http://medical-dictionary.thefreedictionary.com/cardiac gati...
cardiac gating: selective acquisition of cardiac function information at specific points in the cardiac cycle by using information from the electrocardiographic signal to time the cardiac cycle and control image sampling. It has been used in digital subtraction angiography, computed tomography, nuclear cardiology, and magnetic resonance imaging.

https://www.imaios.com/en/e-Courses/e-MRI/Cardiac-MRI/ecg-ga...
Cardiac synchronization methods (cardiac gating)
The methods used to synchronize with cardiac motion essentially rely on the electrocardiogram (ECG). The peripheral pulse is only used as a last resort.