Conducting interslice stimulation for concurrent TMS-fMRI

Background: Transcranial magnetic stimulation (TMS) concurrent with functional magnetic resonance imaging (fMRI) can provide insights into the causal relationships between brain activity and behaviour. TMS pulses can cause artifacts in fMRI data, but these can be avoided if they are presented in short gaps between MRI slice acquisitions (interslice TMS-fMRI).
New method: We collected TMS-fMRI data to provide 1) guidance on the gap required and 2) a higher-level framework and code for researchers to test their own protocols. We quantified signal dropout and temporal signal-to-noise ratio in fMRI data (spherical phantom) for TMS pulses presented from up to 100 ms before and after slice excitation. We delivered up to 3 pulses per volume with interslice gaps of 37.5 ms/100 ms (slice time 62.5 ms), two 7-channel TMS-dedicated surface coils, and a multiband sequence (factor=2), on a Siemens 3 T Prismafit scanner. We repeated a subset of parameters with a human participant.
Results: We observed minimal data contamination when pulses were applied at least −20 ms/+ 50ms from slice excitation, and confirmed this approach can be used with 10 Hz TMS.
Comparison with existing methods: Compared to other strategies that avoid TMS pulse-related artifacts, interslice allows for greater flexibility in terms of timing of the TMS pulse, MRI read out and any stimulus presentation.
Conclusion: A 10 Hz TMS interslice protocol is possible with minimal data contimination. A stimulation frequency faster than 10 Hz would require a shorter gap or shorter slice acquisition times. Further, stimulator intensity, slice orientation, and the number of TMS pulses affected data quality and are important considerations for researchers when setting up their own protocol.