Coil model comparison for cerebellar transcranial magnetic stimulation

Objective. Transcranial magnetic stimulation (TMS) has been increasingly used for investigating the function of different areas of the cerebellum. Experimental studies have shown variable effectiveness of different types of coils in cerebellar TMS. Our objective is to systematically investigate the effects of coil type on the electric field (E-field) induced in the cerebellum. Approach. Induced E-fields of cerebellar TMS were modelled using individualised head models of 15 subjects. A double-cone, a figure-of-8 and a round coil were modelled, and E-field measurements were performed to validate the coil models. The E-fields induced in anatomical models are statistically analysed to investigate the effects of the coil type and placement on the stimulation strength, depth and focality. Main results. The E-field measurements showed that the applied thin-wire approximation is valid and useful for computational modelling of TMS coils. Inter-subject analysis of the induced E-fields showed that the round coil systematically produced the weakest E-fields. The difference between the double-cone and figure-of-8 coils was not systematic; instead, the most efficient coil type depended on the targeted region of the cerebellum. Further, despite inter-subject differences, the double-cone coil appeared more suitable for stimulating deeper regions of the cerebellum, whereas the figure-of-8 coil produces more focal stimulation, hence providing precise stimulation of regions closer to the scalp surface. Significance. The double-cone, round and figure-of-8 coils have different advantages over each other in different situations. The choice of the coil geometry and placement should be made with care for the desired stimulation outcome. Moreover, inter-subject anatomical differences affect the E-field induced in the cerebellum, and anatomical modelling using magnetic resonance imaging data can enhance the precision of cerebellar TMS.