Stimulus-induced Rotary Saturation (SIRS): A potential method for the detection of neuronal currents with MRI

We introduce a potential method for detecting neuronal currents though their resonant T1ρ saturation during a spin-lock preparation period. The method is insensitive to the temporal and spatial cancellation effects since it utilizes the multi-phasic nature of the neuronal currents and thus is not sensitive to the sign of the local field. To produce a T1ρ reduction, the Larmor frequency in the rotating frame, which is set by γB1lock (typically 20 Hz-5 kHz), must match the major frequency components of the stimulus-induced neuronal currents. We validate the method in MRI phantom studies. The rotary saturation spectra showed a sharp resonance when a current dipole within the phantom was driven at the Larmor frequency in the rotating frame. A 7 min block-design experiment was found to be sensitive to a current dipole strength of 56 nAm, an approximate magnetic field of 1 nT at 1.5 mm from the dipole. This dipole moment is similar to that seen using the phase shift method in a similar experimental setup by Konn et al. [Konn, D., Gowland, P., Bowtell, R., 2003. MRI detection of weak magnetic fields due to an extended current dipole in a conducting sphere: a model for direct detection of neuronal currents in the brain. Magn. Reson. Med. 50, 40-49], but is potentially less encumbered by temporal and spatial cancellation effects.