Retrospective ECG-gated spiral scanning

•  For "retrospectively ECG-gated spiral scanning" a continuous spiral scan is acquired with the ECG-signal recorded simultaneously. 
•  The acquired scan data is selected for image reconstruction with respect to a pre-defined cardiac phase. 
•  Similar to ECG-triggered sequential scanning a certain R-wave delay time defines the start point of data that is used for image reconstruction. 
•  It has several advantages over ECG-triggered sequential scanning as the continuous acquisition allows for reconstruction of overlapping slices. But due to the retrospective analysis of the ECG, the technique is less sensitive to arrhythmia. 

Prospective ECG-triggered sequential scanning

• Cardiac CT applications require the synchronization of data acquisition to the cardiac cycle, i.e. to the movement of the heart. 
• For sequential imaging, a prospective trigger is derived from the ECG-trace to initiate the CT-scan with a certain delay time after the R-wave.
• The true delay time is calculated from a given phase parameter (e.g. a percentage of the RR-interval time) for each cardiac cycle individually based on a prospective estimation of the RR-intervals. 
• Usually, the delay is defined such that the scans are acquired during the diastolic phase of the heart.