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Previous work noted that there are significant variabilities in motor response to intermittent theta-burst stimulation (iTBS). It is possible this variability results from individual neuroanatomical differences. The current objective was to investigate the effect of individual neuroanatomy on first dorsal interosseous (FDI) and biceps brachii motor evoked potentials (MEPs) in response to iTBS.
Cross-sectional study using a convenience sample.
Ten healthy individuals (7 females, 23.5 ± 5 years) participated in this study. Participants completed two iTBS sessions (one each targeting the FDI and biceps cortical hotspots) and an MRI on separate days.
Main Outcome Measures
MEPs were calculated using data acquired with a Magstim Super Rapid2 Plus1 stimulator via a 70 mm double air film coil to the primary motor cortex and electromyography signals were measured from dominant FDI and biceps. Head models were generated from T1 & T2 weighted MRI. Diffusion tensor imaging was used to determine fiber tract geometry for FDI and biceps corticospinal tracts. We then established neuroanatomical parameters including: fiber tract surface area (FTSA), tract fiber count (TFC), and brain scalp distance (BSD). Cortical electric field strength (EFS) was calculated using simulated stimulation of head models and finite element analysis.
For the FDI, there was no effect of iTBS (p = 0.223) but individual changes in corticomotor excitability scaled with stimulation type and resting motor potential (p < 0.001), EFS (p = 0.001), BSD (p = 0.004), and FTSA (p = 0.011). iTBS had a faciliatory response on the biceps and was dependent on FTSA (p < 0.001) and TFC (p < 0.001).
Our results show that MRI-based measures of neuroanatomy, specifically cortical architecture and tract anatomy, impact how the motor system responds to iTBS, subject to target-specific cortical control organization. MRI-based modeling of individual neuroanatomy may be a useful approach in selection of motor targets when designing iTBS-based therapies.
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