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Cardiac Imaging |
1 From the Department of Diagnostic Radiology and Organ Imaging (N.M.C.S., W.W.M.L.) and Department of Medicine and Therapeutics (C.W.C, K.Y.C., J.E.S.), Faculty of Medicine, Chinese University of Hong Kong, Prince of Wales Hospital, Shatin, New Territories, Hong Kong. Received November 27, 2001; revision requested January 14, 2002; revision received May 14; accepted July 24. Address correspondence to N.M.C.S. (e-mail: so2173@cuhk.edu.hk).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: The data of 13 patients (mean age, 68 years; range, 40–85 years) at high risk for myocardial infarction who underwent MR imaging at 1.5 T were reviewed. Risk factors were increased troponin T levels in seven, reversible ST depression on an electrocardiogram in four, history of myocardial infarction in two, and presence of heart failure in four. Cine imaging of the left ventricle was performed with a true–fast imaging with steady-state precession (FISP) sequence to assess the regional myocardial contraction and ejection fraction. After injection of 0.1 mmol per kilogram of body weight of gadopentetate dimeglumine, first-pass MR images were obtained by using an inversion-recovery true-FISP sequence at rest and during infusion of adenosine (140 µg/kg/min). Resting and stress images were assessed qualitatively for abnormal regional perfusion (hypoenhancement). The myocardium was divided into three radial segments corresponding to the three coronary artery territories. Delayed (after 15 minutes) contrast material–enhanced images were acquired with use of a segmented inversion-recovery fast low-angle shot sequence. Conventional coronary angiograms were compared with the first-pass images. A more than 50% stenosis in diameter in any coronary artery was considered substantial. Mann-Whitney test was used to assess any significant difference between the left ventricular ejection fraction (LVEF) in patients with and those without myocardial infarct.
RESULTS: Mean LVEF was 51.5% (range, 30%–77%). First-pass stress perfusion studies depicted 25 segments of hypoenhancement in 11 patients. Comparison of first-pass perfusion defects with findings on coronary angiograms indicated an overall sensitivity of 92% (24 of 26) and specificity of 92% (12 of 13) in detection of substantial coronary artery disease. Infarcts detected from hyperenhancement on delayed contrast-enhanced images were present in eight segments (four were transmural) in five patients. No significant difference was noted in the LVEF between patients with and those without infarct (P = .724).
CONCLUSION: Combined stress perfusion and viability MR imaging was feasible in patients with acute coronary syndromes. First-pass MR perfusion defects compare well with the presence of substantial coronary artery stenosis on conventional angiograms.
© RSNA, 2003
Index terms: Coronary vessels, stenosis or obstruction, 54.762 • Magnetic resonance (MR), comparative studies, 54.121413, 54.121416, 54.12143, 54.12149 • Magnetic resonance (MR), contrast enhancement, 54.12143
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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The acute coronary syndromes, which are unstable angina and evolving myocardial infarction, share a common anatomic substrate. Pathologic, angioscopic, and biologic observations have demonstrated that unstable angina and myocardial infarction have different clinical manifestations that result from a common underlying pathophysiologic mechanism, namely, atherosclerotic plaque rupture or erosion, with differing degrees of superimposed thrombosis and distal embolization (5). In patients with an established diagnosis of acute coronary syndrome, the treatment strategy to be selected in a particular patient depends on the perceived risk of progression to myocardial infarction or death.
Results of recent clinical trials indicate that a clinical strategy incorporating careful risk stratification in conjunction with therapeutic agents and revascularization in adequately selected patients may help to improve both immediate and long-term outcomes. The Task Force of the European Society of Cardiology (2) recommended the following methods for risk stratification: clinical, biologic, and angiographic markers of underlying disease to assess the long-term risk and markers of thrombotic risk to assess the immediate risk. These would entail various investigations including echocardiography, exercise electrocardiography, radionuclide scanning, and coronary angiography.
