Clinical cardiac MRI typically requires the use of electrocardiogram (ECG) leads to detect R-waves and synchronize the MRI acquisition accordingly. The magnetohydrodynamic effect can corrupt the ECG signal, sometimes making R-wave detection difficult, especially at higher field strengths. We are proposing here a practical and low cost ultrasound-based solution for detecting cardiac activity in a manner immune to the presence and strength of the B0 field. Equivalence was shown in cases where ECG gating worked well. In the future, the approach should prove most helpful as an alternative to ECG when the latter proves impractical, especially at higher field strengths.