P 233. Robot-assisted and image-guided randomized TMS mapping of human and primate hand muscle

Abstract

Introduction Coil positioning is a crucial factor for transcranial magnetic stimulation (TMS). Manual positioning of flat TMS coil against the curved head allows for errors in translation and rotation. As scalp markings are an ineffective way to ensure consistency in coil positioning, image-guided neuronavigation is used to reduce errors. However, as manual neuronavigation increases time for coil positioning it is less suitable when the coil has to be moved between pulses during TMS mapping. The use of a robot for coil positioning may overcome these drawbacks of manual neuronavigation. Objectives We aimed at assessing the practicability and efficacy of the using a robot for a neuronavigated TMS mapping of the cortical representation of the first dorsal interosseus (FDI) of humans and rhesus monkey. Materials and methods For FDI mapping, 12 supra-threshold stimuli (120% RMT) were applied in a randomized order to each grid point of a 7 × 7 grid with a spacing of 10 mm in 8 subjects (mean age 26 y, 6 male) with a Magstim 200 stimulator and a standard 70 mm figure-8 coil. Surface EMG was recorded from contralateral FDI. An Adept Viper s850 robotic arm moved the coil to each grid point on an MRI-based head model in a random order. An IR camera obtained actual positional data of the head. This allowed tracking of head movements and respective online movement compensation by the robot. Centers of gravity were projected onto the brain surface and co-registered to an MNI152 T1 1 mm brain template. A similar setup was used for FDI mapping in a male adult rhesus monkey on a 4x5 grid with 5 mm spacing. TMS was applied with a Magstim 25 mm figure-8 coil. The mapping was performed during multiple sessions during which the monkey’s head was attached to a frame with frontal and occipital head posts. Relaxation of FDI was rewarded with water after each pulse. Results TMS Mapping sessions took about 80–90 min in each subject with a pulse interval of 8 s. Preparations lasted about 20 min. A closely spaced TMS representation of FDI could be identified in each subject within the precentral gyrus. The shape of the representation area resembled ovals with a mean maximum MEP amplitude of 1.35 mV (SD: 0.52). Mean stereotaxic coordinates of the centers of gravity were (−36, −12, 67) in MNI152 space. In the animal experiment, the monkey adapted to the setup within 4 weeks. The preparation time for each session was about 10 min. Across all sessions muscle relaxation was present during 64% of the pulses. The pulse interval was 13.4 s. Four repetitions of a complete grid could be recorded in a single session of about 45 min duration. MEPs from contralateral FDI were elicited from a highly defined elliptical area (1 × 1.5 cm) within the precentral gyrus. Conclusion Due to the combination of fast high-precision coil positioning and online head tracking, robot-assisted and image-guided TMS is exquisitely suited to perform randomized TMS mappings in the human with acceptable preparation times. In addition, this method allows for precise TMS mapping of the considerably smaller rhesus brain where high-precision coil positioning with a high inter-session reliability is required for multiple session mappings with grid sizes in the millimeter scale.