Short-interval intracortical inhibition (SICI) evoked by transcranial magnetic stimulation (TMS) is emerging as a sensitive and specific tool in the diagnosis of Amyotrophic Lateral Sclerosis (ALS) (Tankisi et al. 2022). Using novel PC-driven threshold tracking software high quality measurements employing automated standard recording and analysis paradigms can now easily be obtained by a single operator with little experience in the routine clinical setting (Tankisi et al. 2021) . The standard instrument in the field, the Magstim 2002 in Bistim mode has been used extensively to collect a large body of normative and patient data (Tankisi et al. 2022). However, it is currently unclear whether these results are similar to those obtained with other instruments. We therefore compared the results from three commonly used TMS devices.
Recordings were obtained from the right FDIO muscle of 12 healthy volunteers (6 female, 6 male, 27-60 years) after stimulation of the contralateral cortex with figure of eight coils: Magstim 2002, D702 coil, Deymed DuoMAG MP, 70 BF-LQC coil and Neurosoft-MS, FEC-03-100 coil. The same protocol was delivered with each of the instruments in a balanced Latin square design. Using stimuli every 4-5s we measured resting motor thresholds (RMT) 50µV, 200µV, 1000µV, stimulus response function and SICI (interstimulus intervals (ISI) of  1-3.5 ms, 70% conditioning stimulus of RMT200) in conventional amplitude (A-SICI) and parallel threshold SICI (T-SICIp) mode.
Bistim and DuoMAG yielded similar RMTs whereas measurements with Neurosoft devices had significantly lower RMTs. There were no significant differences in the SR curves (80 to 170% of RMT200) between the three devices. T-SICIp measurements obtained with BiStim and DuoMag did not differ at any single ISI or in different averages taken over different ISIs. By contrast T-SICIp was significantly higher when measured with the Neurosoft device. Conventional A-SICI averages of 1-3.5ms or 2-3 ms showed similar corresponding findings. There were not statistically significant differences between Bistim and DuoMAG, but recordings taken with Neurosoft devices differed significantly from BiStim.
We conclude that results obtained with BiStim, D702 and DuoMAG, 70 BF-LQC are very similar and can therefore be used interchangeably. By contrast recordings with the Neurosoft-MS, FEC-03-100 show many significant differences and it is not possible to use the reference data from Bistim investigations for diagnostic purposes.