In performing cardiac MRI, the accurate peak detection in the ECG is critical to obtaining good image quality during the study. ECG electrodes should be placed on the patient in a cross-shaped or triangular pattern. Prior to the application of these electrodes, the skin should be cleaned with an alcohol pad or an abrasive gel to ensure effective surface contact. If necessary, the patient's chest may be shaved to achieve better surface contact. Depending on the desired study, images can be obtained by using either prospective or retrospective ECG data. Retrospective ECG gating allows reconstruction of images throughout the cardiac cycle, whereas prospective gating typically excludes the late diastole of the cardiac cycle.
Magnetic resonance imaging cine gradient-echo acquisition studies rely on robust and consistent ECG gating during the examination.13 ECG gating involves a start pulse from the ECG that triggers the beginning of the image acquisition. Therefore, before proceeding with the examination, the MRI technologist should check ECG gating by ensuring a high, constant-amplitude R wave and a low T wave. The time between consecutive R waves on the ECG, or the R-R interval, is used to coordinate ECG gating. The R-R interval is the duration of 1 heartbeat, and is typically expressed in milliseconds. If a high, consistent R wave and low T wave cannot be effectively established, then the technologist should reposition the ECG electrodes. When performing prospective triggered techniques, it is necessary to consider the trigger window, which is the short interval between the end of data sampling and the next expected R wave. ECG-triggered sequences also require a consideration of trigger delay, which refers to the delay between the detection of the R wave and the initiation of imaging.
Newer MRI systems that offer advanced triggering modules based on vectorcardiography (VCG) are available to improve R-wave detection and streamline cardiac MRI set-up. The VCG technology synchronizes the MRI image acquisition with cardiac motion by using temporal and spatial information about the cardiac electrical activity to better differentiate between true signals and signal artifacts that result from a magnetohydrodynamic effect and other noise stemming from physiologic processes.
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