The methods used to synchronize with cardiac motion essentially rely on the electrocardiogram (ECG). The peripheral pulse is only used as a last resort.
ECG recording during an MRI examination is perturbed by:
ECG signal degradation will result in a lack of synchronization, lack of diagnostic value of the ECG monitoring trace during MRI, and the need for greater care in placing the electrodes. Indeed it is vital to have an R wave that clearly stands out from the rest of the trace to be machine-detectable.
Once this prerequisite is obtained, ECG synchronization can be carried out in two ways:
Either the R wave serves to trigger MRI acquisitions, which will then all occur at the same moment in the cardiac cycle: this is prospective gating. The TR is a multiple of the length of the cardiac cycle (1 or 2 cardiac cycles).
Or MRI acquisition is continuous, with a simultaneous ECG recording to reorganize the data during image reconstruction: this is retrospective gating. With each R wave, the phase encoding gradient changes.
Cardiac synchronization limits the artifacts linked to the motion of the heart and blood flow, thus enabling the different phases of the cardiac cycle to be sampled.
The advantage of retrospective gating is the possibility of imaging the entire cardiac cycle, whereas in prospective gating, there is a lapse of time at the end of the diastole.
In cine imaging with prospective gating, the first image has a stronger signal, (flash artifact) because longitudinal magnetization has had an added interval in which to recuperate. Partial saturation and the balancing of longitudinal magnetization only occur in the subsequent images. This drawback can be overcome by continuing to apply radiofrequency pulses and gradients during the lapse of free time at the end of the diastole, without recording a signal, to keep longitudinal magnetization in equilibrium. Retrospective gating is not subject to flash artifacts as there is no lapse of free time in the cardiac cycle.