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UBC Theses and Dissertations

Concurrent transcranial magnetic stimulation and magnetic resonance spectroscopy in phantoms and humans Sporn, Leo


Transcranial magnetic stimulation (TMS) is an increasingly popular treatment for psychiatric diseases including depression, the most common cause of psychiatric disability worldwide. TMS is an effective treatment for depression even in patients where conventional pharmacological and therapeutic treatments for the disease have been unsuccessful. As the mechanism of TMS relies on electromagnetic induction, it is a completely noninvasive treatment. While TMS is known to be effective in treating depression, exactly how TMS affects the brain is an active area of research. To properly study the effects of TMS in vivo, non-invasive methods are required. As such, magnetic resonance (MR) modalities present an attractive option due to their non-invasive nature. There is interest in studying the effects of TMS using MR concurrently with TMS, however, concurrent TMS/MR is challenging due to TMS-related distortions to the MR scanners magnetic field. MR Spectroscopy (MRS) is ideal for TMS/MR applications as not only does MRS provide information about brain chemicals thought to change during TMS, it also is sensitive enough to characterize distortions that are present in other MR images but not as easy to recognize. In this thesis, non-water-suppressed MRS experiments were performed concurrently with TMS to characterize distortions to the MRS water signal. Phantom experiments were performed under a wide range of experimental conditions, varying MRS voxel positioning and parameters related to TMS pulsing. Distortions investigated included signal-noise ratio, free induction decay spikes, B0 inhomogeneity, eddy currents, and frequency modulation sidebands. As a proof of concept, concurrent TMS/MRS results from a human experiment are presented. The dominant source of signal distortion was found to be related to the presence of the TMS coil itself, and the magnitude of the distortions depended most strongly on the position of the MRS voxel relative to the TMS coil. To a lesser extent, TMS pulses further distorted MRS signal, particularly at higher pulse amplitudes and when there was a smaller time delay between the TMS pulse and 90◦ radiofrequency pulse. This work presents results from the first concurrent TMS/MRS experiments reported. It is intended that these results provide guidance for future research using concurrent TMS/MRS.

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