During coronary CT angiography, clear delineation of the coronary arteries is essential for diagnosing calcification, plaque, stenosis and anatomical aberrations and for evaluating the function of stents. Problems in visualizing individual coronary segments may be caused, for example, by cardiac motion artifacts (especially in patients with high heart rate) and by difficulties in the timing of image acquisition (due to heart rate variations). Imaging problems may also be caused by the iodinated contrast medium (CM) used to opacify the coronary lumen. Generally speaking, a high iodine concentration in the blood provides high attenuation and thus good delineation of the coronary anatomy. However, contrast medium with osmolarity higher than that of plasma (high-osmolar CM) tends to be diluted in blood vessels by an inflow of extravascular fluid and thus may not adequately opacify all coronary segments. Moreover, when high-osmolar CM is injected intra-arterially (as during invasive angiography), it can induce acute heart rate fluctuations and thereby reduce image quality. However, the extent to which these heart rate changes occur after an intravenous injection of CM, as during CT angiography, is still a matter of debate.