Marked differences in the thermal characteristics of figure-of-eight shaped coils used for repetitive transcranial magnetic stimulation Thomas Weyh a , Kerstin Wendicke a , Claudia Mentschel b , Hannes Zantow a , Hartwig R. Siebner b,c,d, * a Heinz Nixdorf-Lehrstuhl fu ¨r Medizinische Elektronik, Technische Universita ¨t Mu ¨nchen, Arcisstrasse 21, 80333 Munich, Germany b Department of Neurology, Technische Universita ¨t Mu ¨nchen, Moehlstrasse 28, 81675 Munich, Germany c Department of Neurology, Christian-Albrechts-University, Niemannsweg 147, 24105 Kiel, Germany d NeuroImageNord Hamburg-Kiel-Lu ¨beck, Martinistrasse 52, 20246 Hamburg, Germany Accepted 9 February 2005 Available online 26 March 2005 Abstract Objective: To compare the heating behaviour of three figure-of-eight shaped coils during repetitive transcranial magnetic stimulation (rTMS). Methods: A custom-made coil (referred to as test coil) with a resistance-optimized conductor geometry was compared with two commercially available eight-shaped coils. Each coil was attached to the same energy source, which generated trains of 50 biphasic magnetic pulses every 20 s. Coil temperature was continuously measured during nine rTMS protocols using various combinations of stimulus frequencies (5, 10 or 20 Hz) and intensities (40, 50 or 60% of maximum stimulator output). A heating curve relating coil temperature and the number of applied stimuli was generated for each coil and rTMS condition. In eleven healthy volunteers, we evaluated the effectiveness of motor cortex stimulation. For each coil, we determined the motor threshold (MT) in the right first dorsal interosseus muscle. Results: The slope of the heating curves of the test coil was markedly flattened relative to the heating curves of the two standard coils. This allowed the application of at least twice as many stimuli until the temperature of the coil reached 40 8C. Based on these data, we showed that a one-mass model could be used to accurately describe the heating behaviour of each coil. MTs determined with the test coil were comparable to or lower than the MTs that were determined with the standard coils. Conclusions: The efficacy of the test coil to stimulate the M1 was comparable to the efficacy of the two standard coils, yet thermal characteristics were markedly improved. Significance: Overheating of figure-of-eight shaped coils can be markedly delayed without reducing the efficacy of rTMS. q 2005 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. Keywords: Coil temperature; Efficacy; Figure-of-eight coil; Motor threshold; Overheating; Repetitive transcranial magnetic stimulation 1. Introduction Since its introduction in 1985 (Barker et al., 1985), transcranial magnetic stimulation (TMS) has emerged as a safe and painless method for stimulating the human cortex through the intact scalp (Maccabee et al., 1991; Rothwell et al., 1999). A device for TMS consists of a transducing coil connected to a high-voltage, high-current discharge system which produces a strong magnetic field around the transducing coil for up to a few 100 ms (Barker, 1999). When the coil is placed on the scalp, the induced magnetic field passes without attenuation through the skull and induces an electrical current in the brain (Barker et al., 1985). The induced electrical current can excite cortical neurons depending on the intensity of stimulation. The introduction of stimulating devices that can produce trains of magnetic stimuli at rates up to 50 Hz has considerably expanded the applications of TMS, since repetitive transcranial magnetic stimulation Clinical Neurophysiology 116 (2005) 1477–1486 www.elsevier.com/locate/clinph 1388-2457/$30.00 q 2005 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.clinph.2005.02.002 * Corresponding author. Address: Department of Neurology, Christian- Albrechts-University, Niemannsweg 147, 24105 Kiel, Germany. Tel.: C49 431 597 2703; fax: C49 431 597 2712. E-mail address: h.siebner@neurologie.uni-kiel.de (H.R. Siebner).