P 212. A novel high sensitivity MR-device for combined TMS/fMRI experiments

Abstract

Introduction Studies with simultaneous functional magnetic resonance imaging (fMRI) and transcranial magnetic stimulation (TMS) have become increasingly popular as the combination of these techniques allows investigation of TMS effects on the brain and could help answer unresolved questions concerning causality in fMRI ( Kobayashi, 2003 ). So far, experiments are being performed using rather large, standard MRI head coils, with the TMS positioned between head and MR coil, resulting in poor sensitivity for the MRI experiment, while also preventing visualization of activation spots directly below the TMS focus. Objective To gain sensitivity for combined TMS and fMRI experiments, we developed a dedicated, slim MR coil on a curved, form-fitted surface that can be placed between TMS and the head. This way, the TMS is close enough to the brain to achieve efficient stimulation, and the MR coil can provide far higher sensitivity than in the conventional arrangement due to its proximity to the head. This way, the TMS coil itself does not cause artifacts in the fMRI maps. Materials and methods This dedicated receive coil is designed for 3T MR systems, with a field of view of 15 cm and a target depth of 5.0–7.5 cm. It consists of seven hexagonally arranged loop elements of 6 cm diameter each. To decouple adjacent elements, loops were overlapped. The array is placed on a spherical support with a curvature to fit the average human head. An illustration of the device positioned for investigations in the dorsolateral prefrontal cortex is shown in ( Fig. 1 ). All elements of the MR coil are passively and actively detunable for decoupling from the MR system’s body coil during signal transmission. A second stage matching network ( Reykowski, 1995 ) connects the coil elements to low noise preamplifiers, additionally improving mutual decoupling between array elements. Preamplifiers are not placed directly on the coil, minimizing coil thickness to assure efficient TMS stimulation, and avoiding damage to the preamplifiers by TMS pulses. Results Mutual coupling between elements of the MR coil is below-9 dB for all channels. Additional-20 dB decoupling was achieved by preamplifier decoupling, minimizing noise correlation between elements, and thus, enabling parallel imaging with low g-factors. The dedicated MR coil is well isolated from the transmitting MR body coil, proven by field distortions well below 5%. This avoids image artifacts and ensures patient safety. A gradient echo image of a phantom acquired with one of the seven elements is shown in ( Fig. 2 ), demonstrating the high sensitivity of the basic elements in cortical areas. Conclusion A dedicated 7-channel MR coil array for simultaneous fMRI/TMS measurements is presented. Individual coil elements were efficiently decoupled from each other and from the transmit body coil. Due to the substantial gain in SNR and good isolation between channels, parallel imaging can also be implemented to speed up TMS-fMRI measurements. This novel hardware development will boost sensitivity of fMRI in combined fMRI/TMS experiments, enabling fast and high-resolution fMRI while efficiently and safety stimulating the brain with TMS.