The main components of a CT scanner are the gantry (containing the detector row/photon source unit), table, and the computer software/hardware to process the raw data. The gantry is donut-shaped and contains a photon source opposite a panel of detectors that both rotate around the patient/table in the x–y plane as a fixed unit opposite each other (Fig. 1.1). Dual source scanners have two such units at roughly 90° offset in the x–y plane. Typically the speed of this rotation ranges from 0.2 to 0.35 s/rev in most modern scanners and is the most important determinant of true temporal resolution in CT, which is calculated as half the time for a full rotation plus an angular correction for the fan beam geometry (Fig. 1.2). This yields temporal resolutions of roughly 100–175 ms with single source and 75 ms with dual source scanners. The source detector unit is continuously moving throughout the entire exam; however the delivery of photons can be turned off and on quite rapidly, as well as the mA (amount of photons) adjusted (e.g., automatic tube current modulation and EKG gated tube current modulation). Although slower, the kVp can also be changed to different energies on dual energy scanners.

Similar to MRI, when describing a CT scanner, it is customary to consider the long-axis of the table the z-axis, and the short axis the x–y plane. The photon source is a point source, in that it creates an Elliptical cone of photons from a focal spot. The angle of the elliptical cone in the x–y plane is called fan angle, and in the z-plane is called cone angle. Fan angle is typically around 40–50° and determines the maximum x–y plane of image reconstruction or scan field of view (SFOV) of the study. Manufacturers typically allow you to choose SFOV (sometimes called calibrated field of view) for a scan. Ranges vary from 18 to 50 cm. For CCTA, typically 320–400 mm is used as this covers the majority of patient x–y axis size. Cone angle is very small with 64 detector row CT scanners, but become substantially larger with the use of 256 × 0.625mm or 320 × 0.5 mm collimation. Cone angle and the number of detectors in the z-axis determine the amount of z-axis coverage acquired with one gantry rotation (Fig. 1.3) [1].

There are a few scanning techniques available on current CT scanners. A scout tomogram is obtained without rotation of the gantry, just with table movement and a few detector rows. The image has an appearance grossly similar to a standard AP, PA, or lateral radiograph. It is performed on all exams to plot the z-axis and x–y plane coverage needed. Once the gantry is spinning, two acquisition methods are available, depending on whether or not the table is moving during scanning. If the table is moving during image acquisition then it is called “helical” or “spiral” acquisition. If the table is not moving during imaging acquisition then it is called “step and shoot,” “sequential,” “axial,” or “volume scanning.” Currently (except for in dual-source scanners), the prospectively triggered “step and shoot” acquisition is preferred in most routine applications of CCTA [2].

CCTA fundamentally differs from most routine CTs in that on top of it being respiratory gated (utilizing a breath hold) – it is EKG gated. EKG gating simply means synchronizing CT data acquisition to a specific cardiac “phase.” Phase is defined as a percentage of the R-R interval, so that for a person with a heart rate of 60 bpm 30 % would be near end systole and 75 % would be near end diastole. Although it is arbitrary, it is a more specific nomenclature than the traditional phases of the cardiac cycle taught in physiology. There are two modes of ECG synchronized acquisition: