Comparison of stimulus intensity in hand held and robotized motion compensated transcranial magnetic stimulation

Transcranial Magnetic Stimulation (TMS) is based on a changing magnetic field passing through the skull and inducing an electric field in the cortex [1,2]. The latter results in cortical stimulation and needs to be aligned with the target region. Conventionally, the TMS coil is mounted to a static holder and the subject is asked to avoid head motion. Additionally, head resting frames have been used [3]. In contrast, our robotized TMS system employs active motion compensation (MC) to maintain the correct coil position [4]. To assess the potential impact of patient motion, we study the induced electric field for the different setups. We recorded 30 min of head motion for six subjects in three scenarios: (a) using a coil holder and avoiding head motion, (b) using a coil holder and a head rest, and (c) using the robotized system with motion compensation. The motion traces were fed into a second robot to mimic head motion for a field sensor integrated in a head phantom. We found that after 30 minutes the induced electric field was reduced by 32.0% and 19.7% for scenarios (a) and (b), respectively. For scenario (c) it was reduced by only 4.9%. Furthermore, the orientation of the induced field changed by 5.5°, 7.6°, and 0.4° for scenarios (a), (b), and (c), respectively. None of the scenarios required rigid head fixation [5], which is often considered impractical and uncomfortable. We conclude that active motion compensation is a viable approach to maintain a stable stimulation during TMS treatments.