Lung and heart cyclical imaging, simplified

Cyclical imaging by the technique of synchronization or “gating” allows time-phase resolution of repetitive cyclical movement in a scanned object. For example, the lungs in a breathing mouse and the same animal’s beating heart. Gating can be either “prospective,” in which X-ray CT projection images are acquired at selected timepoints corresponding to a single phase of the cycle, or “retrospective,” in which a large number of redundant images are acquired during the scan and, after the scan, sorted into different phases of the cycle.
There are two ways in which the multiple projections per step acquired in a listmode scan can be sorted into different phases of the cycle. These are time-based and image-based sorting.

Time-based listmode sorting depends on an accurate record of physiological time-points of the cycle of breathing or the heart beat: each inhalation, or each systole. Breathing monitoring is by video movement detection or pressure sensing; cardiac monitoring is by ECG.

Image-based or Intrinsic listmode sorting assigns projection images to cyclical intervals by image analysis. No physiological time signal is needed.

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