MRI measurements of field modulations: Extended frequency range of spin-lock preparation with off-resonance pulses

Motivation: Mapping current flow during temporal interference stimulation (kHz range) or the application of tumor treating fields (100 kHz - 300 kHz) can improve target localization and dose control for these techniques. Goal(s): Explore novel encoding strategies to measure high-frequency magnetic field modulations caused by injected currents. Approach: We use Bloch simulations to explore novel encoding strategies and phantom measurements with currents that create magnetic fields modulations for validation. Results: We show that with off-resonance preparation pulses it is possible to be sensitive to high frequency field modulations while avoiding SAR and B1 limits, albeit with a lower sensitivity than previously suggested spin-lock methods. Impact: This work demonstrates the possibility of measuring time-varying fields at a much greater frequency range than previously possible, which can be crucial for high-frequency brain stimulation techniques such as temporal interference stimulation and tumor treating fields.