Magnetic resonance (MR) imaging is used to assess cardiac structure and function, ventricular mass and volume, and myocardial perfusion and to determine infarct size (6). One key to the advancement of cardiac MR imaging as a clinical tool in the evaluation of patients with ischemic heart disease is the development of an integrated examination. In patients early after acute myocardial infarction, rapid MR imaging techniques have been used individually to assess left ventricular structure, global and regional function, infarction artery patency, or contrast material uptake. MR imaging could be used for a comprehensive evaluation in patients after acute myocardial infarction to study all these parameters in less than 1 hour. MR perfusion studies had been performed successfully in patients with established myocardial infarction (7) within a few days of symptom onset. But so far no published data are available for a less severe group of patients with unstable angina and acute coronary syndromes. The purpose of our study was, therefore, to assess the feasibility of combined perfusion and viability testing by using MR imaging in one setting in patients with non–ST segment-elevation acute coronary syndromes.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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The diagnosis of acute coronary syndrome was determined in the conventional manner by using history, electrocardiographic findings (flat or downslope ST depression or T-wave inversions) (8), and plasma troponin T levels (
0.1 ng/mL). All patients had a normal creatine kinase level. The patients’ clinical characteristics are listed in Table 1. Eleven patients had chest pain compatible with myocardial ischemia at presentation. Two patients had dyspnea due to heart failure at presentation. They all carried high-risk features for the subsequent cardiovascular events of death and myocardial infarction; these features were increased troponin T level in seven, reversible ST depression on electrocardiograms in four, history of myocardial infarction in two, and presence of heart failure in four patients.
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MR Imaging Protocol
All MR images were obtained with a 1.5-T unit (Sonata Magnetom; Siemens, Erlangen, Germany) with use of a phased-array body coil. Electrocardiograms and heart rate were monitored with the physiologic monitor of the imager (MR-compatible monitoring system). All patients were examined in the supine position and had their blood pressure and pulse rate measured by the nursing staff before the examination. A 20-gauge cannula was placed in an antecubital vein for injection of contrast material. A 22-gauge cannula was placed in a peripheral vein in the other arm for injection of adenosine.
Scout images were obtained to determine the exact position and axis of the left ventricle. Cine short-axis images of the left ventricle from base to apex were obtained by using the true–fast imaging with steady-state precession (FISP) two-dimensional sequence (repetition time msec/echo time msec, 32–48/1.6; flip angle, 60°; field of view (FOV), 263 x 350 mm2; matrix, 120 x 256). These cine images were used to calculate the left ventricular ejection fraction (LVEF) by using the standard ventricular analysis software (Argus N3.5 VA13; Siemens, Princeton, NJ) on the imaging and satellite consoles.
Three short-axis sections were chosen for perfusion imaging with an electrocardiographically triggered T1-weighted inversion-recovery true-FISP sequence (repetition time msec/echo time msec/inversion time msec, 634/0.86/84; flip angle, 8°; FOV, 285 x 380 mm2; matrix, 72 x 128; imaging time, >9 seconds; section thickness, 10 mm). The locations of the three sections were at the base, at the level of the midpapillary muscles, and at the apex. During an expiratory breath hold, a bolus of gadopentetate dimeglumine (Magnevist; Schering, Berlin, Germany), 0.05 mmol per kilogram of body weight, was injected at 5 mL/sec and flushed with 20 mL of normal saline by using a power injector (Spectris; Medrad, Indianola, Pa). Sixty to 80 dynamic images were acquired simultaneously at each of the three levels during the first and second pass of the contrast agent. Patients were instructed to hold their breath as long as possible and to breathe quietly and slowly when necessary.
Immediately after the first set of dynamic images was obtained, 140 µg/kg/min of adenosine (Adenoscan; Sanofi Winthrop Industries, Notre Dame De Bondeville, France) was administered by using a syringe pump (Terumo, Tokyo, Japan). During adenosine infusion, the patient’s heart rate was monitored continuously with the physiologic monitor of the imager. Acquisition of the second set of perfusion images was started 3 minutes after the beginning of the adenosine infusion. After the second series of perfusion images was completed, another bolus of gadopentetate dimeglumine, 0.1 mmol/kg, was injected. Delayed (after 15 minutes) contrast-enhanced images were acquired with the use of a segmented inversion-recovery three-dimensional fast low-angle shot (FLASH) sequence (244/1.64/260; flip angle, 10°; matrix, 128 x 256; FOV, 263 x 350 mm2) through the whole left ventricle on the short-axis view. All patients completed the MR imaging studies without complications.
Coronary Angiography
Coronary angiography was performed with 6-F catheters and imaging in multiple projections. The images were interpreted by an experienced cardiologist (C.W.C.) who was blinded to the MR imaging results. The level and degree of stenosis in the main branches of the coronary arteries were visually estimated. A more than 50% stenosis in diameter in any coronary artery was considered to be substantial. The presence of substantial coronary stenosis was compared with the first-pass perfusion defects on MR images.
