Multi slice CT (MSCT) scanners are becoming popular as a non-invasive imaging modality for screening coronary artery disease. The era of MSCT started with 4 slice scanners and progressed to the 64 slice MSCT which is commonly used now. 256 slice and 320 slice CT scanners are also being evaluated for clinical use. One of the difficulties with cardiac CT for imaging coronaries is the constant motion of the heart. This can be partly overcome by using ECG gating of the signals. Conventionally this has been done by retrospective ECG gating. This would mean that CT data is acquired through out the cardiac cycle and only those signals during a particular part of the cardiac cycle, usually a diastolic frame is used for final reconstruction of the coronaries. This would mean a much higher dose of radiation. Now prospective ECG gating is being evaluated in MSCT to reduce the radiation dose so that data acquisition is done only during the selected phase of the cardiac cycle. Prospective ECG gating can reduce the radiation dose up to 90% as compared to retrospective ECG gating. But the quality of images will come down if the heart rate or heart rate variability is high in prospective ECG gating. So also, higher body mass index (BMI) also causes lower image quality. Prospective gating can cause stair step artifacts due to patient motion on the CT table as multiple acquisitions may be needed to cover the z-axis with 64 slice CT. This is avoided in 320 slice CT as the gantry can cover the whole heart in one cycle due to better z-axis coverage. Dual source CT also gives better results.
Imaging of the coronaries is ideally done at lower heart rates and beta blockers are often administered to lower the heart rate during MSCT if the basal heart rate is high. As the number of slices increased, the resolution of the system to pick up more and more details of the coronary anatomy has improved. Main coronary arteries as well as side branches can be visualized well with ECG gated 64 slice CT. 320 slice CT will improve the quality of images further. The whole imaging can be completed in one cardiac cycle, even with prospective ECG gating. This would mean that patients with cardiac arrhythmias are no longer excluded from cardiac CT evaluation of the coronaries.