Coronary endothelial dysfunction and non-contrast multidetector computed tomography.

Previous authors have evaluated endothelial dysfunction after intracoronary infusion of acetylcholine by Doppler flow wire (coronary blood flow) or invasive coronary angiography (coronary artery vasodilation). 4–6 More recent reports have shown that non-invasive assessment of myocardial blood flow (MBF) by positron emission tomography (PET) can effectively evaluate coronary endothelial dysfunction using a coldpressor test (CPT)2,7 or pharmacological stimuli.3 CPT induces coronary vasodilation directly through a mechanism involving the sympathetic nervous system (β-adrenergic activation and increased rate-pressure product [RPP]), or indirectly through endothelial-derived nitric oxide signaling.2,7 On the other hand, adenosine, which is usually used as a pharmacological stimulus, induces direct vasodilation in resistance vessels and increases MBF independently of myocardial oxygen consumption.8 Contrast-enhanced multidetector row computed tomography (MDCT) has been widely used in clinical practice as coronary CT angiography for the diagnosis of CAD.9 In addition, myocardial perfusion CT has been useful in assessing myocardial ischemia in patients with CAD after pharmacological stress-induced MBF changes.10,11 Use of a contrast medium and relatively high radiation exposure are still the main limitations of the technique. In this issue of the Journal, Dunet et al12 propose a unique application of non-contract MDCT to assess changes in the coronary cross-sectional area (CSA) before and during CPT. They report the feasibility of non-contrast, low-dose MDCT in the assessment of coronary vasoreactivity in comparison with 82Rb myocardial perfusion PET. They show that relative changes in the coronary CSA can be assessed by MDCT, reflecting vasodilation during CPT, which correlate to changes in MBF by 82Rb PET. The conclusion rests on the following evidence: (1) MBF significantly increased from 0.88±0.26 ml · min–1 · g−1 at rest to 1.01±0.31 ml · min–1 · g−1 during CPT in a PET study (absolute and relative mean increase MBF: 0.13±0.18 ml · min–1 · g−1 and 16±18%, respectively); (2) CPT-induced MBF changes significantly correlated with RPP changes; (3) mean coronary CSA, as measured by MDCT, significantly increased from 20.3±6.5 mm2 at rest to 22±6.7 mm2 during CPT (absolute and relative mean change of coronary CSA: 1.7±1.5 mm2 and 8.9±7.1%, respectively); and (4) CPTinduced MBF increase moderately correlated with relative changes in coronary CSA. Although there was statistical significance, a few methodological issues remain to be solved. First, as mentioned above, CPT and adenosine induce coronary vasodilation through distinct mechanisms. Because adenosine slightly decreases systemic blood pressure, at the same time increases heart rate,13 it is not entirely acceptable to compare CPTand adenosineinduced RPP changes, using the latter as the standard reference. Second, coronary CSA was manually measured. Although the method had excellent reproducibility, parameters should be evaluated with greater objectivity, for instance, by using iterative reconstruction to improve the quality of the image, automated software to detect the contour, and the full-width at half-maximum method. Third, the temporal resolution of MDCT achieved in the present study (0.35-s rotation and half reconstruction algorithm) is not robust enough for quantitative assessment of coronary artery dilation along with the CPTinduced higher heart rate. As the authors mention, it would be better to use dual-source CT with higher temporal resolution. Finally, because of its potent vasodilation effect, adenosine may lead to better results than with CPT. In addition to coronary calcium scoring14 and lipid-rich plaque assessment by computer-aided technique,15 non-contrast and low-dose MDCT imaging may be also used to assess the changes of reactive coronary dilation as a surrogate investigation item for coronary endothelial dysfunction in near future. But clinical application of the present study’s results may be still limited because of the significant but small amount of change of reactive coronary dilation in comparison with its spatial resolution. This study, however, proposes a unique and potential application of non-contrast and low-dose MDCT coronary imaging.