Comparison of Spiral Multidetector CT Angiography and Myocardial Perfusion Imaging in the Noninvasive Detection of Functionally Relevant Coronary Artery Lesions: First Clinical Experiences

Patient Selection

Twenty-five consecutive patients who had undergone MPI within 40 d of MDCT angiography were enrolled in this study; patients who had had a cardiac intervention within this period were excluded. The examinations were performed between February 2002 and August 2004 according to the following protocols.

Myocardial Scintigraphy

A same-day stress/rest electrocardiography-gated MPI protocol was performed on all patients, using weight-adjusted doses of 4 MBq/kg (at least 300 MBq of ^99mTc-methoxyisobutylisonitrile) at peak ergometric or pharmacologic stress and 10 MBq/kg (at least 700 MBq) at rest at least 3 h after the first injection.

The ergometric stress test was performed on an electronically braked bicycle ergometer and was terminated according to standard criteria: reaching age-predicted submaximal heart rate (heart rate ≥ [220 − age in years × 0.85]), severe chest pain, ST-segment depression > 0.2 mV, severe cardiac arrhythmia, hypertension (>240/120 mm Hg), or a fall of >40 mm Hg in systolic blood pressure. Pharmacologic stress was obtained by administration of 0.56 mg of dipyridamole per kilogram of body weight over 5 min. If systolic blood pressure was >120 mm Hg, 0.8 mg of nitroglycerin was administered sublingually to the patients before injection of the radiopharmaceutical for the rest image.

Gated images were acquired on a triple-head camera system (Prism 3000 XP; Philips) using a low-energy, high-resolution, parallel-hole collimator with a 360° rotation in continuous mode. An electrocardiogram R-wave detector provided a gate to acquire 12 frames per cardiac cycle during the poststress and postrest acquisitions. The summed image set was normalized conventionally for comparison of stress and rest images. For each study, short and long axes were reconstructed. A standardized filter (low-pass fourth-power; cutoff frequency, 0.26) was used.

Scintigraphic stress and rest images were evaluated visually by 2 experienced observers, who reached a consensus on the findings. The observers were unaware of the findings of MDCT angiography but aware of the size, weight, and sex of the patients. Raw data cines, gated SPECT wall motion, and wall thickening were analyzed, and quantitative analysis software (Hermes Perfit; Nuclear Diagnostics) was used for defect detection and quality control. Perfusion defects were allocated to coronary arteries according to the coronary perfusion type of the subject as described previously (27). Defects in the anterior wall and septal region were allocated to the left anterior descending coronary artery (LAD); defects in the lateral wall, to the left circumflex coronary artery (LCX); and inferior defects, to the right coronary artery (RCA). Apical defects were considered to be in the LAD region, unless the defect extended to the lateral (LCX) or inferior (RCA) wall. In the watershed regions, the extension of a defect to the anterior, lateral, or inferior wall was decisive for the allocation of a coronary artery to the vascular bed. Furthermore, distinct defects affecting both the LAD region and the LCX region were rated as left main artery (LM) disease.

To ensure that reversible perfusion defects on MPI were true-positive findings, all patients underwent conventional coronary angiography and revascularization therapy in case of coronary artery stenosis.

Spiral MDCT Angiography

The MDCT angiography datasets were acquired using the 12 inner detectors of a 16-MDCT scanner (Sensation 16; Siemens Medical Solutions) and a previously described protocol (15).

