Novel functional imaging technique for the brachial plexus based on magnetoneurography

Assessing ulnar neuropathy at the elbow using magnetoneurography

OBJECTIVE

To measure neuromagnetic fields of ulnar neuropathy patients at the elbow after electrical stimulation and evaluate ulnar nerve function at the elbow with high spatial resolution.

METHODS

A superconducting quantum interference device magnetometer system recorded neuromagnetic fields of the ulnar nerve at the elbow after electrical stimulation at the wrist in 16 limbs of 16 healthy volunteers and 21 limbs of 20 patients with ulnar neuropathy at the elbow. After artifact removal, neuromagnetic field signals were processed into current distributions, which were superimposed onto X-ray images for visualization.

RESULTS

Based on the results in healthy volunteers, conduction velocity of 30 m/s or 50% attenuation in current amplitude was set as the reference value for conduction disturbance. Of the 21 patient limbs, 15 were measurable and lesion sites were detected, whereas 6 limbs were unmeasurable due to weak neuromagnetic field signals. Seven limbs were deemed normal by nerve conduction study, but 5 showed conduction disturbances on magnetoneurography.

CONCLUSIONS

Measuring the magnetic field after nerve stimulation enabled visualization of neurophysiological activity in patients with ulnar neuropathy at the elbow and evaluation of conduction disturbances.

SIGNIFICANCE

Magnetoneurography may be useful for assessing lesion sites in patients with ulnar neuropathy at the elbow.

SQUID magnetoneurography: an old-fashioned yet new tool for noninvasive functional imaging of spinal cords and peripheral nerves

We are engaged in the development and clinical application of a neural magnetic field measurement system that utilizes biomagnetic measurements to observe the activity of the spinal cord and peripheral nerves. Unlike conventional surface potential measurements, biomagnetic measurements are not affected by the conductivity distribution within the body, making them less influenced by the anatomical structure of body tissues. Consequently, functional testing using biomagnetic measurements can achieve higher spatial resolution compared to surface potential measurements. The neural magnetic field measurement, referred to as magnetoneurography, takes advantage of these benefits to enable functional testing of the spinal cord and peripheral nerves, while maintaining high spatial resolution and noninvasiveness. Our magnetoneurograph system is based on superconducting quantum interference devices (SQUIDs) similar to the conventional biomagnetic measurement systems. Various design considerations have been incorporated into the SQUID sensor array structure and signal processing software to make it suitable for detecting neural signal propagation along spinal cord and peripheral nerve. The technical validation of this system began in 1999 with a 3-channel SQUID system. Over the course of more than 20 years, we have continued technological development through medical-engineering collaboration, and in the latest prototype released in 2020, neural function imaging of the spinal cord and peripheral nerves, which could also be applied for the diagnosis of neurological disorders, has become possible. This paper provides an overview of the technical aspects of the magnetoneurograph system, covering the measurement hardware and software perspectives for providing diagnostic information, and its applications. Additionally, we discuss the integration with a helium recondensing system, which is a key factor in reducing running costs and achieving practicality in hospitals.

Neuromuscular Magnetic Field Measurement Based on Superconducting Bio-Sensors

These years, disease-causing and disabling diseases have caused great concern. Neurological musculoskeletal disorders are diverse and affect people of a wide range of ages. And the lack of comprehensive diagnostic methods places a huge burden on healthcare systems and social economies. In this paper, the current status of clinical research on neuromuscular diseases is introduced, and the advantages of magnetic field measurement compared with clinical diagnostic methods are illustrated. A comprehensive description of the related technology of superconducting quantum interference devices (SQUIDs), magnetic field detection noise suppression scheme, the development trend of the sensor detection system, and the application and model establishment of the neuromuscular magnetic field is also given in this paper. The current research and development trends worldwide are compared simultaneously, and finally the conclusions and outlook are put forward. Based on the description of the existing literature and the ideas of other researchers, the next development trends and my own research ideas are presented in this paper, that is, starting from the establishment of a neuromuscular model, combining medical and industrial work, designing a sensor system that meets clinical needs, and laying the foundation for the clinical application of a bio-magnetic system. This review promotes a combination between medicine and industry, and guides researchers on considering the challenges of sensor development in terms of clinical needs. In addition, in this paper, the development trends are described, including the establishment of the model, the clinical demand for sensors, and the challenges of system development so as to give certain guidance to researchers.

Visualization of axonal and volume currents in median nerve compound action potential using magnetoneurography

OBJECTIVE

Reconstruct compound median nerve action currents using magnetoneurography to clarify the physiological characteristics of axonal and volume currents and their relationship to potentials.

METHODS

The median nerves of both upper arms of five healthy individuals were investigated. The propagating magnetic field of the action potential was recorded using magnetoneurography, reconstructed into a current, and analyzed. The currents were compared with the potentials recorded from multipolar surface electrodes.

RESULTS

Reconstructed currents could be clearly visualized. Axonal currents flowed forward or backward in the axon, arcing away from the depolarization zone, turning about the subcutaneous volume conductor, and returning to the depolarization zone. The zero-crossing latency of the axonal current was approximately the same as the peak of its volume current and the negative peak of the surface electrode potential. Volume current waveforms were proportional to the derivative of axonal ones.

CONCLUSIONS

Magnetoneurography allows the visualization and quantitative evaluation of action currents. The currents in axons and in volume conductors could be clearly discriminated with good quality. Their properties were consistent with previous neurophysiological findings.

SIGNIFICANCE

Magnetoneurography could be a novel tool for elucidating nerve physiology and pathophysiology.

Magnetoneurography to investigate the mechanisms underlying the P9 far-field potential

OBJECTIVE

The mechanism underlying the generation of P9 far-field somatosensory evoked potentials (SEPs) is unresolved. Accordingly, we used magnetoneurography to visualize the current distribution in the body at the P9 peak latency and elucidate the origin of P9 generation.

