Scan acquisition & gating

Axial vs. helical acquisition, what pitch actually means, and when to choose prospective vs. retrospective ECG gating.

ECG gating and cardiac CT acquisition modes — prospective vs retrospective gating and four acquisition strategies — Prospective vs retrospective ECG gating, and the four acquisition strategies (step-and-shoot, helical, high-pitch dual-source, volume).

Acquisition is the choreography of the table, the gantry, and (for cardiac work) the patient’s heartbeat. Two decisions matter most: helical vs. axial, and prospective vs. retrospective gating.

1 Acquisition modes

Axial / sequential mode. The scanner acquires data during a rotation, pauses, moves the table, and acquires again.
Helical (spiral) mode. The table translates continuously while the gantry rotates. Characterized by pitch — the table translation per rotation divided by the detector collimation. A low pitch (e.g. 0.25) allows the overlapping data essential for cardiac CT.

2 ECG gating

ECG gating synchronizes X-ray acquisition with the cardiac cycle to minimize motion artifact.

Prospective gating. X-ray is only triggered during a predetermined cardiac phase — typically mid-diastole, 65–75% of the R-R interval. Cuts radiation by 70–90% vs. retrospective. Requires a regular, slow heart rate.
Retrospective gating. Continuous acquisition throughout the cycle; phases are picked later. Highest radiation dose, but critical for irregular rhythms (e.g. atrial fibrillation) and for functional analysis like ejection fraction.