Magnetoneurography of evoked compound action currents in human cervical nerve roots

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.

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.

Visualization of electrical activity in the cervical spinal cord and nerve roots after ulnar nerve stimulation using magnetospinography

OBJECTIVE

To establish a method for magnetospinography (MSG) measurement after ulnar nerve stimulation and to clarify its characteristics.

METHODS

Using a 132-channel magnetoneurography system with a superconducting quantum interference device, cervical MSG measurements were obtained for 10 healthy volunteers after stimulation of the ulnar nerve at the elbow and the wrist, and neural current distribution was calculated and superimposed on the cervical X-ray images.

RESULTS

Neuromagnetic signals were obtained in all participants after applying the stimulus artifact removal algorithm. The measured magnetic field intensity after elbow stimulation was about twice that after wrist stimulation. Calculated neural currents flowed into the intervertebral foramina at C6/7 to T1/2 and propagated cranially along the spinal canal. The conduction velocity from the peak latency of inward currents at C5-C7 was 73.4 ± 19.6 m/s.

CONCLUSIONS

We successfully obtained MSG measurements after ulnar nerve stimulation. The neural currents flowed into the spinal canal from more caudal segments after ulnar nerve stimulation compared with median nerve stimulation, and these MSG measurements were effective in examining the spinal tracts at C5/6/7.

SIGNIFICANCE

This is the first report on the use of MSG to visualize electrical activity in the cervical spinal cord and nerve root after ulnar nerve stimulation.

Novel functional imaging technique for the brachial plexus based on magnetoneurography

OBJECTIVE

To visualize neural activity in the brachial plexus using magnetoneurography (MNG).

METHODS

Using a 124- or 132-channel biomagnetometer system with a superconducting quantum interference device, neuromagnetic fields above the clavicle and neck region were recorded in response to electrical stimulation of the median and ulnar nerves in five asymptomatic volunteers (four men and one woman; age, 27-45 years old). Equivalent currents were computationally reconstructed from neuromagnetic fields and visualized as pseudocolor maps. Reconstructed currents at the depolarization site and compound nerve action potentials (CNAPs) at Erb's point were compared.

RESULTS

Neuromagnetic fields were recorded in all subjects. The reconstructed equivalent currents propagated into the vertebral foramina, and the main inflow levels differed between the median nerve (C5/C6-C7/T1 vertebral foramen) and the ulnar nerve (C7/T1-T1/T2). The inward current peaks at the depolarization site and CNAPs showed high linear correlation.

CONCLUSIONS

MNG visualizes neural activity in the brachial plexus and can differentiate the conduction pathways after median and ulnar nerve stimulations. In addition, it can visualize not only the leading and trailing components of intra-axonal currents, but also inward currents at the depolarization site.

SIGNIFICANCE

MNG is a novel and promising functional imaging modality for the brachial plexus.

Magnetospinography visualizes electrophysiological activity in the cervical spinal cord

Diagnosis of nervous system disease is greatly aided by functional assessments and imaging techniques that localize neural activity abnormalities. Electrophysiological methods are helpful but often insufficient to locate neural lesions precisely. One proposed noninvasive alternative is magnetoneurography (MNG); we have developed MNG of the spinal cord (magnetospinography, MSG). Using a 120-channel superconducting quantum interference device biomagnetometer system in a magnetically shielded room, cervical spinal cord evoked magnetic fields (SCEFs) were recorded after stimulation of the lower thoracic cord in healthy subjects and a patient with cervical spondylotic myelopathy and after median nerve stimulation in healthy subjects. Electrophysiological activities in the spinal cord were reconstructed from SCEFs and visualized by a spatial filter, a recursive null-steering beamformer. Here, we show for the first time that MSG with high spatial and temporal resolution can be used to map electrophysiological activities in the cervical spinal cord and spinal nerve.

Conductive neuromagnetic fields in the lumbar spinal canal

OBJECTIVE

To measure neuromagnetic evoked fields in the lumbar spinal canal.

METHODS

Using a newly developed superconducting quantum interference device (SQUID) fluxmeter, neuromagnetic fields of 5 healthy male volunteers were measured at the surface of the lower back after stimulation of the tibial nerves at the ankles. For validation, we inserted a catheter-type electrode percutaneously in the lumbar epidural space in 2 of the subjects and measured cauda equina action potentials after tibial nerve stimulation.

