In CT, images are created by rotating an x-ray source emitting a fanshaped beam of x-rays, which then pass through the body. Some x-rays are absorbed or scattered, but others are transmitted and subsequently sensed by detectors located directly across the x-ray source. In MDCT, the x-ray tube and detectors are mounted on a gantry that rotates rapidly around the patient as he or she passes through the scanner. As in traditional x-ray radiography, different structures attenuate the x-ray beam to differing extents depending on their atomic composition and density, as well as the energy of the incident photons. The data collected by the detectors then go through a complex set of mathematical reconstruction algorithms that create a set of axial images through the technique of backprojection. Each voxel in the resulting axial image is ascribed a specific attenuation value, which is expressed in Hounsfield units (H.U.). Using a reference of 0 H.U. for water, −1000 H.U. for air, and +1000 H.U. for bony cortex, different points are assigned their respective attenuation values. This information is then converted into a grayscale image that can be manipulated by the interpreting physician.

1. The fast cyclical motion of the heart requires high temporal resolution to avoid blurring or degradation of images due to cardiac motion artifact. In cardiac CT, image acquisition is referenced, or gated, to the cardiac cycle. Although data can be acquired throughout the cardiac cycle, most image data sets are reconstructed during periods of minimal cardiac motion, typically a brief 100- to 300-millisecond interval in late diastole (60% to 75% of the R-R interval).

2. High spatial resolution is required to image relatively small vessels such as the coronary arteries. Current MDCT scanners (64-slice) provide a spatial resolution of 0.4 mm compared with approximately 0.2 mm for invasive angiography, the gold standard.

3. Respiratory motion artifact can be minimized by having the patient hold his/her breath during image acquisition. Most clinically available scanners can cover the entire heart in 10 to 12 seconds, whereas the newest 256-slice MDCT scanners can cover this area in just one or two heartbeats.

4. Rapid improvements in CT technology and protocoling have outpaced research in the field. Many studies investigating the diagnostic and prognostic yield of information gained from cardiac CT are not based on the newest MDCT scanners—but rather on single-beam/detector systems. Further investigation involving randomized controlled studies has been limited by the ethical issue raised by radiation exposure, although this has also been significantly reduced by innovative scanning approaches.

1. MDCT involves using an x-ray tube mounted opposite multiple detector rows on a gantry, which is then rotated around the patient at a rapid rate (220 to 400 ms/rotation). The patient is moved at either a fixed or variable speed, or pitch, through the scanner. An increasing number of detectors allows for an increased z-axis (cranial-caudal) coverage, permitting faster scans with improved image quality due to less cardiac and respiratory motion artifact. Temporal resolution is improved by faster gantry rotation, the use of two x-ray tubes and detector arrays mounted at 90° angles to each other (dual-source MDCT), and special reconstruction techniques. Dual-source/dual-energy scanners provide substantial improvements by utilizing dual-source MDCT technology as well as dual-energy sources to improve temporal resolution and decrease scatter. The fastest scanners provide a temporal resolution of 83 to 105 milliseconds. Spatial resolution is largely determined by detector architecture (typically 0.4 mm isotropic resolution), although thicker slices (1 to 5 mm) can be acquired to reduce radiation dose according to the study indication. MDCT can be used for both
cardiac and noncardiac studies, and it is now the most widely used type of CT hardware for cardiac imaging.

2. Electron beam computed tomography (EBCT), although rarely used today, was specifically developed for cardiac imaging. It involves the use of a rapidly oscillating electron beam reflected onto a stationary tungsten target. Because there is no mechanical motion within the gantry, EBCT is capable of very high temporal resolution (50 to 100 milliseconds). EBCT was used primarily for the quantitative detection of coronary artery calcification (CAC).