Increasing the measurable frequency range of magnetic field oscillations with off-resonance spin-lock preparation pulses

Oscillatory magnetic fields-arising from, e.g., neuronal currents or induced currents during brain stimulation-can be measured with spin-lock preparation pulses, where the frequency of the measured field matches the amplitude of the spin-lock pulse. The upper frequency limit is thus determined by either the maximum RF amplitude or SAR restrictions. This study aimed to extend the measurable frequency range. We outline theoretically how off-resonance preparation pulses lead to an extended measurable frequency range of oscillating fields and how to tune them to the frequency of interest, and we discuss how this is related to the theory of two-photon excitation. We use the MR simulator software KomaMRI to simulate our proposed sequence and compare it to phantom measurements, where the oscillatory magnetic field is created from a current in a cable loop around a phantom. The simulations and measurements were consistent, showing that the RF amplitude can be decreased when the off-resonance detuning is increased while being sensitive to the same frequency of the oscillating field. We additionally observed that the effects of the RF inhomogeneities on the measured signal were reduced as the detuning increased. We have shown that the measurable frequency range of magnetic field oscillations can be increased using off-resonance preparation pulses, albeit with a lower sensitivity. We also observed reduced effects from RF inhomogeneities, which can potentially improve neuronal current detection at low frequencies as well.