Abstract
The use of the double-cone (DC) coil in transcranial magnetic stimulation (TMS) is promoted with the notion that the DC coil enables stimulation of deeper brain areas in contrast to conventional figure-of-8 (Fo8) coils. However, systematic comparisons of these two coil types with respect to the spatial distribution of the magnetic field output and also to the induced activity in superficial and deeper brain areas are limited. Resting motor thresholds of the left and right first dorsal interosseous (FDI) and tibialis anterior (TA) were determined with the DC and the Fo8 coil in 17 healthy subjects. Coils were orientated over the corresponding motor area in an angle of 45 degrees for the hand area with the handle pointing in posterior direction and in medio-lateral direction for the leg area. Physical measurements were done with an automatic gantry table using a Gaussmeter. Resting motor threshold was higher for the leg area in contrast to the hand area and for the Fo8 in contrast to the DC coil. Muscle by coil interaction was also significant providing higher differences between leg and hand area for the Fo8 (about 27%) in contrast to the DC coil (about 15%). Magnetic field strength was higher for the DC coil in contrast to the Fo8 coil. The DC coil produces a higher magnetic field with higher depth of penetration than the figure of eight coil.
Highlights
The use of the double-cone (DC) coil in transcranial magnetic stimulation (TMS) is promoted with the notion that the DC coil enables stimulation of deeper brain areas in contrast to conventional figure-of-8 (Fo8) coils
Transcranial magnetic stimulation (TMS) has traditionally been considered to be a method that enables the direct modulation of neuronal activity in superficial cortical areas, but not in deeper brain areas, as the strength of the produced magnetic field strongly declines with increasing distance from the coil
resting motor threshold (RMT) differences between the Fo8 and DC coil were higher for the tibialis anterior (TA) in contrast to the first dorsal interosseous (FDI)
Summary
The use of the double-cone (DC) coil in transcranial magnetic stimulation (TMS) is promoted with the notion that the DC coil enables stimulation of deeper brain areas in contrast to conventional figure-of-8 (Fo8) coils. More recently new coil geometries (double-cone coil, batwing coil, H-coil) have been developed, which differ in the spatial configuration of the produced magnetic field and enable direct stimulation of skull-distant areas[5,6]. This is for example of high relevance in treatment of depression with repetitive TMS (rTMS). Behavioural[13], neuroimaging[14,15] and clinical studies[16,17,18,19] suggest that the DC coil modulates cingulate or medial prefrontal cortex function even if it is still under debate whether these regions are reached directly or transsynaptically via stimulation of superficial brain areas. Some parameters were not reported at all[21]
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