Gating for CT

First of all, we must know a basic assumption of 3D-reconstruction: The algorithm assumes that the object should be always static during scanning. However, the human body is not a stone, even patients don't move, you cannot stop breathing or heart beating. If we directly use these ungated projection data, the motion artefact will exist in reconstructed volume.

ECG gating and respiratory gating is used to remove the artefact caused by cardiac and respiratory motions.

Take ECG gating as example. All projects are synchronized with ECG signals and classified into several groups according to different ECG phases. Then we reconstruct volume 1 by projections in phases 1, volume 2 by projections in phase 2, and so on. In a word, each volume is reconstructed by the project in the same phase and motion artefact is almost removed. Someone call gated it also 4-D CT, namely, volume1, volume2,...volume n, because they not only record spatial information, but also present the motion of organ along the time-axis.