networks. Experiments performed so far are using very large, standard birdcage MRI head coils to provide adequate space for positioning of the TMS system ...
A new MR device for combined TMS/fMRI experiments Lucia Navarro de Lara1,2, Christian Windischberger1,2, Jürgen Sieg1,2, Ewald Moser1,2, and Elmar Laistler1,2 1
MR Centre of Excellence, Medical University of Vienna, Austria
2
Center for Medical Physics and Biomedical Engineering, Medical University of Vienna, Austria
Introduction There is an increasing interest to perform studies with simultaneous fMRI and transcranial magnetic stimulation (TMS)[2003]. These techniques together offer the possibility to directly study the effects of local TMS-induced alterations of cortical excitability [2008]. Also, combined TMS/fMRI allows for localized investigation of TMS effects and could help answer questions of causality in neuronal networks. Experiments performed so far are using very large, standard birdcage MRI head coils to provide adequate space for positioning of the TMS system between head and MR coil. Such a setup results in poor sensitivity for the MRI experiment due to the large distance between brain and MR coil. To improve MR sensitivity, we have developed a dedicated, very slim MR coil on a curved surface that can be placed between the TMS device and the head. In this case, the TMS is very close to the brain to achieve efficient stimulation, and the MR coil is directly attached to the head providing much higher sensitivity compared to conventional setups. Materials and methods The dedicated receive-only coil was designed for 3 T MR systems (125 MHz), with a total diameter of 15 cm and a target depth of 5.0 - 7.5 cm. It consists of seven surface loops, in a hexagonal arrangement, placed on a spherical support with a curvature to allow for flexible positioning around the head. Adjacent elements were decoupled by appropriate spatial overlapping.
All basic elements of the MR coil are passively and actively detunable for decoupling from the MR system's body coil during signal transmission. To improve mutual decoupling between the elements, asecond stage matching network [1995] connects the coil elements to low noise preamplifiers. 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. An illustration of the housing of the MR device with the TMS system is shown on the left side of Figure 1. The figure also shows the design of the electronic part.
Results Mutual coupling between elements of the MR coil is below -9 dB for all channels. Additional -15 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 as shown maximum field distortions below 5%. This avoids image artifacts and ensures patient safety. Cable traps were also built for each channel to avoid shield currents.
In vivo T1 weighted images(standard MPRAGE sequence) were acquired with the new MR-coil (see Fig. 2 top), demonstrating the high sensitivity of the device, still enabling scanning of large parts of the brain. The corresponding SNR (signal-to-noise ratio) maps of the measurements were calculated as image intensity divided by the standard deviation of noise in a large background region and are also depicted in figure 2 (bottom).
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, although at the cost of SNR. This novel hardware development will boost the local sensitivity of fMRI results in combined TMS/fMRI experiments, enabling fast and high-resolution fMRI during efficient and safety stimulation of the brain with TMS.
Acknowledgements This work was funded by the Austrian BMWFJ, FFG Project Nr 832107 References [1] Kobayashi K, 2003, The Lancet, 2:145-156 [2] Bestmann,S.,2008,Exp Brain Res,191:383-402 [3] Reykowski S.M, 1995, MRM, 848-852
