Peripheral Nerve Magnetoneurography With Optically Pumped Magnetometers

Abstract

Electrodiagnosis is routinely integrated into clinical neurophysiology practice for peripheral nerve disease diagnoses, such as neuropathy, demyelinating disorders, nerve entrapment/impingement, plexopathy, or radiculopathy. Measured with conventional surface electrodes, the propagation of peripheral nerve action potentials along a nerve is the result of ionic current flow which, according to Ampere’s Law, generates a small magnetic field that is also detected as an “action current” by magnetometers, such as superconducting quantum interference device (SQUID) Magnetoencephalography (MEG) systems. Optically pumped magnetometers (OPMs) are an emerging class of quantum magnetic sensors with a demonstrated sensitivity at the 1 fT/√Hz level, capable of cortical action current detection. But OPMs were ostensibly constrained to low bandwidth therefore precluding their use in peripheral nerve electrodiagnosis. With careful OPM bandwidth characterization, we hypothesized OPMs may also detect compound action current signatures consistent with both Sensory Nerve Action Potential (SNAP) and the Hoffmann Reflex (H-Reflex). In as much, our work confirms OPMs enabled with expanded bandwidth can detect the magnetic signature of both the SNAP and H-Reflex. Taken together, OPMs now show potential as an emerging electrodiagnostic tool.

Magnetoencephalography with optically pumped magnetometers (OPM-MEG): the next generation of functional neuroimaging

“…Arrays can be designed to target specific brain regions with high sensor density, for example, if high spatial resolution is desired in a specific area (recently published examples include the language network [19], hippocampus [20], and cerebellum [21]). It is also becoming apparent that OPM use is not limited to the brain, with arrays also having been used to measure electrophysiological signals in the muscles [22], peripheral nerves [23], spinal cord [24], retina [25], and the foetus [26]. Another advantage is that, whereas SQUIDs typically measure the magnetic field in one orientation (usually radial to the A participant sits with their head in a static helmet (see inset photo, adapted from [16]), containing an array of field sensors (blue circles).…”

Alignment of magnetic sensing and clinical magnetomyography

“…The new generation of MMG sensors, such as TMR, do not have a limitation for frequency bandwidth 12 . However, there is a trade-off between the bandwidth and the sensitivity of a magnetic sensor, which means that they cannot be optimized simultaneously, as increasing the bandwidth would reduce the sensitivity of the sensor ( Bu et al, 2022 ).…”

A scoping review of current and emerging techniques for evaluation of peripheral nerve health, degeneration, and regeneration: part 1, neurophysiology

“…Wikswo [155] produced some of the first magnetic field measurements of nerve impulses followed by numerous incremental improvements in application to the human nerve that continue to be ongoing [156][157][158]. Ionic current flow (as measured by conventional surface electrodes) also generates a tiny magnetic field, which can be detected by magnetometers, the most applied of which being superconducting quantum interference device (SQUID) magnetoencephalography (MEG) systems [158] and more recently optically pumped magnetometers [159,160] (OPMs, figure 9). The ability to non-invasively produce high resolution pictures of intra-axonal as well as inward currents in deep nerves (depth-independent) throughout the entire neural pathway, including plexus, roots, spine, and brain would clearly be of significant value [161].…”