Non-invasive magnetoneurography for 3D-monitoring of human compound action current propagation in deep brachial plexus

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.

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.

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.

Diagnosis of incomplete conduction block of spinal cord from skin surface using spinal cord evoked magnetic fields

BACKGROUND

We previously reported the usefulness of neuromagnetic recordings for the diagnosis of disorders in peripheral nerves or the spinal cord. However, there have been no reports on incomplete conduction block of the spinal cord, which is clinically common in conditions such as cervical myelopathy. Here, we estimated the usefulness of measuring spinal cord evoked magnetic fields for evaluating incomplete conduction block.

METHODS

Incomplete conduction block models of the spinal cord of the rabbit were established using a Fogarty balloon catheter that was inserted into the epidural space of the cervical spine. Electrical stimuli were applied to the lower thoracic spinal cord with an epidural catheter electrode. Spinal cord evoked potentials were recorded using epidural electrodes. Spinal cord evoked magnetic fields were recorded over the skin surface of the neck using a biomagnetometer.

RESULTS

The decrease in the conduction velocity and amplitude at the compression site could be detected by spinal cord evoked potentials from the epidural space, confirming the spinal cord lesion. The waveforms of the magnetic fields showed a biphasic configuration. The distribution of magnetic fields showed a characteristic quadrupolar pattern propagating from caudal to cranial. After compression, the amplitude and the conduction velocity of the magnetic fields decreased, and the distribution of magnetic fields were attenuated and decelerated near the compression site especially in the trailing magnetic fields. Diagnosis of the incomplete conduction block was thus possible.

CONCLUSIONS

We report the first measurement of the spinal cord evoked magnetic field in the intact spinal cord from the skin surface and that it can be applied to incomplete conduction block of the injured spinal cord. The use of a biomagnetometer is promising as a less-invasive method for clinically evaluating spinal cord function.

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.

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.

Wide-range visualization of compound nerve action magnetic fields in the human median and ulnar nerves from the forearm to Erb's point

We successfully visualized the compound nerve action magnetic fields (CAFs) in the human median and ulnar nerves from the forearm to Erb's point. To observe the CAFs, we used a superconducting quantum interference device gradiometer system that was developed for human peripheral nerves. The CAFs were visualized as a quadrupole pattern consisting of leading and trailing dipoles. The CAFs propagated along the anatomical pathway and extended longitudinally in the proximal segment. The most reasonable explanation is that the peak separation in the trailing dipole appeared when the leading dipole reached the proximal segment after stimulation of the median nerve at the wrist. A temporal dispersion of the CAFs was suggested to be visualized.

[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.

Visualization of signal propagation from sciatic nerve to spinal cord in canine

The neuroelectric activity that ascends the sciatic nerve and moves to the spinal cord was visualized by measuring the magnetic compound action fields (CAFs) with a superconducting quantum interference device gradiometer. The sciatic nerve of a dog was stimulated electrically, and propagating evoked CAFs were measured non-invasively. Isomagnetic field maps were made on the basis of this data, and the signal propagation was visualized. The evoked magnetic fields presented a quadrupole consisting of two elements: depolarization and repolarization. Measuring the magnetic CAFs of the sciatic nerve on the body surface enabled us to visualize the non-invasively the signal movement continuously from the sciatic nerve to the spinal cord.

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.

Magnetoneurography of evoked compound action currents in human cervical nerve roots

OBJECTIVE

A measurement protocol for magnetoneurography (MNG) is established which allows the non-invasive localization and tracing of evoked compound action currents propagating along cervical nerve roots in man.

METHODS

Inside a magnetically shielded room either both median or both ulnar nerves of healthy subjects were conventionally electrostimulated in alternation. Evoked magnetic responses were recorded using a multichannel SQUID-detector with a planar measuring area centered over the neck. Simultaneously, electric surface potentials were recorded using cervical bipolar electrode montages.

RESULTS

Upon median (ulnar) nerve stimulation somatosensory evoked magnetic fields up to 20 fT (10 fT) amplitude were detected propagating over the cervical transforaminal root entry zone, with corresponding electrical surface potentials of 1.5 microV (0.5 microV). Furthermore, the signal-to-noise ratio of the spatiotemporal magnetic field mappings in median nerve stimulation experiments allowed dipolar source reconstructions and tracing of the propagation of the compound action currents along nerve root fibers.

CONCLUSION

Magnetoneurography allows tracing of the propagation of evoked compound action currents along cervical roots in healthy subjects with millisecond temporal and high spatial resolution. Thus, MNG offers a sensitivity appropriate to serve as a clinical diagnostic tool for localizing focal neuropathies of cervical nerve roots.