In brief, a bolus of 120 mL of contrast agent (Solutrast 300, 300 mg/mL; Altana) was injected intravenously (5 mL/s). As soon as the signal density level in the ascending aorta reached a predefined threshold of 100 HU, the acquisition of the CT data and the electrocardiogram trace was started. Detector collimation was 12 × 0.75 mm; rotation time, 420 ms; and tube voltage, 120 kV at a current of 500 mAs during the diastolic phase of the cardiac cycle, and tube current was reduced by 80% during the remaining phase of the R-R interval, leading to an estimated mean effective radiation dose of approximately 4.3 mSv (28). Images were reconstructed with an acquisition time of 210 ms in diastole 350–450 ms before the R wave using retrospective electrocardiogram gating. Motion artifacts can be avoided only in the presence of a heart rate of <65 beats per minute (29); therefore, all patients with heart rates of >65 beats per minute were pretreated with 50–100 mg of oral metoprolol 1 h before the scan or intravenously with 10–20 mg of metoprolol directly before the scan. The voxel size of the resulting CT images was 0.6 × 0.6 × 1 mm.

The CT datasets were analyzed by 2 independent, experienced readers using a Leonardo workstation (Siemens), who reached a consensus on the findings. In a first step, image quality was determined by the investigators on the basis of the presence of motion artifacts and vessel calcifications. Image quality was graded as excellent (no motion artifacts present), good (minor motion artifacts present), moderate (substantial motion artifacts present, but luminal assessment of significant stenosis still possible), heavily calcified (vessel lumen obscured by calcification), or blurred (only contrast visualization inside the vessel possible; no luminal assessment of significant stenosis possible). Results were documented separately for all coronary segments using a modified American Heart Association classification as described recently by Kuettner et al. (20). Each bypass graft was counted as an additional segment. Sections containing an intracoronary stent could not be evaluated, but the stent location was noted. The observer subjectively identified a prior myocardial infarction by considering blockage of contrast material and decreased CT attenuation. The latter was seen with the help of CT density values (HU), which were measured for areas of suspected infarction and representative noninfarcted areas in the same patient, with standard-sized (1 cm²), circular regions of interest placed over corresponding areas as described previously (30). Suspected areas of myocardial infarction were allocated to respective coronary vessels as described for myocardial scintigraphy.

The readers visually estimated whether stenoses were ≥50% or <50% and allocated them to the LM, LAD, LCX, or RCA. Bypass grafts were rated as belonging to their respective coronary arteries. Lesion structure was stated as calcified, not calcified, or mixed when appropriate (9).

Results of MDCT Angiography

Mean heart rate for MDCT angiography acquisition was 60 ± 10 beats per minute (range, 47–85 beats per minute). Image quality was assessed for 330 coronary segments on MDCT angiography (324 coronary artery segments and 6 bypass grafts). The number of segments classified as excellent, good, or moderate was 231 (70%). Twenty-three segments (7%) were rated as heavily calcified and 30 (9%) as blurred; 23 (7%) were not evaluable because of motion artifacts and 23 (7%) because of intracoronary stents. Thus, vessel diameter could not be evaluated for 99 (30%) of 330 segments.

MDCT angiography detected 47 stenoses in 44 of 99 coronary arteries. Stenoses were classified as ≥50% in 17 of 99 coronary arteries: 1 LM, 9 LAD, 3 LCX, and 4 RCA. Four patients showed myocardial areas with decreased CT attenuation suspected of representing prior myocardial infarction: 3 in the LAD and 1 in the territory supplying the RCA. In 1 of these 4 patients, additional blockage of contrast material was detected just posterior to a ≥50% stenosis in the middle segment of the RCA (Fig. 1). Stenoses were rated as <50% for 30 coronary arteries: 8 LM, 6 LAD, 5 LCX, and 11 RCA; 3 coronary arteries showed stenoses of both ≥50% and <50%. In 55 coronary arteries, no luminal narrowing could be detected, nor was luminal narrowing found in any of the 6 bypass grafts. Twenty of the 47 detected stenoses had plaque that was classified as calcified; 23, as mixed; and 4, as noncalcified.

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FIGURE 1.

