. Mean coronary arterial flows by catheterization and PC-MRI (x axis) and the difference between catheterization and PC-MRI measurements (y axis) for the 12 subjects. For each subject, two data points are displayed: baseline and after infusion of adenosine. The mean difference (solid line) and ±2 SD from the difference (dashed lines) are shown.

View larger version (12K): [in this window] [in a new window]   Figure 5. Mean coronary reserve by catheterization and PC-MRI (x axis) and the difference between catheterization and PC-MRI measurements (y axis) for the 12 subjects. Each symbol represents the data from one subject. The mean difference (solid line) and ±2 SD from this difference (dashed lines) are shown. There is excellent agreement between PC-MRI and catheterization measurements of coronary reserve.

	Discussion

Top Abstract Introduction Methods Results Discussion References

 
In patients with symptomatic atherosclerotic coronary artery disease, coronary flow reserve measurements can be used to determine the physiological importance of epicardial stenoses³ and to assess the efficacy of coronary angioplasty⁴ or bypass grafting.⁵ In addition, processes other than epicardial atherosclerotic coronary artery disease attenuate the ability of coronary resistance vessels to dilate when exposed to pharmacological or exercise-induced stress. For example, coronary flow reserve may be blunted in subjects with atherosclerosis of the coronary vasculature not visible on contrast coronary arteriography²⁷ and in those with hypertrophic^{6 7} or dilated⁸ cardiomyopathies. Importantly, in experiments with animals and in humans with concentric left ventricular hypertrophy, disease regression leads to a normalization of coronary flow reserve.²⁸ In short, measurement of absolute coronary arterial flow and flow reserve provides a unique method of assessing disease processes that affect the coronary circulation at the epicardial and microcirculatory levels.

At present, absolute coronary arterial flow and flow reserve can be measured with a surgically placed flow probe, a Doppler velocity catheter or guide wire, or a coronary sinus thermodilution catheter.⁹ Each of these techniques is invasive and therefore impractical for repetitive assessments. The most commonly used noninvasive methods of measuring coronary flow reserve in humans can be quantitative (positron emission tomography²⁹ and Doppler transesophageal echocardiography³⁰ ) or qualitative (nuclear scintigraphy³¹ with thallium or technetium sestamibi radioisotopes). Although positron emission tomography can directly quantify myocardial perfusion, it requires the proximity of a cyclotron and is relatively expensive.²⁹ Doppler transesophageal echocardiography has been used to estimate coronary flow reserve,³⁰ but it is somewhat uncomfortable (because it requires esophageal intubation), and localizing the Doppler sample volume distal to a stenosis or a major side branch is difficult. Importantly, neither positron emission tomography nor Doppler transesophageal echocardiography quantifies absolute flow within a coronary vessel.³² Currently, the most widely used method for assessing myocardial perfusion is radionuclide perfusion imaging, but this process requires intravenous administration of a radioisotope and estimates regional myocardial blood flow relative to other regions of the heart, rather than quantifying absolute coronary arterial flow and flow reserve.^{31 32}

Velocity-encoded PC-MRI measurements of flow in small-caliber phantom models and coronary arteries in animals have proved accurate and reproducible.^{15 21} They are noninvasive and do not require the use of ionizing radiation or the administration of radioactive isotopes. Unlike Doppler echocardiography, which samples a velocity profile over a limited sample volume and assumes a velocity profile within the lumen of the vessel of interest, PC-MRI directly samples the entire velocity profile within the vessel. Our data allow us to reach several important conclusions. First, PC-MRI can determine absolute coronary arterial flow in humans, and PC-MRI measurements correlate well with those made invasively (see the Table and Figs 2 and 4). Second, PC-MRI can accurately assess pharmacologically induced changes in coronary arterial flow, thereby providing a reliable noninvasive assessment of flow reserve (see the Table and Figs 3 and 5). Additionally, with proper hemodynamic monitoring in the MRI suite, intravenous adenosine can be administered safely to patients undergoing an MRI examination. It is well tolerated by patients during 20 to 25 seconds of breath holding. As reflected by our catheterization data, 20- to 25-second periods of breath holding do not substantially alter coronary arterial flow or flow reserve compared with normal respiratory patterns.

