Noninvasive Detection of Myocardial Ischemia From Perfusion Reserve Based on Cardiovascular Magnetic Resonance

Nidal Al-Saadi, MD, Eike Nagel, MD, Michael Gross,  MD, Axel Bornstedt, PhD, Bernhard Schnackenburg,  PhD, Christoph Klein, MD, Helmut Oswald, PhD, Eckart Fleck, MD

Abstract
Background: Myocardial perfusion reserve can be non-invasively assessed with cardiovascular magnetic resonance. In this study the diagnostic accuracy of this technique for the detection of significant coronary artery stenosis was evaluated.
Methods and Results: In 15 patients with single vessel and five patients without significant coronary artery disease the signal intensity time curves of the first pass of a gadolinium-DTPA bolus injected via a central vein katheter were evaluated before and after dipyridamole infusion to validate the technique. A linear fit was used to determine the upslope and a cut off value for the differentiation between the myocardium supplied by stenotic and nonstenotic coronary arteries was defined. The diagnostic accuracy was then examined prospectively in 40 patients with coronary artery disease and was compared with coronary angiography.
A significant difference in myocardial perfusion reserve between ischemic and normal myocardial segments (1.08± 0.23 and 2.33± 0.41; p<0.001) was found which resulted in a cut off value of 1.5 (mean minus 2 standard deviations of normal segments).
In the prospective analysis sensitivity, specificity and diagnostic accuracy for the detection of coronary artery stenosis (³ 75%) were 93%, 89%, and 91%. Inter- and intra-observer variability for the linear fit were low (r= 0.96 and 0.99).
Conclusions:MR first pass perfusion measurements yielded a high diagnostic accuracy for the detection of coronary artery disease. Myocardial perfusion reserve can be easily and reproducibly determined by a linear fit of the upslope of the signal intensity time curves.

In principle, the reduction of myocardial perfusion is a sensitive indicator for myocardial ischemia, as myocardial blood flow is directly correlated to myocardial oxygen supply. Such measurements are superior to coronary angiography for the detection of myocardial ischemia, as the functional relevance, rather than the morphological appearance, of a stenosis is assessed. Clinical routine measurements of myocardial perfusion are performed with single photon emission computed tomography (SPECT) or with positron emission tomography (PET). To induce myocardial ischemia in myocardial regions supplied by stenotic coronary arteries physical or pharmacological stress is applied. Sensitivity and specificity for the detection of significant coronary artery disease with SPECT or PET range from 83% to 95% and 53% to 95%. However, these techniques have a rather low spatial resolution and are not suitable for the detection of subendocardial perfusion defects, which by themselves are extremely sensitive to the occurrence of myocardial ischemia. In addition, the requirement of radioactive markers prohibits the use of these techniques for follow up examinations. The long washout time of some tracers makes repeated measurements on the same day difficult and, finally, due to the characteristics of the tracers used, SPECT imaging is limited by artifacts that arise mainly in obese patients and women especially in the inferolateral and anterior myocardial region. PET has a higher sensitivity and specificity than SPECT, but in addition to its resolution problems it is burdened by its limited availability.

Ultrafast magnetic resonance (MR) tomography allows an analysis of myocardial perfusion by the use of the first pass of a T1 shortening contrast agent bolus. The advantages of MR tomography are its high spatial resolution, the independence from radioactive tracers, and thus attenuation artifacts, as well as its capability for repeat examinations for follow-up. Several studies have shown in principle that an analysis of myocardial perfusion with MR is possible and may even permit a quantitative assessment of myocardial blood flow.
The aim of this study was to define a threshold value for ischemic regions by myocardial perfusion reserve. This was measured by cardiovascular magnetic resonance to differentiate the myocardium supplied by a stenotic coronary artery from the myocardium supplied by a nonstenotic coronary artery. Also we aimed to determine prospectively the diagnostic accuracy of this cut off value for the detection of significant coronary artery stenosis in patients with suspected coronary artery disease.

