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Cardiac Imaging |
1 From the Departments of Radiology (N.I., H.S., K.T.) and Thoracic Surgery (B.P.C., T.S., T.T., I.Y.), Mie University School of Medicine, 2-174 Edobashi, Tsu, Mie 514-8507, Japan; and the Department of Radiology, University of California Medical Center, San Francisco (C.B.H.). Received July 20, 2000; revision requested September 7; final revision received February 15, 2001; accepted February 26. Address correspondence to N.I. (e-mail: nanaka@clin.medic.mie-u.ac.jp).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Twenty-six consecutive patients who had undergone coronary artery bypass surgery were examined. Breath-hold velocity-encoded cine MR images were obtained at the midpoint of the internal mammary artery between its origin from the subclavian artery and the distal anastomosis to the left anterior descending artery.
RESULTS: MR images were obtained successfully in 24 patients. At conventional angiography, no significant stenosis was observed in 17 patients (group A), and significant stenosis (diameter > 70%) was observed in seven patients (group B). The mean diastolic-to-systolic peak velocity ratio in group B (0.61 ± 0.44 [SD]) was significantly lower than that in group A (1.88 ± 0.96; P < .01). Evaluation of graft stenosis with the diastolic-to-systolic peak velocity ratio revealed a sensitivity of 86% and a specificity of 88%. The mean blood flow rate at baseline in group B (16.9 mL/min ± 5.5) was significantly lower than that in group A (79.8 mL/min ± 38.2; P < .01). The sensitivity and specificity of MR blood flow measurement in predicting significant stenosis were 86% and 94%, respectively. The mean pharmacologic flow reserve ratios were 2.00 ± 1.43 in group A and 1.39 ± 1.46 in group B (P > .05).
CONCLUSION: Fast MR blood flow measurement at baseline is highly useful for predicting significant stenosis in internal mammary arterial grafts.
Index terms: Arteries, MR, 54.121416, 54.12144, 949.129416, 949.12944 • Arteries, stenosis or obstruction, 949.721 • Coronary vessels, flow dynamics, 54.91, 949.91 • Coronary vessels, MR, 54.121416, 949.129416 • Magnetic resonance (MR), flow studies, 54.121416, 54.12144, 949.129416, 949.12944 • Stents and prostheses, 949.1268
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Assessments of blood flow volume and flow reserve (8,9), in addition to enabling the demonstration of the anatomic stenosis of the graft seen at conventional angiography, enable the evaluation of the function of the bypass conduit. The results of several studies (10–12) have demonstrated the feasibility of flow measurements in coronary artery bypass grafts at velocity-encoded cine MR imaging. However, the value of MR flow parameters of internal mammary arterial grafts, such as blood flow rate, systolic and diastolic flow pattern, and flow reserve ratio, in predicting the significant graft stenosis revealed at coronary angiography has not been established.
The purposes of the current study were to evaluate the sensitivity and specificity of breath-hold MR flow measurement in the detection of significant stenosis in the internal mammary artery–to–coronary artery bypass conduit, with conventional angiography as the reference standard, and to determine whether pharmacologic stress testing can improve the MR detection of graft stenosis.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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MR Imaging
MR images were acquired by using a 1.5-T MR imaging unit (Signa Horizon; GE Medical Systems, Milwaukee, Wis). The mean time (± SD) between internal mammary artery bypass graft surgery and MR imaging was 25 days ± 11. MR flow measurements were obtained in all patients within 1 week after selective internal mammary and coronary angiography. The subjects were situated in a supine position, with a 3-inch (7.6-cm) surface coil on the anterior chest wall. All patients had a sinus rhythm.
Fast velocity-encoded cine MR images were acquired during a single breath hold in a double oblique imaging plane perpendicular to the internal mammary artery. The following imaging parameters were used: 14/5 (repetition time msec/echo time msec), section thickness of 5 mm, field of view of 20–24 cm, 96 phase-encoding steps, four views per segment, true temporal resolution of 112 msec, effective temporal resolution of 56 msec, and a view-sharing reconstruction (11). The spatial resolution for data acquisition was 1.56 x 0.78 mm, and the pixel size of the reconstructed images was 0.78 x 0.78 mm. Image data were acquired for 24 heartbeats (20–25 seconds). Velocity-encoding gradients were applied in a section-selective direction with a velocity window of plus or minus 80 cm/sec. The maximal flow velocity of a pixel within the internal mammary artery in the entire group of patients was 75.7 cm/sec, so no aliasing was observed in any patient.