Patient with stenosis of LAD classified as ≥50% on MDCT angiography and associated with reversible perfusion defects in anterior wall. Blockage of contrast material posterior to stenosis of ≥50% was detected in RCA, and corresponding fixed perfusion defect with residual ischemia was found in posterior wall. (A) Maximum-intensity projection MDCT angiogram (left anterior oblique view) shows proximal LAD with partly calcified, reduced lumen (arrow). (B) Axial MDCT angiogram shows proximal RCA with partly calcified, reduced lumen (arrow). (C) Axial (top 2 rows) and sagittal (bottom 2 rows) MPI slices at stress and rest show reversible perfusion defects in anterior wall (white arrowheads) and fixed perfusion defects with residual ischemia in posterior wall (yellow arrowheads).

Results of MPI

Myocardial perfusion images could be analyzed for 25 of 25 patients. Five reversible and 6 fixed defects were detected in 10 of 25 patients, 1 of whom showed both reversible and fixed perfusion defects. Of the 5 reversible perfusion defects, 4 were in the LAD and 1 in the LCX, and of the 6 fixed defects, 3 were in the LAD, 2 in the RCA, and 1 in the LCX.

Comparison of MDCT Angiography and MPI

Eight (47%) of 17 stenoses ≥ 50% as detected by MDCT angiography showed perfusion defects in the corresponding myocardial area on MPI. Five of these perfusion defects were reversible and 3 were fixed. Blockage of contrast material was detected posterior to 1 of these 8 stenoses, as was CT attenuation in the corresponding myocardial area. MPI showed a fixed perfusion defect with residual ischemia (Fig. 1). One patient with stenosis ≥ 50% and without blockage of contrast material showed decreased CT attenuation in the supplied myocardial region. In 2 patients with suspected myocardial infarction, no stenosis ≥ 50% was found in the supplying coronary artery, but 1 of the 2 patients had an intracoronary stent.

Nine (53%) of 17 stenoses ≥ 50% showed no perfusion defects on MPI (Fig. 2). None of the stenoses < 50% as detected by MDCT angiography in 30 coronary arteries was associated with a perfusion defect.

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FIGURE 2.

Patient with LAD stenosis of ≥50% and RCA stenosis of ≥50%, neither showing hemodynamic relevance on MPI. (A) Axial MDCT angiogram shows proximal RCA with calcified, reduced lumen (white arrow) and coronal view of mixed plaque in reduced lumen of LAD (yellow arrow). (B) Sagittal MDCT angiogram shows proximal LAD with partly calcified, reduced lumen (arrow). (C) Axial (top 2 rows) and sagittal (bottom 2 rows) MPI slices at stress and rest show homogeneous perfusion without perfusion defects.

Sensitivity, specificity, and negative and positive predictive values for MDCT angiography to detect reversible defects on MPI were 100%, 87%, 100%, and 29%, respectively, with an accuracy of 88%. When information from MDCT angiography about the myocardial infarction was added, specificity and positive predictive value increased to 89% and 33%, respectively.

Sensitivity, specificity, and negative and positive predictive values for MDCT angiography to detect any perfusion defect in the target area on MPI were 73%, 90%, 96%, and 47%, respectively. Accuracy was 88% (Table 2).

In 5 of 5 patients with reversible perfusion defects, the appropriate coronary artery stenosis could be detected on MDCT angiography. In 4 of 6 patients with fixed perfusion defects on MPI, CT attenuation was detected in the corresponding myocardial area, but in only 3 of 6 patients could the appropriate lesions be found. In the remaining 3 of 6 patients, the appropriate lesions were not found because of heavy calcifications in 2 patients and an intracoronary stent in 1 patient. Generally, in cases of heavy calcifications or intracoronary stents, the readers were not able to grade the luminal narrowing as ≥50% or <50%, but the perfusion defects could be allocated to the appropriate lesions.

When we focused on the 15 patients without perfusion defects on MPI, MDCT angiography showed 5 stenoses ≥ 50% and 22 stenoses < 50% in 25 coronary arteries, and 15 calcified and 12 mixed plaques were registered. The locations of these stenoses and the consistency of the plaque are listed in Table 3. No relationship was found between plaque consistency and the appearance of perfusion defects.