Three technical differences exist between this and previous MRI assessments of coronary flow reserve in humans. First, instead of lying prone on the surface coil, our subjects were positioned supine, with the surface coil resting on the chest; this technique was well tolerated. Second, cine MRI with PEG was used to obtain flow information throughout the cardiac cycle instead of during a single diastolic frame. At baseline, the majority of flow occurred during the first and last frames of our cine sequences, whereas with adenosine infusion, the greatest percentage change in flow was seen during the second and third frames of our sequence. The sampling of multiple frames during the cardiac cycle appeared to be important for accurate measurement of coronary flow reserve in our subjects. Third, we acquired images with 0.82x0.82- to 0.89x1.00-mm² pixel resolution. Tang et al³³ showed that the error of measured volume flow rate is <10% if the ratio of in-plane pixel dimension to vessel radius is <0.5, thereby implying that 9 to 16 pixels would have to be sampled within the vessel lumen to obtain accurate flow measurements. With the imaging parameters used in this study, the data of Tang et al would suggest that accurate flow measurements could be obtained only in >3.5- to 4-mm-diameter vessels. However, more recently, Hofman et al³⁴ showed that accurate PC-MRI flow information can be obtained in vessels in which only four pixels are sampled within the vascular lumen. Our data suggest that accurate coronary arterial flow and flow reserve measurements can be obtained in vessels >2 to 2.5 mm in diameter with as few as four to eight pixels sampled within the vascular lumen.

Using PC-MRI to obtain routine clinical measurements of coronary arterial flow and flow reserve in humans is appealing for several reasons. First, it is safe and easy to perform in an outpatient setting without an intravenous contrast agent. Second, it directly visualizes the coronary vessel of interest and provides quantitative epicardial flow information in a single procedure. Third, MRI is widely available. Fourth, because MRI is noninvasive, serial quantitative assessments are performed easily, a helpful feature when patients are followed long term and therapeutic interventions are monitored. Fifth, the technique described can be combined with other cardiovascular MRI examinations, such as determination of myocardial mass and ventricular volumes, ejection fraction, or systolic function. Finally, the use of accessory materials is minimal. MRI is not associated with the procurement, handling, and administration of radioactive isotopes or the one-time use of intracoronary Doppler guide wires.

Our study has limitations. First, all our subjects were in sinus rhythm. None had frequent ventricular ectopy or atrial fibrillation. We are uncertain whether this technique provides reliable results in subjects with irregular rhythms. Second, we used intravenous adenosine to induce coronary vasodilation. Although it is widely used clinically and causes coronary artery vasodilation similar in magnitude to that caused by intracoronary injection of papaverine in normal subjects, its effects in subjects with disease processes affecting the coronary microcirculation (ie, diabetes mellitus, concentric hypertrophy, or hypercholesterolemia) are not well described. Third, although MRI data are acquired rapidly, processing and analysis may be time-consuming when performed manually. However, with automated analysis programs, these times are reduced substantially (<1 minute for flow measurements).³⁵ Fourth, we measured coronary arterial flow only in the LAD or its diagonal branches. Previous MRI coronary angiographic studies³⁶ noted decreased sensitivity and specificity for detecting vessels on the lateral wall of the left ventricle; however, special coils or phased array units that improve signals from the lateral and posterior walls may improve these results.³⁷ Fifth, in three subjects (subjects 9 through 11, the Table), MRI flow measurements tended to be larger than invasive measurements. In these subjects, the matrix size was decreased (pixel sizes were 0.95 mm²), and undersampling was used to decrease the duration of the breath hold. Partial volume effects (causing overestimation of coronary arterial area) or wraparound noise artifact may have introduced errors into the MRI flow calculations. Finally, while Doppler-derived assessments of coronary velocity and flow reserve are used widely, they are not a perfect gold-standard measurement technique. Doppler wire velocity measurements assume a parabolic velocity profile within the vascular lumen, and coronary area determinations are derived from formulas that assume a circular vessel lumen.

In conclusion, cine velocity-encoded PC-MRI can noninvasively measure absolute coronary arterial flow in the LAD in humans. PC-MRI can detect pharmacologically induced changes in coronary arterial flow and reliably distinguish between those subjects with normal and abnormal coronary arterial flow reserve. Without the use of ionizing radiation or intravenous contrast agents, it provides a unique, widely available, well-tolerated method in humans for directly visualizing coronary arteries and assessing the physiological response of the coronary circulation to pharmacological stimuli.

	Selected Abbreviations and Acronyms

 

APV = time-averaged instantaneous spectral peak velocity LAD = left anterior descending coronary artery PC-MRI = phase-contrast magnetic resonance imaging PEG = phase-encoding grouping

	Acknowledgments

 
This research was supported in part by grants from the Society of Cardiac Angiography and Intervention and Bracco Diagnostics, NIH Special Center of Research (Ischemic SCOR grant HL-17669), NHLBI (Training in Cardiovascular Research grant HL-07360-18), and Picker International, Inc. Adenosine used for this project was kindly supplied by Fujisawa, Inc.

Received September 20, 1995; revision received November 1, 1995; accepted November 5, 1995.

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