Now you see it, now you don't: optimal parameters for interslice stimulation in concurrent TMS-fMRI

Abstract

The powerful combination of transcranial magnetic stimulation (TMS) concurrent with functional magnetic resonance imaging (fMRI) provides rare insights into the causal relationships between brain activity and behaviour. Despite a recent resurgence in popularity, TMS-fMRI remains technically challenging. Here we examined the feasibility of applying TMS during short gaps between fMRI slices to avoid incurring artefacts in the fMRI data. We quantified signal dropout and changes in temporal signal-to-noise ratio (tSNR) for TMS pulses presented at timepoints from 100ms before to 100ms after slice onset. Up to 3 pulses were delivered per volume using MagVenture's MR-compatible TMS coil. We used a spherical phantom, two 7-channel TMS-dedicated surface coils, and a multiband (MB) sequence (factor=2) with interslice gaps of 100ms and 40ms, on a Siemens 3T Prisma-fit scanner. For comparison we repeated a subset of parameters with a more standard single-channel TxRx (birdcage) coil, and with a human participant and surface coil set up. We found that, even at 100% stimulator output, pulses applied at least -40ms/+50ms from the onset of slice readout avoid incurring artifacts. This was the case for all three setups. Thus, an interslice protocol can be achieved with a frequency of up to ~10 Hz, using a standard EPI sequence (slice acquisition time: 62.5ms, interslice gap: 40ms). Faster stimulation frequencies would require shorter slice acquisition times, for example using in-plane acceleration. Interslice TMS-fMRI protocols provide a promising avenue for retaining flexible timing of stimulus delivery without incurring TMS artifacts.

TMS Does Not Increase BOLD Activity at the Site of Stimulation: A Review of All Concurrent TMS-fMRI Studies

“…To do so, we started with the 69 studies reported by Bergmann et al (2021) and then, using equivalent search criteria, looked for additional studies published in 2020 and 2021, which resulted in five additional studies (Cobos Sánchez et al, 2020;Navarro de Lara et al, 2020;Jackson et al, 2021;Rafiei et al, 2021;Scrivener et al, 2021). Second, we excluded two sets of articles: (1) papers that primarily focused on methodological issues and only reported data from a single subject (Bestmann et al, 2003a(Bestmann et al, , 2006Peters et al, 2013;Navarro de Lara et al, 2015;Oh et al, 2019), and (2) papers that did not provide information that allows the determination of whether or not TMS led to an increase in local BOLD activation because relevant analyses were not reported (Ruff et al, 2006(Ruff et al, , 2008Guller et al, 2012;Chen et al, 2013;Hanlon et al, 2016;Fonzo et al, 2017;Cobos Sánchez et al, 2020;Eshel et al, 2020;Hermiller et al, 2020;Navarro de Lara et al, 2020;Jackson et al, 2021;Oathes et al, 2021;Scrivener et al, 2021). Among the remaining studies, two pairs of studies shared the same underlying dataset (Leitão et al, 2013and Shitara et al, 2011 and, therefore, we kept only the ones that explicitly mention results related to the presence or absence of activation at the site of stimulation (Shitara et al, 2011;Leitão et al, 2015).…”

Concurrent TMS-fMRI: An international consensus and functional guide for current and future researchers

“…between TMS and subsequent slice 99 and this approach has been used with repetitive TMS protocols such as theta burst volleys 23 , 2-Hz 99,100 , 4-Hz 72 , and 10-Hz Hz 99,100 stimulation.…”

“…The latter is more dependent on coil orientation relative to B0, which is, in turn, determined by the targeted brain area, and the associated physical properties with placing the coil over the area. For new coil types, coil positions, or new coil holder solutions, the temporal gap between the pulse and the subsequent MR acquisition would need to be determined to avoid image artefacts while keeping the length of the gap short to improve temporal SNR 72,99 . Coil vibration can also be dampened to some extent by the TMS coil holder, as well as TMS coil design adjustments.…”