Imaging the coronary arteries with CT

In the 1980s CT imaging of coronary arteries began after the introduction of electron beam CT (EBCT), with the main application being quantification of calcifications in the coronary arteries for risk stratification. In the late 1990s helical multidetector row CT (MDCT), starting with the four-detector row, was used for noninvasive evaluation of suspected coronary artery disease. Initially MDCT had limited performance for visualising coronary arteries due to long scan time, with the required breath-hold often exceeding 30 seconds. The scanners had a long rotation time with limited temporal resolution of 250ms and x-ray beam coverage of approximately 8mm (four detector rows, each of 2mm thickness), insufficient for motion-free coronary imaging. Clinical performance improved considerably with the introduction of firstly 16-detector row rapidly followed by 64-detector row CT. The new scanners could perform helical acquisition of coronary arteries within one breath-hold (less than 15 seconds), with increased x-ray beam coverage up to 40mm and reconstructed slice thickness down to 0.5mm with improved temporal resolution. Helical cardiac MDCT, in contrast to regular helical scanning, has a very low pitch and is acquired over several cardiac cycles with electrocardiogram (ECG) signal recorded simultaneously. A very low pitch requires very low table speed to scan the heart during one complete cardiac cycle. The recorded ECG is used to select, or gate, the scan data corresponding to cardiac rest phase, which is the phase interval with least motion blurring – termed retrospective reconstruction. Retrospective ECG-gated reconstruction allows not only the cardiac rest phase, but any cardiac phase to be reconstructed; thus, the phase with the least motion can be selected retrospectively and, when necessary, additional phases can be reconstructed to obtain optimised diagnostic image quality for each coronary artery independently (figure 1). In addition, reconstructions at the end-systolic and end-diastolic phases can be made to evaluate ventricular function (figure 2). The wider beam coverage allowed introduction of sequential prospective triggering acquisition of the heart. This socalled step-and-shoot allowed coverage of the entire heart in three to five shots and the ECG signal is used to trigger the axial acquisition at exactly the desired cardiac rest phase. This technique allows a decrease in radiation exposure of 80%. Data is acquired during a specific, predefined cardiac rest phase. At a low and stable heart rate, the middiastolic phase is optimal for coronary artery evaluation, usually at 65-75% of R-R interval. At higher heart rates (>65 beats per min) the phase with least coronary motion is often end-systolic phase, usually at 35-40% of R-R interval (figure 3). Recent technical advances include dual-source CT scanners equipped with two x-ray tubes (and two detectors) at 90° to each other. By combining information from both detectors, images are reconstructed with a temporal resolution corresponding to only a quarter of the rotation of both detectors with a gantry rotation time of 330ms. Temporal resolution thus improves to 330/4=83ms, and with the second-generation dual-source CT scanners, temporal resolution of 75ms is achieved. However, dual-source CT still relies on a helical acquisition. Another innovation is development of volume CT with 320 detector rows. The volumetric CT scanner covers 160mm and thus allows the entire heart to be scanned in a single axial acquisition that takes only a fraction of a second. Volumetric axial CT allows dose reduction to be achieved, short scan times resulting in less arrhythmia related artifacts and use of smaller volumes of contrast material.