METHODS

We studied five healthy male volunteers without neurological abnormalities. We recorded far-field SEPs after median nerve stimulation at the wrist to identify the P9 peak latency. Using magnetoneurography, we recorded the evoked magnetic fields in the whole body under the same stimulus conditions as the SEP recording. We analyzed the reconstructed current distribution at the P9 peak latency.

RESULTS

At the P9 peak latency, we observed the reconstructed current distribution dividing the thorax into two parts, upper and lower. Anatomically, the depolarization site at the P9 peak latency was distal to the interclavicular space and at the level of the second intercostal space.

CONCLUSIONS

By visualizing the current distribution, we proved that P9 peak latency originates in the change in volume conductor size between the upper and lower thorax.

SIGNIFICANCE

We clarified that magnetoneurography analysis is affected by the current distribution due to the junction potential.

Detailed magnetoelectric analysis of a nerve impulse propagation along the brachial plexus

OBJECTIVE

To visualize impulse conduction along the brachial plexus through simultaneous electromagnetic measurements.

METHODS

Neuromagnetic fields following median nerve stimulation were recorded above the clavicle with a superconducting quantum interference device biomagnetometer system in 7 healthy volunteers. Compound nerve action potentials (CNAPs) were obtained from 12 locations. Pseudocolor maps of equivalent currents reconstructed from magnetic fields and isopotential contour maps were superimposed onto X-ray images. Surface potentials and current waveforms at virtual electrodes along the brachial plexus were compared.

RESULTS

In magnetic field analysis, the leading axonal current followed by a trailing backward current traveled rostrally along the brachial plexus. The spatial extent of the longitudinal intra-axonal currents corresponded to the extent of the positive-negative-positive potential field reflecting transmembrane volume currents. The peaks and troughs of the intra-axonal biphasic current waveforms coincided with the zero-crossings of triphasic CNAP waveforms. The amplitudes of CNAPs and current moments were linearly correlated.

CONCLUSIONS

Reconstructed neural activity in magnetic field analysis visualizes not only intra-axonal currents, but also transmembrane volume currents, which are in good agreement with the surface potential field.

SIGNIFICANCE

Magnetoneurography is a novel non-invasive functional imaging modality for the brachial plexus whose performance can surpass that of electric potential measurement.

Tutorial: A guide to techniques for analysing recordings from the peripheral nervous system

The nervous system, through a combination of conscious and automatic processes, enables the regulation of the body and its interactions with the environment. The peripheral nervous system is an excellent target for technologies that seek to modulate, restore or enhance these abilities as it carries sensory and motor information that most directly relates to a target organ or function. However, many applications require a combination of both an effective peripheral nerve interface and effective signal processing techniques to provide selective and stable recordings. While there are many reviews on the design of peripheral nerve interfaces, reviews of data analysis techniques and translational considerations are limited. Thus, this tutorial aims to support new and existing researchers in the understanding of the general guiding principles, and introduces a taxonomy for electrode configurations, techniques and translational models to consider.

Noninvasive measurement of sensory action currents in the cervical cord by magnetospinography

OBJECTIVE

To obtain magnetic recordings of electrical activities in the cervical cord and visualize sensory action currents of the dorsal column, intervertebral foramen, and dorsal horn.

METHODS

Neuromagnetic fields were measured at the neck surface upon median nerve stimulation at the wrist using a magnetospinography system with high-sensitivity superconducting quantum interference device sensors. Somatosensory evoked potentials (SEPs) were also recorded. Evoked electrical currents were reconstructed by recursive null-steering beamformer and superimposed on cervical X-ray images.

RESULTS

Estimated electrical currents perpendicular to the cervical cord ascended sequentially. Their peak latency at C5 and N11 peak latency of SEP were well-correlated in all 16 participants (r = 0.94, p < 0.0001). Trailing axonal currents in the intervertebral foramens were estimated in 10 participants. Estimated dorsal-ventral electrical currents were obtained within the spinal canal at C5. Current density peak latency significantly correlated with cervical N13-P13 peak latency of SEPs in 13 participants (r = 0.97, p < 0.0001).

CONCLUSIONS

Magnetospinography shows excellent spatial and temporal resolution after median nerve stimulation and can identify the spinal root entry level, calculate the dorsal column conduction velocity, and analyze segmental dorsal horn activity.

SIGNIFICANCE

This approach is useful for functional electrophysiological diagnosis of somatosensory pathways.

Brachial and lumbosacral plexopathies: A review

Diseases of the brachial and lumbosacral plexus are uncommon and complex. The diagnosis of plexopathies is often challenging for the clinician, both in terms of localizing a patient's symptoms to the plexus as well as determining the etiology. The non-specific clinical features and similar presentations to other root, nerve, and non-neurologic disorders emphasize the importance of a high clinical index of suspicion for a plexopathy and comprehensive clinical evaluation. Various diagnostic tests, including electrodiagnostic (EDX) studies, neuroimaging (including ultrasound, MRI, or PET), serologic studies, and genetic testing, may be used to confirm a plexopathy and assist in identifying the underlying etiology. EDX testing plays an important role in confirming a plexopathy defining the localization, pathophysiology, chronicity, severity, and prognosis. Given the complexity of the plexus anatomy, multiple common and uncommon NCS and an extensive needle examination is often required, and a comprehensive, individualized approach to each patient is necessary. Treatment of plexopathies often focuses on symptomatic management although, depending on the etiology, specific targeted treatments may improve outcome. This article reviews the clinical features, EDX approaches, and evaluation and treatment of brachial and lumbosacral plexopathies.