Browsing by Author "Nieminen, Jaakko, Dr., Aalto University, Department of Neuroscience and Biomedical Engineering, Finland"

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Implementing advanced transcranial magnetic stimulation technology
(2017) Koponen, Lari
School of Science | Doctoral dissertation (article-based)
Transcranial magnetic stimulation (TMS) is a non-invasive method to stimulate the human brain. In TMS, a brief, strong magnetic field pulse induces an electric field that can cause individual neurons to fire. A typical TMS pulse lasts less than one millisecond; with a suitable coil design, focal stimulation of the superficial parts of the brain can be obtained. TMS has better than centimetre-scale spatial resolution and millisecond-scale temporal resolution. Thus, it has found use in basic brain research and clinical applications—TMS has been used, for example, to study inhibitory and facilitatory circuits in the human motor cortex, and to treat depression. The spatiotemporal resolution of TMS is, however, limited: to change the locus of stimulation, the TMS coil must be moved. This limits the stimulation to predetermined targets. This thesis consists of six publications that advance TMS technology, methodology, and instrumentation. Publication 2 mostly considers instrumentation: it describes a probe for automatic characterisation and calibration of the stimulation output of a TMS device—a TMS-coil characteriser. Publications 4 and 5 consider methodology: they, respectively, describe a method to increase the temporal resolution of TMS to study the dynamics of the brain in the microsecond scale, and a pipeline to compute the TMS-induced electric field in small mammals. Publications 1, 3, and 6 consider technology. Publications 1 and 3 present a systematic approach to design the most energy-efficient TMS coil with the desired focality. Then, based on this coil-design method, Publication 6 introduces an algorithm to design a set of coils, with which the spatial properties of the TMS-induced electric field can be electronically controlled. In this thesis, the TMS-coil characteriser was used to measure the output from several different commercial TMS devices; differences were found even within devices from one manufacturer. The field-computation pipeline was used to study the effects of the skull geometry to stimulation; in the small mammals, the geometry influences the stimulation location and causes large, unexpected changes in the stimulation intensity and direction. The higher temporal resolution due to new TMS-pulse waveforms was used to study the non-linear properties of excitable membrane in the microsecond-scale. Finally, the TMS coils developed in this thesis demonstrate a considerable improvement over the state of the art: The optimised coil presented in Publication 3 had two times the efficiency of a common, commercial figure-of-eight coil. The two-coil transducer in Publication 6, in combination with a two-channel TMS device, allowed to build the first TMS device with electronic stimulation targeting—an ability to adjust the locus of stimulation without any physical coil movement, which, in addition to allowing much higher spatiotemporal resolution, may reduce the manual labour required in clinical TMS.