Electric field properties of two commercial figure-8 coils in TMS: calculation of focality and efficiency.

OBJECTIVE To compare two commonly used TMS coils, namely the Medtronic MC-B70 double coil and the Magstim 70 mm double coil, with respect to their electric field distributions induced on the cortex. METHODS Electric field properties are calculated on a hemisphere representing the cortex using a spherical head model. The coil designs are characterised using several parameters, such as focality, efficiency and stimulation depth. RESULTS Medtronic and Magstim coils exhibit similar focality values and stimulation depths, despite very different coil designs. However, the Medtronic coil is about 1.2 times more efficient compared to the Magstim coil. This difference corresponds to different motor and visual phosphene thresholds obtained in previous physiological studies, thereby validating the chosen coil modelling approach. Focality of the Medtronic coil changed less with varying coil-cortex distance compared to the Magstim coil, whereas both coils exhibited similar dependencies on changes in cortex radius. CONCLUSIONS The similar values for focality and stimulation depth indicate that both coil types should evoke similar physiological effects when adjusting for the different efficiencies. The different physiological thresholds of the two coils can be traced back to differences in coil design. Ideally, focality should depend neither on coil-cortex distance nor on cortex radius in order to allow for an inter-subject comparability. In particular, in motor mapping experiments the size of the resulting maps is affected by these two parameters. Consequently, they are at least partially the cause of the variability across subjects seen in these experiments.