  Purpose: In the recent past, spin-locking MRI experiments were successfully
applied for the direct detection of sinusoidal magnetic field oscillations in
the sub-nT range. In the present study, this detection approach was extended to
non-sinusoidal pulsed magnetic fields based on the Rotary Excitation (REX)
mechanism. Methods: The new detection concept was examined by means of Bloch
simulations, evaluating the interaction effect of spin-locked magnetization and
low-frequency pulsed magnetic fields. The REX detection approach was validated
under controlled conditions in phantom experiments at 3T. Gaussian and
Sinc-shaped stimuli were investigated. In addition, the detection of artificial
fields resembling a cardiac QRS complex, which is the most prominent peak
visible on a Magnetocardiogram, was tested. Results: Bloch simulations
demonstrated that the REX method has a high sensitivity to pulsed fields in the
resonance case, which is met when the spin-lock frequency coincides with a
non-zero Fourier component of the stimulus field. In the experiments, we found
that magnetic stimuli of different durations and waveforms can be distinguished
by their characteristic REX response spectrum. The detected REX amplitude was
proportional to the stimulus peak amplitude (R2>0.98) and the lowest field
detection was 1 nT. Furthermore, the detection of QRS-like fields with varying
QRS durations yielded significant results in a phantom setup (p<0.001).
Conclusion: REX detection can be transferred to non-sinusoidal pulsed magnetic
fields and could provide a non-invasive, quantitative tool for spatially
resolved assessment of cardiac biomagnetism. Potential applications include the
direct detection and characterization of cardiac conduction.