RESULTS

Neuromagnetic fields propagating from the intervertebral foramina into the spinal canal were measured, and the latencies of the magnetic fields corresponded largely with those of the cauda equina action potentials.

CONCLUSIONS

We successfully measured ascending neuromagnetic fields originating at the nerve root and the cauda equina with high spatial resolution. Future studies will determine whether neuromagnetic field measurement of the lumbar spine can be a useful diagnostic method for the identification of the disordered site in spinal nerves.

SIGNIFICANCE

We successfully measured neuromagnetic fields in the lumbar spinal canal, which have previously been difficult to verify. Future studies will determine whether neuromagnetic field measurement of the lumbar spine can be a useful diagnostic method for identifying disorders of spinal nerves.

Functional imaging of spinal cord electrical activity from its evoked magnetic field

This paper investigates dynamic source imaging of the spinal cord electrophysiological activity from its evoked magnetic field by applying the spatial filter version of standardized low-resolution brain electromagnetic tomography (sLORETA). Our computer simulation shows that the sLORETA-based spatial filter can reconstruct the four current sources typically associated with the elicitation of the spinal cord evoked magnetic field (SCEF). The results from animal experiments show that significant changes in the latency and intensity of the reconstructed volume current arise near the location of the artificial incomplete conduction block. The results from the human SCEF show that the SCEF source imaging can visualize the dynamics of the volume currents and other nerve electrical activity propagating along the human spinal cord. These experimental results demonstrate the potential of SCEF source imaging as a future clinical tool for diagnosing cervical spinal cord disorders.

Evaluation of segmental spinal cord evoked magnetic fields after sciatic nerve stimulation

OBJECTIVE

We have previously reported that the measurement of spinal cord evoked magnetic fields (SCEFs) could be a helpful method for evaluating spinal cord function or detecting conduction blocks in the spinal cord. However, there have been no reports about segmental-SCEFs as a complex of axonal and synaptic activities in the spinal cord. The purpose of this study is to record and evaluate segmental-SCEFs.

METHODS

The segmental-SCEFs were measured over the lumbar dural tubes of adult rabbits using our SQUID system following sciatic nerve stimulation; spinal cord evoked potentials (SCEPs) were also measured to compare the results.

RESULTS

SCEPs showed conductive sharp waves following gentle waves, suggesting action potentials and synaptic potentials, respectively. The isomagnetic field maps of SCEFs showed a quadrupolar pattern propagating from the caudal to the cranial region within a short latency time, and after the conductive magnetic fields passed, stationary dipolar fields appeared and were sustained at some vertebral levels.

CONCLUSIONS

The quadrupolar magnetic fields were estimated to be generated from conducting action potentials, and the dipolar fields were thought to be caused by synaptic activities.

SIGNIFICANCE

Through the measurement of segmental-SCEFs, the conductive neural and synaptic activities in the spinal cord can be visualized and distinguished. This is the first report to record and visualize the sequence of events ranging from the axonal activities of peripheral nerves and the spinal tract to the synaptic activities in the spinal cord.

Magnetoneurography: theory and application to peripheral nerve disorders

Magnetoneurography (MNG) is a non-invasive method to trace and visualize three-dimensionally the propagation path of compound action currents (CAC) along peripheral nerves. The basic physical and physiological principle is the mapping of extremely weak magnetic fields generated by the intraaxonal longitudinal ion flows of evoked nerval CAC using SQUID sensors (Superconducting Quantum Interference Devices). During recent years, MNG protocols have been established which allow for a non-invasive spatiotemporal tracing of impulse propagation along peripheral nerves in humans and in particular along proximal nerve segments in a clinical setting. Thereby, the three-dimensional path, the local nerve conduction velocity, the length and strength of the CAC de- and repolarization phase have been reconstructed. First recordings in patients demonstrated that the method is sensitive enough to detect and to localize nerve conduction anomalities along nerve roots, as, e.g. caused by lumbosacral disc herniation. This review on MNG will focus on those studies which provide data from humans and thereby reveal perspectives for its future clinical applications.

Visualization of incomplete conduction block by neuromagnetic recording

OBJECTIVE

We previously reported on evoked compound action magnetic fields (CAFs) in isolated sciatic nerves with complete conduction block. In this study, we examined evoked CAFs of the nerve with incomplete conduction block, which is clinically common.

