What is gating?
Gating is the process of synchronizing the CT acquisition to the cardiac cycle. ECG leads are placed on the patient and connected to the scanner. Images can be acquired either prospectively or retrospectively. The aim is to leverage the temporal resolution of the scanner to maximal advantage by acquiring data during phases in the cardiac cycle when cardiac motion is least; this is usually in late diastole.
In retrospective gating , the patient is scanned in a continuous helical fashion. The x-ray tube is on throughout the cardiac cycle, although tube current modulation may be used to decrease the radiation dose during phases of the cardiac cycle where motion is most prominent (e.g., systole). Images are then reconstructed from data acquired during specific portions of the cardiac cycle. Advantages include the ability to reconstruct images from any phase of the cardiac cycle, which may help in evaluation of a coronary artery segment that is blurred during the preferred late diastolic phase. In addition, if images are generated from all phases of the cardiac cycle, functional information is available regarding ejection fraction, valve motion, and wall motion. Retrospective gating is, however, accompanied by a relative radiation dose penalty compared with prospective gating of about 2 to 3 : 1.
In prospective gating (also called prospective triggering), the x-ray tube is turned on only during a specific phase of the cardiac cycle, usually in late diastole when cardiac motion is at a minimum. During the remainder of the cycle, the table moves, and the patient is repositioned for the next set of images, which are taken during the same phase of the next heartbeat. Prospective gating exposes the patient to a lower radiation dose than retrospective gating. Strict heart rate control is required for a prospective scan, however, because if there is motion blur on an image, there are no data available from other points in the cardiac cycle to attempt reconstruction at a different phase.