MR Image and Data Analysis
Two radiologists (N.M.C.S., W.W.M.L.), each with more than 9 years of experience, who were blinded to the clinical and coronary angiographic data assessed the cine, perfusion, and viability images, and the results were reached with consensus. End-diastolic and end-systolic areas of the left ventricle on the cine images were contoured manually by using the software, which automatically calculated the LVEF. Degree of myocardial wall thickening was assessed visually on the cine images. First-pass perfusion contrast-enhanced MR images were analyzed qualitatively for the presence or absence of regions of reduced contrast material uptake. Myocardium was divided into three radial segments corresponding to the three coronary artery territories at three levels (base, middle, and apex) (Fig 1). Presence of hypoenhancement in these territories was considered positive for perfusion defect and indicative of substantial stenosis affecting the corresponding coronary artery.
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Statistical Analysis
The Mann-Whitney test was used to calculate any significant difference between the LVEF in patients with and those without infarct. A P value of less than .05 was considered to indicate a statistically significant difference.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Perfusion Studies
In two patients, resting and stress perfusion images were normal. In another patient, resting perfusion images were normal but hypoenhancement of the segments was observed during stress. One patient had resting perfusion defects, which normalized after stress. The other nine patients had resting perfusion defects that enlarged after adenosine stress. There were 25 perfusion defects in 11 patients (Fig 2a, 2b). Table 2 summarizes the catheterization findings and MR perfusion study results.
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Infarct Detection
Five patients showed evidence of delayed hyperenhancement compatible with infarct (Fig 2c). Four patients had transmural infarct, and one had subendocardial infarct. All transmural infarcts were associated with myocardial wall thinning. The mean infarct size was 249.9 g (range, 50.3–445.5 g). Four patients with infarct demonstrated reduced contrast material uptake on resting first-pass perfusion images at the corresponding infarct area. There was no significant difference in the LVEF between patients with and those without infarcts (mean, 47% vs 57%; P = .724).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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The prognostic value of end-systolic volume and ejection fraction after acute myocardial infarction has been well established (10,11). MR imaging is an excellent method for evaluating global left ventricular structure and function (12,13). It is the optimal method for quantifying ejection fraction after myocardial infarction because of its ability to be used to acquire information in three-dimensional space and its excellent contrast and spatial resolution (14). Rapid breath-hold MR imaging techniques (15) are available to assess global and regional left ventricular function (16).
MR imaging has been used for more than a decade for evaluation of myocardial perfusion (17). Cardiac MR imaging allows the assessment of myocardial perfusion by an analysis of the first-pass kinetics of a contrast agent bolus (18). The application of multisection mode to highly selected patient populations with documented single-vessel disease yielded sensitivities of 100% (19,20). However, the application of a multisection mode to a mixed unselected study population either yielded low sensitivity (44% in 10 patients) (21) or low specificity (44% in 45 patients) (22). In a recent prospective study with a single-section approach, sensitivity and specificity for detection of stenoses of 75% or greater were 90% and 83%, respectively (23). Others sought to measure the myocardial perfusion reserve from the alterations of the upslope of a first-pass gadopentetate dimeglumine bolus after pharmacologic stress by using a linear fit (23).
Manning et al (17) used maximal signal intensity, whereas Lauerma et al (19) measured the upslope of the perfusion curve. In a study, a hybrid echo-planar readout allowed evaluation of perfusion indexes quantitatively within distinct myocardial layers (24). This approach provides information on the amount of compromised myocardium even when perfusion abnormalities are confined to the subendocardial layer. Although various successes have been achieved with these methods, they all involve additional software for secondary analysis of the perfusion data, which in itself is complicated and time-consuming, and may introduce further error.
So far, there is no consensus regarding which method is the best to analyze the signal intensity curve, be it the maximum signal intensity, the area under the curve, input function correction, maximum upslope, or the slope by linear fit. In our study, we applied a semiquantitative method by grading the raw data (ie, the perfusion images) by visual observation, which is relatively easy and simple). By combining the perfusion defects both at rest and after stress, we detected significant coronary stenosis with high sensitivity and specificity.
