On the importance of specialized radiofrequency filtering for concurrent TMS/MRI

The concurrent application of TMS and MRI is challenging due to the MR-image artifacts, which are produced by using the two techniques in combination. One such artifact arises from the introduction of radio frequency noise through the lead of the TMS-coil into the scanner. Here we describe four methods used in the literature to integrate TMS into the MR environment and quantify in detail the efficacy of one approach in filtering RF interference. We show that RF filtering has a dramatic effect on the overall signal-to-fluctuation-noise ratio (SfNR) of the acquired echo-planar imaging data. The reduction in SfNR when integrating a TMS system into the MR scanner varies from 20% up to 80% (compared with MR scanner in the absence of TMS system), depending on the configuration used. Using an RF-filter in-line with the TMS-coil eliminates much of this loss in SfNR. However the RF filter also causes a ∼7% decrease in the functional efficacy of TMS. Overall, this study highlights the importance of RF-filtering when designing and installing a concurrent TMS/MRI system.