The study population consisted of 60 patients (Table 1) who were referred for coronary angiography. Written and informed consent was obtained from all patients. Initially, 15 patients with single vessel disease and five patients with chest pain, but without significant stenoses of the coronary arteries, were examined to define the cut off values in perfusion measurements for the detection of significant coronary artery stenosis (group A). This group served for the validation of the technique and for the determination of inter- and intra-observer variability. Then, the subsequent 40 patients with suspected single or double vessel disease, who were referred for a coronary angiography because of new chest pain or progressive symptoms, were prospectively examined by the use of the previously defined threshold value (group B). In this group the diagnostic accuracy of MR perfusion reserve measurement in comparison with angiography was assessed. Patients were excluded if they were <18 years old, had a history of prior myocardial infarction, unstable angina, hemodynamic relevant valvular disease, ventricular extrasystoly ³ Lown III, atrial fibrillation, ejection fraction <30%, blood pressure >160/95 mmHg or <100/70 mmHg, obstructive pulmonary disease, known claustrophobia, or a contraindication for an MR-examination such as incompatible metal implants.

Table 1

After the MR examination all patients underwent left-sided cardiac catheterization and bi-plane selective coronary angiography in Judkins technique. Coronary stenoses were filmed in the center of the field from multiple projections, and as much as possible overlap of side branches and foreshortening of relevant coronary stenoses was avoided. Coronary angiograms were quantitatively assessed for high grade coronary artery stenoses (³ 75% area stenosis). The examiner was blinded to the MR examination.

The patients were examined in the supine position with a 1.5 Tesla whole body MR tomograph (ACS NT, Philips, Best, The Netherlands). After two rapid surveys to determine the exact position and axis of the left ventricle, a short axis slice at the height of the origin of the papillary muscles was chosen for perfusion imaging using an ECG triggered T1-weighted inversion recovery single shot turbo gradient echo sequence (inversion pulse, pre-pulse delay 360 ms, acquisition duration 360 ms, flip angle 15°, echo time 1.7 ms, repetition time 9 ms). Slice thickness was 8 mm with a spatial resolution of 1.7 x 1.9 mm. A bolus of 0.025 mmol gadolinium DTPA/kg body weight (Magnevist, Schering AG, Berlin) was rapidly injected by hand and flushed with 10 ml of 0.9% NaCl. Sixty dynamic images (one image per heart beat) were acquired during the first and second pass of the contrast agent. The dipyridamole infusion was discontinued prematurely upon patient request or when chest discomfort indicative of progressive or severe angina, dyspnea, decrease in systolic pressure >40 mmHg, severe supra-ventricular or ventricular arrhythmias or other adverse effects occurred. Aminophylline was administered as required.

In all images the endo- and epicardial contours were traced by an examiner blinded to the angiographic results. The myocardium was divided into 6 equiangular segments. An additional region of interest was placed within the cavity of the left ventricle excluding myocardial segments or papillary muscles (Figure 1). Images acquired after premature ventricular beats or insufficient cardiac triggering were excluded from the analysis to guarantee steady state conditions. Signal intensity (SI) was determined for all dynamics and segments (Figure 2). The native SI was subtracted and the upslope of the resulting SI-time curve was determined by the use of a linear fit. The results of the myocardial segments were corrected for the input function by dividing the upslope of each myocardial segment through the upslope of the left ventricular SI curve which was regarded as a measure of the input function. Perfusion reserve was calculated by dividing the results at maximal vasodilation by the results at rest. In group A the segment with the lowest myocardial perfusion reserve within the territory of the stenotic coronary artery was defined as ‘ischemic’. All segments of the patients without significant coronary artery disease and the contralateral segment opposite to the ischemic segment in the 15 patients with single vessel disease were defined as ‘nonischemic’. The absolute upslope at rest, after dipyridamole infusion, as well as myocardial perfusion reserve of ischemic and nonischemic segments, were compared. The cut off value was defined as the mean perfusion reserve minus 2 standard deviations of all nonischemic segments. In group B myocardial perfusion reserve was calculated for all segments. If the myocardial perfusion reserve was less than the defined cut off value, the segment was classified as pathological (MPR+), if it was more than the cut off value it was defined as normal (MPR-). If at least one segment within the territory of a coronary artery was found to be ischemic, MR was regarded as positive for that region.

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