Combination of fMRI and TMS: timing issues on 3 Tesla

Research output: Chapter in Book/Report/Conference proceedingConference paperResearch

Abstract

Introduction:
Transcranial magnetic stimulation (TMS) is a method to noninvasively disrupt brain activity and which can be combined with fMRI to study brain activity and connectivity. The feasibility of this combination of techniques has been shown earlier [1] and problems due to the interference of electromagnetic fields, have been explored [2]. This study focuses on the timing of the TMS stimulation pulse during the fMRI measurements.
Methods:
Measurements were performed on a Philips Achieva 3T MR scanner on a spherical phantom (diameter = 18cm) filled with distilled water doped with sodium acetate and lithium lactate. TMS pulses were applied using an MR-compatible butterfly TMS coil (20cm x 10cm) and a Rapid stimulator, both from The Magstim Company. The coil was connected to the stimulator outside the RF-shielded room with an 8m cable running through one of the waveguides. No RF filter was used. Timing of the TMS pulses was controlled with microsecond accuracy using a USB-6251 data acquisition device from National Instruments. Due to the limited space inside the standard RF volume head coil, an elliptical two element flexible surface coil (14cm x 17cm) was used instead to accommodate the TMS coil. The surface coils were positioned to the left and right side of the phantom mimicking the clinical situation. The TMS coil was placed horizontally in two different positions: inferior and posterior to the phantom (Figure 1). For both positions of the TMS coil, images were acquired of single slices parallel to the coil at 2cm and 4cm distance using a single shot EPI sequence (TR=1000ms, TE=35ms, 64x64 matrix, 3.6x3.6x5 resolution). For each acquisition of the slice, the time between the RF excitation pulse and a single TMS pulse with maximum stimulator output was increased with 1ms steps.
Results:
Figure 2 shows the signal intensities in the measured slices as a function of the delay between a single TMS pulse and the RF excitation pulse of the EPI sequence.
Conclusions:
When the TMS pulse is applied less than 65ms before or less than 50ms after the RF excitation pulse of the EPI sequence, signal loss occurs in sections at both distances. This results in a period of about 115ms during which no TMS pulses should be applied. For the posterior positioning of the TMS coil, a stronger signal loss is noted.
References:
[1] Bohning, D.E., 1998, Investigative Radiology, 33:336-340
[2] Bestmann, S., 2003, J. Magn. Reson. Imaging, 17:309-316.
Original languageEnglish
Title of host publicationProceedings ESMRMB 2008
PublisherESMRMB
Pages329-329
Number of pages1
Publication statusPublished - 2 Oct 2008
EventFinds and Results from the Swedish Cyprus Expedition: A Gender Perspective at the Medelhavsmuseet - Stockholm, Sweden
Duration: 21 Sep 200925 Sep 2009

Conference

ConferenceFinds and Results from the Swedish Cyprus Expedition: A Gender Perspective at the Medelhavsmuseet
CountrySweden
CityStockholm
Period21/09/0925/09/09

Keywords

  • rTMS
  • fMRI
  • interlaced

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