After breath-hold velocity-encoded cine MR images were obtained at baseline, 0.56 mg of dipyridamole per kilogram of body weight was injected into the antecubital vein for 4 minutes. Velocity-encoded cine MR images were obtained 2 minutes after finishing the dipyridamole injection. The heart rate and blood pressure were monitored and recorded during the entire examination. The MR imaging procedure lasted approximately 45 minutes. Any complication related to intravenous administration of dipyridamole was recorded.
Flow velocity and volume were analyzed with computer software (XPHASE; Stephan Maier, MD, PhD, Brigham and Women’s Hospital, Boston, Mass) without knowledge of the contrast-enhanced coronary angiographic results. MR flow measurements were obtained at the approximate midpoint of the internal mammary artery between its origin from the subclavian artery and the distal anastomosis to the left anterior descending artery, because the internal mammary artery has a relatively straight course in this segment. One author (N.I.) manually placed regions of interest with dimensions equal to the graft diameters on interpolated images with matrices of 1,024 x 1,024 to evaluate the area of and mean velocity in the graft. We also measured the velocity in the anterior mediastinal fat tissue adjacent to the graft or in the sternum to perform baseline phase correction.
The volume flow rate was quantified by multiplying the area by the mean velocity in the internal mammary arterial graft. The vasodilator flow reserve ratio was calculated as a ratio of the hyperemic to baseline flow volumes in the internal mammary arterial graft. The diastolic/systolic peak velocity ratio was calculated by placing a small region of interest (1.12–2.18 mm2; mean ± SD, 1.88 mm2 ± 0.57) within the internal mammary arterial graft. The time required to analyze the blood flow curve and volume flow rate in the graft was less than 10 minutes after the MR images were transferred to the workstation (Ultra 5; Sun Microsystems, Palo Alto, Calif).
Conventional Angiography and Flow Measurement with a Doppler Guide Wire
Selective conventional angiography of the internal mammary arterial graft was performed in all patients. Cine angiographic results were used as the reference standards for graft patency and stenosis. All bypass grafts were visually examined in at least two projections by one investigator (H.S.), who was unaware of the MR data. The stenosis was considered to be significant when greater than 70% luminal diameter narrowing was seen in at least one projection. In 10 patients, flow velocity in the internal mammary arterial grafts was measured with a Doppler guide wire (FloWire; EndoSonics, Rancho Cordova, Calif) to validate the MR flow measurements. A 0.0018-inch guide wire with a 12-MHz piezoelectric ultrasound transducer at its tip was advanced into the internal mammary artery through a 5-F catheter. Flow velocity was recorded by using the frequency shift between the transmitted and returning signals. Doppler flow velocity data were obtained at baseline in the middle portion of the internal mammary arterial graft that corresponded approximately to the location at which MR flow measurements were obtained. Systolic and diastolic peak velocities were computed from the phasic blood flow velocity recordings. The maximal blood flow velocity within the vessel lumen during the diastolic phase measured at MR imaging was compared with the diastolic peak velocity measured by using the Doppler guide wire in the resting state. Any complication related to selective angiography or insertion of the Doppler guide wire was recorded.
Statistical Analysis
All values were expressed as mean values plus or minus the SD. The statistical significance of differences in mean values between two groups (patients with and those without significant stenosis) was evaluated with a two-tailed t test. The statistical significance of differences between the MR and Doppler flow velocities was assessed with a paired two-tailed t test. P values of less than .05 were considered to indicate a statistically significant difference.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Angiographic Analysis of Internal Mammary Arterial Grafts
The internal mammary arterial graft was patent at selective conventional angiography in all patients. The 24 patients with successful MR studies were assigned to two groups according to the assessment of stenosis at cine angiography: Group A comprised 17 patients without significant graft stenosis, and group B comprised seven patients with significant graft stenosis (>70% luminal diameter narrowing). No patient had complications related to selective angiography or insertion of the Doppler guide wire.
Hemodynamics at Rest and after Dipyridamole Administration during MR Imaging
The mean systolic blood pressure, mean diastolic blood pressure, and mean heart rate in patients with internal mammary arterial grafts were 124.8 mm Hg ± 12.9, 70.5 mm Hg ± 10.5, and 82.5 beats per minute ± 18.0, respectively, at baseline and 122.8 mm Hg ± 11.8, 68.0 mm Hg ± 11.8, and 87.6 beats per minute ± 16.5, respectively, after dipyridamole administration. There was no significant difference between the systolic and diastolic blood pressures before and those after dipyridamole administration. The heart rate increased significantly after dipyridamole administration (P < .05). No patients experienced adverse effects from dipyridamole administration.
