In this work, we demonstrate a method to detect low-frequency, low-amplitude AC magnetic fields in an ultra-low-field (ULF) MRI system using a steady-state implementation of the Stimulus-Induced Rotary Saturation (SIRS) method. The method optimizes SNR efficiency by applying the SIRS mechanism in a bSSFP scan. This approach takes advantage of the low SAR and small absolute B0 deviations of the ULF system. We describe simulation results, show a clear signal response in phantoms, and describe an in vivo protocol for using the method to detect response from an auditory stimulus.
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