The RF Cap: A 26-channel flexible RF coil cap for optimized concurrent TMS/fMRI experiments at 3T

Combining brain imaging methods with non-invasive brain stimulation such as transcranial magnetic stimulation (TMS) is a rapidly expanding field with the potential to drastically improve the understanding of brain function. However, currently there is no generally applicable hardware solution optimized for these types of acquisitions. To make concurrent TMS/fMRI experiments at 3 T feasible without sacrificing imaging quality, we have designed, constructed, and tested the first of its kind "RF Cap": a 26-channel flexible RF coil cap. The RF Cap achieves full brain coverage with high sensitivity while allowing the administration of TMS at most targets over the scalp with an easy setup and possibility of using a neuronavigation system. The RF Cap consists of a FLEXIBLE and a RIGID part. The FLEXIBLE part is a neoprene cap with 26 flexible RF coaxial cable loops sewn onto it and distributed following a soccer ball layout. The RF elements were interfaced using flexible PCBs and incorporating a BALUN to minimize common modes on the short cables connecting the elements to their preamplifiers placed on the RIGID part. This solution provides a user-friendly approach for concurrent TMS/fMRI acquisitions while ensuring optimal patient comfort. We show that the RF Cap offers at least 4 times more SNR than a birdcage coil at the center of the brain and 10-16 times more SNR on the cortex. The effects of the TMS on the SNR of the RF Cap are between 10% and 25% loss over the region where the TMS coil is placed. The RF Cap has the potential to transform concurrent TMS/fMRI into a practical and useful neuroscientific tool as well as to pave the way for future clinical applications.