Magnetic vector field mapping of the stimulated abductor digiti minimi muscle with optically pumped magnetometers.

Mapping the myomagnetic field of a straight and easily accessible muscle after electrical stimulation using triaxial optically pumped magnetometers (OPMs) to assess potential benefits for magnetomyography (MMG). Approach: Six triaxial OPMs were arranged in two rows with three sensors each along the abductor digiti minimi (ADM) muscle. The upper row of sensors was inclined by 45° with respect to the lower row and all sensors were aligned closely to the skin surface without direct contact. Then, the electromagnetic muscle activity was electrically evoked utilizing stepwise increasing currents at the cubital tunnel at the ulnar nerve. Evoked myomagnetic activity was recorded with 18 channels, three per sensor. As the measurements were performed in PTB's magnetically shielded room (BMSR-2) no averaging and only moderate filtering was applied. Main results: The myomagnetic vector field could be successfully mapped. The obtained spatial structure with a radial symmetry corresponds to the expectations from the ADM's parallel muscle architecture. The temporal evolution exhibits an up to four-phasic shape. Implications for future experiments are derived and needs for sensor performance improvements are identified. Significance: The use of an OPM array with small (~3 mm edge length) sensing voxels enabled the mapping of the magnetic vector field of the ADM. This allowed visualization of the spatiotemporal evolution of the muscle's evoked magnetic field and gives implications for future experiments. In the future, high density OPM grids may enable high-accuracy determination of muscle parameters such as innervation zone position, pennation angle, and propagation velocities. .