Most advances in cardiac computed tomography (CT), particularly coronary CT angiography (CTA), have come from the development of protocols consistent with rapid incremental improvements in CT technology. The evolution of cardiac CT from early technology has been paralleled with evolving protocols that have extended cardiac CT beyond the imaging of coronary arteries alone: current applications include the assessment of coronary bypass grafts, cardiac valve analysis, cardiac function, and chest pain imaging. While the details of these more advanced protocols are beyond the scope of this article, all advanced cardiac CT protocols are founded on basic CTA. This article introduces coronary CTA by breaking down the examination into its core components.
Building a CT Protocol
Cardiac motion separates cardiac and coronary CT from the CT assessment of other body parts. Ultimately, successful coronary imaging by any modality relies on the ability of the hardware (for CT, the scanner) to produce motion-free images, or to scan faster than the heart beats. Thus, coronary CT relies on faster imaging or slowing cardiac motion. The imaging speed is measured by the temporal resolution and is determined by the CT gantry rotation time, defined as the time required for the CT gantry to make one full revolution. The temporal resolution is half the gantry rotation time, owing to the fact that image reconstruction requires CT data acquired from approximately one-half of a gantry rotation. Among commercially available scanners, gantry rotation times are now as low as 330 milliseconds, yielding a temporal resolution as low as 165 milliseconds. (Note that spatial resolution, described below, refers to the image voxel size and is independent of temporal resolution.) Manufacturers have steadily developed faster CT gantries, but even with these advances, imaging during 165 milliseconds of the cardiac cycle yields motion artifact for a significant fraction of patients. Thus, using current technology, heart rate control remains a critical component of the examination.
Beta-blockade for Heart Rate Control
In patients who do not routinely take beta-blockers, administration of metoprolol at the time of scanning is essential. One rule of thumb for the target heart rate is 'the first number is a fiveÔÇÖ, i.e. an ideal heart rate between 50 and 59 beats per minute. While this goal is not achieved in every patient, it provides a useful reference frame. With cardiac monitoring, intravenous (IV) metoprolol is routinely and safely administered by both cardiologists and radiologistsÔÇö5-mg increments given every five minutes to a total dose between 15mg and 25mg. Routine IV delivery has supplanted oral administration, which has the disadvantages of a longer serum half life and the fact that premedication requires patient compliance before reaching the examination site.