Fig. 3.

Clinical setup (left), block diagram (center) and physiological waveforms (right) for (a) conventional ECG gating and (b) acoustic cardiac triggering (ACT) for synchronization of MR imaging with the cardiac cycle. Signal traces were recorded simultaneously with ECG and ACT sensors located in the isocenter of a 7.0-T magnet. Interference by electromagnetic fields and magneto-hydrodynamic effects cause severe distortion in the vector ECG waveform acquired during free breathing, resulting in erroneous trigger recognition, which manifests itself in a severe jitter in the R-wave recognition. In comparison, ACT is free of interferences from electromagnetic fields and magneto-hydrodynamic effects, and provides a reliable trigger signal free of jitter even in the presence of free breathing. Note that neither the limited ECG in the MR scanner nor the acoustic waveforms shown here should be treated as reliable indicators of patient emergency conditions