METHODS

Rabbits' isolated nerves were electrically stimulated in a chamber containing Ringer's solution. Compound action potentials (CAPs) and CAFs were recorded before and after the incomplete conduction block induced by a vascular clip. The positions of the lesion were estimated by dipole localization.

RESULTS

Before the nerve clipping, magnetic contour maps showed CAFs with a characteristic quadrupolar pattern. After the clipping, CAFs attenuated in the amplitude and decelerated through the lesion. Estimated position of the lesion was 0.12+/-3.23 mm (mean+/-SD, n=10) assuming that the real position of the clip was 0 mm.

CONCLUSIONS

The time-course of changes of CAFs in the incomplete conduction block was visualized by magnetic contour maps, and the lesions were closely localized focusing on the velocity change of the leading dipole.

SIGNIFICANCE

The neural conduction with incomplete conduction block was visualized and the lesion was closely localized by neuromagnetic recordings.

[Magnetoneurographic registration of evoked summation action fields over lumbar vertebrae following transcutaneous tibial nerve stimulation]

Goal of this study was the development of a protocol for the registration of evoked magnetic fields over the lumbar spine using off-the-shelf equipment. Three subjects in a sitting position with their torso bent slightly forward were stimulated at the tibial nerve with a commercially available stimulator. Neuromagnetic fields were registered over a circular, 800 cm2 area of the lumbosacral spine using a 61-channel 4D-Neuroimaging biomagnetometer. After appropriate signal processing, dipolar magnetic fields with a field strength 5-17 fT peak-to-peak amplitude were detected in three out of four registrations. Location and orientation of these fields concurred with the expected evoked compound action currents along the course of the nerve fibers.

Imaging of neural conduction block by neuromagnetic recording

OBJECTIVE

For the clinical application of neuromagnetic recordings in neural conduction block, the patterns of magnetic fields in the region should be clarified. Using an experimental in vitro model, the spatiotemporal course of the neuromagnetic fields at the site of complete conduction block was examined. Additionally, the magnetic compound action fields (CAFs) and electric compound action potentials (CAPs) were compared and correlated.

METHODS

In a chamber containing Ringer's solution, 10 isolated sciatic nerves of rabbits were electrically stimulated. Both evoked CAPs and CAFs were measured before and after the ligation of the nerve. The sequential positions of the current dipoles and the location of the conduction block were estimated by the least-squares search.

RESULTS

The magnetic contour maps of the CAFs showed a characteristic quadrupolar pattern propagating along the nerve. The peak of the leading magnetic field ceased and disappeared at the position of the nerve ligation, while the trailing magnetic field became attenuated before reaching that position. The positions of the conduction blocks were localized by magnetic recordings within a difference of 2mm.

CONCLUSIONS

The neuromagnetic recordings could visualize the change of the magnetic fields at the site of the complete conduction block and closely localize that position.

SIGNIFICANCE

The neural conduction block was visualized and localized by neuromagnetic recordings.

Tracing of proximal lumbosacral nerve conduction--a comparison of simultaneous magneto- and electroneurography

OBJECTIVE

The reconstruction of nerve impulse conduction along proximal lumbosacral plexus and nerve roots is compared using simultaneous magneto- and electroneurography.

METHODS

In 3 healthy subjects the left tibial nerve was electrostimulated at the ankle. Evoked magnetic fields and electric surface potentials were measured simultaneously over the lumbosacral spine using a multichannel SQUID-detector with a planar measuring area and 25 surface electrodes covering a comparable area centered around L4. Based on either magnetic field or electric potential maps the depolarization front of the evoked compound action currents (CAC) was spatio-temporally reconstructed using a simple equivalent current dipole model in a half-space volume conductor.

RESULTS

The mean signal-to-noise ratio in the magnetic (electric) recordings was around 4 (8). Yet, the localization quality for the propagating CAC was lower for electric than magnetic recordings. The local nerve conduction velocity was around 47 m/s (calculated from magnetic data), but fluctuated unphysiologically for electric data.

CONCLUSION

In comparison to electroneurography, an anatomically reasonable localization of evoked compound action currents propagating in lumbosacral roots can be obtained by magnetoneurography.