Magnetic motor threshold and the power of stimulation Magnetic motor threshold (MT) is defined as the minimal amount of machine power needed to induce a deflection of 50 μV in the electromyographic recordings in 5 out of 10 trials.10 It has been argued that the difference between the MT (ie, electromyographic recording of motor evoked potentials [MEP]) and the twitch threshold (ic, hand movement, that corresponds to the MEP) is minimal and probably clinically irrelevant.11 Inhibitors,research,lifescience,medical However, current, safety guidelines require monitoring by electromyography (EMG) for identification of afterdischarges or spreads of excitation, ic, the established forerunners of seizures.12 In sTMS, magnetic Inhibitors,research,lifescience,medical stimulations are usually
administered at 100% machine power,
whereas in rTMS the power ranges between 80% and 120% MT (usually about 40% to 70% of the stimulator’s maximum). Initial studies with rTMS were performed with the power set at 80% to 90% MT. However, more recent studies generally use around 100% to 110% MT. Stimulation paradigms significantly above MT have been reported to be associated with the induction Inhibitors,research,lifescience,medical of seizures,7 or have been used to induce seizures in a controlled setting.8 It is important, to note that, since the twice introduction of the safety guidelines for the administration of TMS, there has been no new report, of seizures during TMS.13 MT may not, be the best guiding principle for setting the power of Inhibitors,research,lifescience,medical stimulation when rTMS is performed over the frontal cortex. Indeed, what is appropriate for the motor cortex may not be appropriate for the frontal or prefrontal cortex Kozel et al14 and McConnell et al15 pioneered the concept that, the power of stimulation needs to be calculated on the basis of the scalp-to-cortex distance and not just as a function of MT. MT reflects more closely the scalpto-motor cortex distance than Inhibitors,research,lifescience,medical the Imatinib structure scalp-to-prefrontal cortex distance. In elderly patients in whom the scalp-to-frontal cortex is increased due to brain atrophy, calculations of the power of stimulation on the basis of scalp-to-motor cortex distance may underestimate the
power needed to stimulate the frontal cortex in these individuals. Coil Dacomitinib used for administering TMS Two main types of coil are used in TMS: the round coil and the figure-of-eight coil. It is unclear whether one is superior to the other, as positive results have been reported with both types. The round coil is more common in singlepulse and sTMS studies, while the figure-of-eight coil is used more commonly in rTMS studies. The magnetic field produced by the round coil is strongest around the perimeter of the coil and, therefore, it stimulates a larger but more diffuse cortical area, lite magnetic field of the figureof-eight-coil is concentrated over the area where the wings of the coil meet, providing a much more focused stimulation over a smaller area of the cortex.
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