Variables Obtained with MR Flow Measurement
The typical blood flow–versus-time curves acquired in the internal mammary arteries had a biphasic flow pattern, with an initial peak during early systole and a later peak during early diastole. In group A—the patients without significant stenosis—the flow pattern was characterized by predominant flow during the diastolic phase (Fig 1). In contrast, diastolic flow was reduced and systolic flow was predominant in group B—the patients with significant stenosis (Fig 2). The mean diastolic-to-systolic peak velocity ratio in group B was 0.61 ± 0.44 (Fig 3), which was significantly lower than that in group A, 1.88 ± 0.96 (P < .01). With a threshold value of 1.0, the sensitivity and specificity of the diastolic-to-systolic peak velocity ratio in predicting the graft stenosis seen at conventional angiography were 86% (six of seven patients; 95% CI: 48.6%, 99.2%) and 88% (15 of 17 patients; 95% CI: 72.9%, 93.8%), respectively.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Flow Pattern in Coronary Artery Bypass Grafts
Flow measurement with fast velocity-encoded cine MR imaging demonstrated a biphasic flow pattern in internal mammary artery bypass grafts. The blood flow pattern in the normal internal mammary arterial grafts was similar to that in the normal left anterior descending arteries, with a diastolic-to-systolic peak velocity ratio that was higher than 1.0. These results suggest that the grafts direct blood flow into the recipient arteries without stenotic resistance. In contrast, flow during the diastolic phase was decreased in patients with significant graft stenosis, and this resulted in a significantly reduced diastolic-to-systolic peak velocity ratio that was less than 1.0. The diastolic-to-systolic peak velocity ratio measured with MR imaging seems to be an effective diagnostic parameter for detecting graft stenosis: In the current study, both its sensitivity and its specificity were more than 85%.
The diastolic-to-systolic velocity ratio has been measured previously with transthoracic and intravascular Doppler ultrasonography (US). Takagi et al (13) measured the diastolic-to-systolic peak velocity ratio at a proximal site in 56 left internal mammary arterial grafts by using Doppler US from a window at the supraclavicular fossa. They reported that a low diastolic-to-systolic peak velocity ratio (<0.6) predicted severe stenosis (>75% diameter narrowing) of internal mammary arterial grafts with a sensitivity of 80% and a specificity of 100%. The results of the current study are in agreement with their report.
Location of MR Flow Measurement
We measured blood flow in the middle segment of the internal mammary artery, because this artery shows the least tortuosity at this parasternal segment at MR imaging and motion blurring due to cardiac contraction is much less severe than that in the distal segment near the heart. It should be noted that systolic and diastolic blood flow patterns in internal mammary arterial grafts and the optimal cutoff values can be substantially different, depending on the location at which blood flow is measured. Bach et al (14) measured phasic blood flow velocity in 27 patients by using an intravascular Doppler guide wire in the proximal, middle, and distal segments of internal mammary arterial grafts. The flow velocity in the proximal one-third of the normal internal mammary arterial graft demonstrated a predominant systolic peak and lower diastolic peak (diastolic-to-systolic peak velocity ratio, 0.6 ± 0.2). In the middle to distal segments of the graft, the flow pattern showed a transition to the flow pattern of the native left anterior descending artery, with an increasing diastolic peak and decreasing systolic peak (diastolic-to-systolic peak velocity ratios, 1.0 ± 0.3 and 1.4 ± 0.3, respectively). These findings can be explained by the resistance and passive capacitance of the vessel. During systole, blood can flow in the proximal part of the internal mammary artery because of the high systemic blood pressure and capacitance of the vessel. In the distal part, runoff from the distal internal mammary artery to the native coronary artery is hampered by the high resistance in the left ventricular myocardium during the systolic phase.
MR Quantification of Blood Flow Volume in Internal Mammary Arterial Grafts
MR quantification of blood flow volume can be performed by multiplying the flow velocity by the cross-sectional area of the coronary artery during the cardiac cycle. The mean MR flow volume measured in the internal mammary arterial grafts without significant stenosis (79.8 mL/min ± 38.2) in our study was in accordance with previously reported results obtained by using other modalities. The mean baseline blood flow volumes in the internal mammary arterial grafts without significant stenosis measured by Crowley and Shapiro (15) and Takemura et al (16) by using transthoracic echocardiography were 65 mL/min ± 23 and 56 mL/min ± 14, respectively; that measured by Akasaka et al (17) by using an intravascular Doppler guide wire was 62 mL/min ± 17. Hoogendoorn et al (18) used non–breath-hold MR imaging and reported a mean blood flow volume in the normal saphenous vein graft of 71 mL/min ± 17. They observed that the mean blood flow volume in the stenotic or occluded graft was reduced to 9 mL/min ± 8 (P < .001). In contrast to diastolic-to-systolic peak velocity ratio and flow velocity measurements, measurement of blood flow volume is less influenced by the location of the measurement, and this can be a substantial advantage in serial assessments of graft function in patients after coronary bypass graft surgery.