Myocardium that is ischemic but viable shows impaired contractility on cine images and may or may not show hypoenhancement on the perfusion images, with no evidence of delayed hyperenhancement. Identification of viable myocardium is useful in predicting which patients with acute coronary syndromes will have increased LVEFs and improved survival after revascularization. Results of recent studies support the concept of delayed hyperenhancement, which represents predominantly infarcted nonviable myocardium. Simonetti et al (25) report the results with use of a segmented inversion-recovery turbo FLASH MR pulse sequence, and the greatest differences in regional myocardial signal intensity in infarcted myocardium were produced after gadolinium chelate injection.
With a similar technique, contrast-enhanced MR imaging was used to distinguish between reversible and irreversible myocardial ischemic injury regardless of the extent of wall motion or age of the infarct (26). With application of this technique in our patients, we were able to detect hyperenhancement that represented infarcted myocardium in five of our 13 patients. As the transmural extent of hyperenhancement was significantly related to the likelihood of improvement in contractility after revascularization (26), this information is helpful when surgical intervention is being considered.
MR imaging is unique in its variety of applications in imaging the cardiovascular system. A thorough assessment of myocardial structure, function, and perfusion and of coronary artery anatomy and flow, and spectroscopic evaluation of cardiac energetics can be readily performed. Presently, imaging of these cardiac factors in patients after acute coronary syndrome is performed with various techniques, including echocardiography, left ventriculography, nuclear perfusion scanning, and conventional arteriography.
Although echocardiography is the most widely used technique in the evaluation of cardiac function, it is operator dependent and limited by the acoustic window. The quantification of ventricular function with echocardiography involves geometric assumptions that are based on uniform ventricular contraction, which is less reliable in the remodeled heart and the ischemic heart with stunned or hibernating myocardium. Single photon emission computed tomography (SPECT) provides assessment of the ventricular ejection fractions and perfusion but has limited spatial resolution and suffers from attenuation artifacts. Positron emission tomography is more accurate than SPECT but is not widely available. These studies often cannot be performed on the same day and present difficulties in terms of spatially matching perfusion data to regional function.
The concept of "one-stop shop" has been reported by several groups of investigators who performed examinations in patients with myocardial infarction. Kramer et al (7) performed a comprehensive MR imaging assessment of left ventricular structure and function, infarct artery patency, and regional myocardial contrast material uptake in 27 patients with acute myocardial infarction. Sandstede et al (27) reported the use of first-pass perfusion and delayed contrast enhancement in predicting myocardial viability. Lauerma et al (28) combined three MR imaging modalities: dobutamine stress cine, first pass, and late contrast enhancement to assess viability in 10 patients with multivessel disease. Such a combination significantly increased the specificity of MR imaging in the detection of nonviable sectors. The recent advances in the speed of cardiac MR imaging have made it feasible to integrate the examination of all these cardiac factors, which previously required as many as three different clinical cardiac imaging methods.
In our study, we combined cine, resting, and pharmacologic stress first-pass perfusion studies and delayed contrast enhancement to provide a comprehensive evaluation in a group of high-risk patients with acute coronary syndromes. Ideally, the MR imaging study should be performed as soon as possible after initial stabilization, so that patients can be triaged for early revascularization. Because the waiting time for MR imaging is long in our institution, only patients whose conditions were initially stabilized medically were referred for an MR imaging examination as part of their work-up for subsequent specific treatment.
MR angiography of the coronary arteries was not routinely performed because conventional coronary angiography was currently the criterion standard in assessing coronary artery disease and part of the standard assessment in our patients. Findings in this study indicated that it is feasible to conduct a comprehensive evaluation of patients with acute coronary syndromes in a clinically acceptable MR imaging examination time. Future trials will be needed to establish more convincingly the benefits of conservative, noninvasive, and cost-effective diagnostic approaches that offer advantages for risk stratification.
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FOOTNOTES |
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Author contributions: Guarantor of integrity of entire study, C.W.C.; study concepts and design, C.W.C., N.M.C.S.; literature research, C.W.C., N.M.C.S., W.W.M.L., J.E.S.; clinical studies, all authors; data acquisition, N.M.C.S., W.W.M.L., K.Y.C., C.W.C.; data analysis/interpretation, C.W.C., N.M.C.S., W.W.M.L.; statistical analysis, C.W.C., N.M.C.S., W.W.M.L.; manuscript preparation and definition of intellectual content, C.W.C., N.M.C.S., W.W.M.L.; manuscript editing, N.M.C.S., W.W.M.L., J.E.S.; manuscript revision/review and final version approval, all authors.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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