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 five’, 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 radiologists—5-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.

ECG Gating

Despite the importance of improved temporal resolution and heart rate control, it is ECG gating that enables coronary CTA. ECG gating refers to the simultaneous acquisition of both the patient’s ECG tracing and the CT data (see Figure 1). By acquiring both pieces of information, CT images can be reconstructed using only a short temporal segment periodically located in the same location of the R-wave to R-wave (R–R) interval over multiple cardiac cycles. Each temporal segment of the R–R interval is named by its ‘phase’ in the cardiac cycle; the typical nomenclature is to name the percentage of a specific phase with respect to its position in the R–R interval. The number of phases is manufacturerdependent. For example, if a manufacturer enables reconstruction of 20 (equally spaced) phases, they would typically be named 0%, 5%, 10%, up to 95%, beginning with one R-wave and ending with the following R-wave.The period in which the heart has the least motion is usually (but not always) in diastole, near a phase between 55% and 75%.Thus, under the assumption that the position of the heart remains consistent over the R–R intervals during which CT data is acquired, cardiac motion is minimized by producing images from the same phase over multiple cardiac cycles. This explains why ECG gating typically fails to freeze cardiac motion patients with an irregular rhythm such as atrial fibrillation (AF). Consequently,AF patients rarely have CTA examinations that are diagnostic over all coronary segments.

Figure 1a

Current modulation represents one strategy to lower the overall patient radiation by modulating the tube current (expressed as the mA) over the course of the cardiac cycle (see Figure 1c) so that the desired diagnostic tube current is delivered in diastole while the current is reduced for the remainder of the cardiac cycle. While featured on newer generations of CT scanners, there are potentially significant drawbacks. First, once current modulation is used, images subsequently reconstructed during phases with low tube current will be noisy. This is important because there is a subset of patients for whom the most diagnostic images, particularly for evaluation of the right coronary artery, is in systole. Second, current modulation eliminates the potential to reconstruct cine imaging, that is, reconstruction of all phases in the cardiac cycle displayed in a continuous loop. While cardiac CT cine loops are technically inferior to similar magnetic resonance (MR) acquisitions, global function can be routinely computed, and regional wall motion abnormalities seen.This can be particularly important in patients with a contraindication to MR, such as patients with a pacemaker.

ECG gating requires image data ‘oversampling’ because, for the reconstruction of each interval, only a small portion of the cardiac cycle is used. The CT pitch (a unitless parameter) is most accurately characterized as the distance that the patient moves through the scanner in a single gantry rotation divided by the width of the X-ray beam used. Because such a small part of the R–R interval is used to reconstruct an entire image, significant overlap along the craniocaudal extent of the patient is required, translating into a pitch between 0.2 and 0.35, or an oversampling rate of between 5:1 and roughly 3:1.The only other application for which ECG gating is routinely required is CTA of the thoracic aorta, where motion extending from the aortic valves to the arch can limit diagnostic accuracy.The fundamental difference between thoracic CT aortography and coronary CTA is the need for superior spatial resolution, as described below.

The practical consequence of oversampling is that the cardiac scan time (craniocaudal imaging over approximately 15cm) is far greater than non-gated scanning of the same Z-axis region of any other body part. For this reason, one great benefit of cardiac scanners equipped with a larger number of detectors is the ability to cover a larger craniocaudal territory per rotation. For example, while a four-slice coronary CTA may require a 35- second breath-hold, a 64-slice acquisition (performed at the same gantry rotation time or temporal resolution) on the same patient may require only 15 seconds. This is a significant advance, as breathing motion dramatically degrades image quality and, more often than not, eliminates the opportunity to capitalize on the high negative-predictive value of coronary CTA to determine that the patient has no significant coronary artery stenosis.