Computational modelling of white matter activation for transcranial magnetic stimulation

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School of Electrical Engineering | Master's thesis
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Date

2025-06-19

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Biosensing and Bioelectronics

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Master's Programme in Life Science Technologies

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en

Pages

64

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Abstract

Transcranial Magnetic Stimulation (TMS) is a non-invasive neurostimulation technique being widely used in therapy, diagnosis, and brain research. Various studies have investigated the neural activation patterns of TMS aiming to enhance the effectiveness and precision of its usage. However, the majority of these findings were based on neuronal activation in the grey matter, while recent studies have shown that neural structures in the white matter would potentially be activated. In this thesis, an MRI-based computational TMS model was developed. Along with the usage of a tractography-based axon model, effect of different stimulation parameters upon TMS-induced neural activation was investigated by assuming the activation sites are in the white matter. Three simulation scenarios were used and the simulation results were validated through comparison with actual TMS measurements collected from the same participants. It was discovered that using anterior-to-posterior(AP)-TMS, higher electric field intensity was required to activate the axons in comparison to posterior-to-anterior(PA)-TMS, and the activation sites of the cortico-cortical axons were further away from the axon origin. Without restriction from the model, corticospinal tract axons had a lower activation intensity than that of cortico-cortical axons, but the order was reversed after blocking the first few nodes of Ranvier from excitation. There was an inverse relationship between the pulse duration and activation intensity of the axons, while increasing the pulse durations beyond 60-70 µs would have a minimal effect on the activation pattern. Regarding to the axon origin, Brodmann Area 4a (BA4a) originated axons are more likely to be activated using PA-TMS than axons originated from Brodmann Area 4p (BA4p). The simulated results were relatively accurate with some random errors while comparing to the measured MEP threshold and latency.

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Supervisor

Laakso, Ilkka

Keywords

transcranial magnetic stimulation, axon, computational modelling, primary motor cortex, white matter, pulse duration

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https://urn.fi/URN:NBN:fi:aalto-202508186204

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[dipl] Sähkötekniikan korkeakoulu / ELEC