Optical ballistocardiography for gating and patient monitoring during MRI: an initial study

During magnetic resonance imaging (MRI), information about the cardiac activity is required for gating in cardio-vascular MRI (CMR) and for patient monitoring. This can be achieved using electrocardiogram (ECG) signals. However, ECG signals are affected by the different magnetic fields of the MR scanner which can hamper the R-peak detection. Ballistocardiograms (BCG) are an alternative method to gather information about cardiac activity and the cardiac cycle. For this study, displacement BCGs were acquired from four healthy subjects using a Moire phase tracking system. Velocity BCGs were derived from the displacement BCGs. Both BCG types were evaluated with respect to their usage during MRI for gating and monitoring purposes. Simultaneously acquired 12-lead ECGs were synchronised with the BCG signals and used as a gold standard reference. R-peaks in the ECG and corresponding J-peaks in the BCG were annotated and used for the evaluation. Average delays between the R- and J-peaks of 237 ms to 275 ms and of 167 ms to 230 ms occurred in the displacement and velocity BCGs of the different subjects. The jitter of the BCG's J-peaks ranged from 7 ms to 22 ms. Average heart rates estimated from the BCGs agreed with those obtained from the ECG signals. BCG measurements were shown to be sensitive to macroscopic motion artefacts. Heart rate monitoring using the optical BCG is feasible but needs to be validated with arrhythmic patients. Further experiments are required to investigate how the jitter of the BCG affects retrospectively gated CMR sequences.