Public defence in Neuroscience and Biomedical Engineering, MSc Mikko Nyrhinen

Title of the thesis: Combining stimulation and imaging of the brain

Thesis defender: Mikko Nyrhinen 
Opponent: Professor Pasi Karjalainen, University of Eastern Finland  
Custos: Professor Matti Hämäläinen, Aalto University School of Science

Mental disorders affect hundreds of millions of people worldwide and are among the leading causes of disability, disrupting daily life, work, and relationships. To improve our understanding of the causes of mental disorders and how to treat them, scientists have to first understand how the brain works. For this, we need powerful tools. 

Transcranial magnetic stimulation (TMS) is a non-invasive tool that uses magnetic fields to stimulate the brain. Multi-locus TMS (mTMS) is an emerging technology that omits the need for manual coil adjustment by clever coil design, enabling researchers to change the stimulation direction and/or location without moving the coil. Functional magnetic resonance imaging (fMRI) is a non-invasive imaging method that maps brain activity indirectly by measuring changes in blood oxygenation: the more active an area of the brain is, the more oxygen it needs. 

In this thesis, we studied the combination of these two modalities. This can be done by performing TMS and fMRI in separate sessions, or simultaneously—concurrent TMS–fMRI. Both approaches have benefits, but simultaneous TMS–fMRI is particularly advantageous because it enables the study of immediate brain responses to stimulation and how ongoing brain activity affects stimulation. In this thesis, we compared TMS and fMRI in a study conducted in separate sessions to assess how well they correspond, but the main aim was to perform TMS–fMRI concurrently. To this end, we built concurrent (m)TMS–fMRI systems for humans and small animals and used them successfully in experiments. 

Conducting TMS–fMRI concurrently creates unique technical challenges; operating the stimulation coil inside the MRI causes the coil to deform, and that mechanical stress produces acoustic noise. To ensure that concurrent TMS–fMRI experiments can be conducted safely, we measured the emitted acoustic noise. We measured noise levels well above regulatory safety limits but concluded that studies can be conducted safely when using appropriate hearing protection and safe work practices. 

Our work shows that combining (m)TMS and fMRI concurrently is feasible and helps to conduct the experiments safely. In addition, our work outlines the possible future of emerging mTMS technology.

Keywords: TMS, mTMS, fMRI, melu

Thesis available for public display 7 days prior to the defence at Aalto University's public display page.