Objectives Mean transit time (MTT) is inversely related to perfusion pressure. We herein investigated the relationship between baseline myocardial MTT measured by dynamic contrast-enhanced (DCE) CT imaging and the degree and hemodynamic significance of coronary stenosis in patients with coronary artery disease (CAD).
Methods Thirteen CAD patients underwent invasive coronary angiography and CT myocardial perfusion (MP) imaging within 2 weeks. Degree of stenosis in each coronary artery and its branches was qualitatively classified from angiogram as non-significantly stenosed (NS, normal, mildly irregular or <70% luminal narrowing) and significantly stenosed (SS, > = 70% narrowed). For the CT MP study, 8x5 mm of the heart was scanned for 30 s using a GE Healthcare Discovery VCT scanner with 140 kV, 50 mA and 0.4 s gantry period after a bolus injection of contrast (Omnipaque 300, 0.7 mgI/mL) at 4 mL/s. The study was repeated at 3 min after a 4-min infusion of dipyridamole (Persantine, 0.56 mg/kg). DCE cardiac images from each scan were corrected for beam hardening using an image-based correction algorithm before analysed using a model-based deconvolution algorithm (CT Perfusion, GE Healthcare) to generate functional maps of MP and MTT. In each map, myocardium in horizontal long-axis was divided into six segments and assigned to a supply coronary artery according to the AHA schema. Myocardial perfusion reserve (MPR) in each segment was calculated as the ratio of MP at stress to that at rest. Baseline MTT and MPR in segments perfused by NS and SS coronary arteries were averaged over all slices and compared using paired t-tests.
Results Mean baseline MTT in NS myocardial segment was 5.20 ± 0.61 s and was significantly lower than that in SS segment (5.56 ± 0.69 s, p < 0.05). By contrast, the corresponding mean MPR in NS segment was significantly higher than that in SS segment (2.19 ± 0.45 vs. 1.85 ± 0.49, p < 0.05).
Conclusions Flow pressure across SS lesions in coronary arteries was much reduced leading to decrease in the downstream perfusion pressure hence prolonged baseline myocardial MTT and attenuated MPR compared to those in remote NS segments. A single DCE CT protocol acquired at rest without stressing the heart with pharmacologic stimuli is sufficient for assessing the functionally significance of coronary artery stenosis.
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