Vasodilator Flow Reserve Ratio
The results of previous studies (13,19) to measure blood flow only at baseline suggest that the noninvasive determination of the flow reserve ratio would help to identify graft stenosis. In our study, we observed no significant difference in the flow reserve ratios between groups A and B. Akasaka et al (17) measured flow velocities in internal mammary arterial grafts by using a Doppler guide wire and found the mean flow reserves of recently placed internal mammary arterial grafts to be significantly lower than those of grafts that had been in place for a long time (1.8 ± 0.3 vs 2.6 ± 0.3). Internal mammary arterial grafts assessed early after operation were characterized by higher peak velocities at baseline to compensate for the smaller diameter, and the early postoperative flow reserve was low owing to high baseline flow velocity. Because MR flow measurement was performed during the early postoperative period in this study, in actual practice, the restricted flow reserve in the early postoperative state might increase later.
MR Flow Measurement versus Other Noninvasive Techniques
Several investigators have measured the flow velocity in coronary artery bypass grafts by using Doppler US. Success rates in detecting blood flow velocity in the internal mammary artery were not very high in initial studies. Fusejima et al (20) measured blood flow velocities at a distal site in left internal mammary arterial grafts by using a parasternal approach, with a success rate of 79%. In a study by Kyo et al (21), the success rate in measuring flow velocity at the proximal site of the internal mammary artery with the parasternal approach was 55%.
Study Limitations
Sternal wires and metallic clips can cause attenuated signal intensity around the metal, and this can interfere with flow measurement. The solution to this problem in the current study was the use of titanium clips. By using titanium clips, diagnostic velocity-encoded cine MR images of the grafts were obtained in more than 90% of the patients.
There are several potential sources of error in measuring coronary blood flow volume by using the described phase-contrast MR technique, including partial volume averaging and motion blurring due to cardiac contraction (22). The results of previous studies with animal models demonstrate that fast MR phase-contrast MR imaging can enable accurate quantification of coronary arterial blood flow (23,24). In our study, blurring of the internal mammary artery due to cardiac contraction was less than that of the native coronary artery; thus, there should be fewer errors in the MR quantification of blood flow volume in internal mammary arterial grafts than in native coronary arteries.
The flow velocity measurements obtained with velocity-encoded cine MR imaging were significantly lower than those obtained by using the Doppler flow wire method. This is likely due to the fact that with the MR technique, velocity is averaged within the imaging voxel and with the Doppler flow wire technique, the peak of the velocity spectrum distribution occurs within the ultrasound beam. In addition, lower temporal resolution during the cardiac cycle and an incomplete breath hold during data acquisition can result in a decreased flow velocity value at MR imaging.
Another limitation of this study was the relatively limited number of subjects with significant graft stenoses. The clinical symptoms of the patients and the status of native runoff vessels were not assessed in the current protocol.
Clinical Implications
MR blood flow measurement does not eliminate the need for conventional angiography when coronary reinterventions are under immediate consideration. However, MR blood flow measurement in the graft is useful in the following clinical circumstances: (a) In patients with chest pain early after graft operation, MR blood flow measurement can help determine whether the chest pain episodes are associated with graft occlusion or dysfunction (25). (b) Routine MR screening for graft occlusion or narrowing might be useful in patients without anginal symptoms, because occult myocardial ischemia, which can be found in up to one-third of patients after bypass graft operation, adversely affects the postoperative prognosis (26). (c) MR blood flow measurement may be useful for further treatment of patients when graft stenosis is demonstrated at conventional angiography. MR flow parameters can be used to determine the functional importance of mild or intermediate stenosis at conventional angiography. In addition, noninvasive MR flow measurement permits serial assessment of graft function, which is useful in detecting gradual increases in graft stenosis before the onset of total occlusion.
In conclusion, our study results demonstrate that MR quantification of the systolic-to-diastolic peak velocity ratio and volume flow rate has high diagnostic accuracy for predicting significant stenoses in internal mammary arterial grafts. Breath-hold velocity-encoded cine MR imaging is rapid and effective for functional assessment of internal mammary arterial grafts and enables noninvasive detection of significant graft stenosis with high sensitivity and specificity.
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FOOTNOTES |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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This article has been cited by other articles:
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N I Stauder, A M Scheule, U Hahn, M Fenchel, F S Eckstein, U Kramer, C D Claussen, and S Miller Perioperative monitoring of flow and patency in native and grafted internal mammary arteries using a combined MR protocol Br. J. Radiol., April 1, 2005; 78(928): 292 - 298. [Abstract] [Full Text] [PDF]
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