Round Magnetic Coil Research Articles

The effect of magnetic stimulation on unloaded soleus muscle of rat: changes in myosin heavy chain mRNA isoforms

This study assessed the potential application and the effectiveness of functional magnetic stimulation (FMS) for preventing skeletal muscle atrophy in adult rats. FMS using magnetic stimulator was performed to rat soleus muscle by placing a round magnetic coil on the back of 3rd-5th lumbar vertebral level at 20 Hz frequency for 60 min/day up to 10 days. A reverse transcriptase-polymerase chain reaction was applied to evaluate relative amounts of mRNAs specific to four myosin heavy chain (MHC) isoforms [MHCIbeta, MHCIIa, MHCIIb, and MHCIId(x)] in rat soleus muscle during contractile activity by magnetic stimulation. Ten-day unloading by hindlimb suspension induced a drastic decrease of MHCIbeta and MHCIIa mRNA expressions, while MHCIIb and MHCIId(x) mRNA was not decreased. The magnetic stimulation resuscitated the down-regulation of the mRNA levels of MHCIbeta and MHCIIa. These results suggest that magnetic stimulation on acute atrophied muscles is useful for preventing the muscle atrophy.

Functional magnetic ventilation in dogs

Lin VW, Hsiao I, Deng X, Lee Y-S, Sasse S. Functional magnetic ventilation in dogs. Arch Phys Med Rehabil 2004;85:1493–8. Objective To investigate the efficacy of the magnetic stimulation of inspiratory muscles as an alternative to mechanical ventilation and functional electric stimulation. Design A prospective before-after trial. Setting Functional magnetic stimulation laboratory in a Veterans Administration health care system. Animals Six male mongrel dogs, each weighing between 25 and 35kg. Interventions Commercially available magnetic stimulators with a round magnetic coil were used. The center of the magnetic coil was placed posteriorly over the C5–7 vertebrae of the spinal cord transected dogs. Magnetic stimulation parameters were set at 80% intensity, 20Hz, and a 1.2-second on and 3.8-second off pulse train. Main outcome measures The major outcomes were changes in tidal volume (V t), tracheal pressure (P tr), and arterial partial pressure of carbon dioxide (Pa co 2) and oxygen sustained by magnetic stimulation over time. Results The average V t and P tr produced during functional magnetic ventilation (FMV) were .47±.07L and −4.7±.51cmH 2O, respectively. Blood gas data showed that Pa co 2 increased from a baseline of 33 to 75mmHg, whereas pH decreased from 7.33 to 6.99 at the end of the 1-hour FMV period. Conclusions FMV was achieved for 2 hours in dogs with C2 spinal cord transection. Additional refinements in magnetic stimulation are needed to improve ventilation in animals.

Functional magnetic stimulation facilitates gastric emptying

Lin VW-H, Kim KH, Hsiao I, Brown W. Functional magnetic stimulation facilitates gastric emptying. Arch Phys Med Rehabil 2002;83:806-10. Objective: To evaluate the effect of functional magnetic stimulation (FMS) on gastric emptying in able-bodied and spinal cord injury (SCI) subjects. Design: A prospective, nonrandomized clinical experiment. Setting: SCI and disorder center in a Veterans Affairs medical facility. Participants: Five healthy, able-bodied subjects and 4 subjects with SCI. Intervention: A commercially available magnetic stimulator was used; a round magnetic coil was placed along the T9 spinous process. The intensity of the magnetic stimulation was 60%, with a frequency of 20Hz, and a burst length of 2 seconds for the gastric emptying protocol. Man Outcome Measures: Rate of gastric emptying and time required to reach gastric emptying half-time (GEt1/2) with and without FMS. Data fit into linear regression curve. Results: Accelerated gastric emptying was achieved in both able-bodied and SCI subjects. The mean ± standard error of mean of the GEt1/2 at baseline and with FMS was 36±2.9 minutes and 33±3.1 minutes, respectively, for able-bodied subjects, and 84±11.1 minutes and 59±12.7 minutes, respectively, for SCI subjects. Conclusion: Gastric emptying was enhanced by FMS in able-bodied subjects and was greatly enhanced in SCI subjects. FMS can be a useful noninvasive therapeutic tool to facilitate gastric emptying in humans. © 2002 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation

Functional magnetic stimulation for conditioning of expiratory muscles in patients with spinal cord injury

Lin VW, Hsiao IN, Zhu E, Perkash I. Functional magnetic stimulation for conditioning of expiratory muscles in patients with spinal cord injury. Arch Phys Med Rehabil 2001;82:162-6. Objective: To evaluate the effectiveness of functional magnetic stimulation (FMS) in conditioning expiratory muscles patients with spinal cord injury (SCI). Design: A prospective before-after trial. Setting: The Functional Magnetic Stimulation Laboratory of the SCI Health Care Group, VA Long Beach Health Care System, and the Spinal Cord Injury Services, Department of Veterans Affairs, Palo Alto Health Care System. Participants: Eight men with tetraplegia. Intervention: Expiratory muscle training was achieved by placing a magnetic stimulator with a round magnetic coil along subjects' lower thoracic spine. Main Outcome Measures: Measures taken were the maximal expired pressure at total lung capacity (MEP-TLC) and at functional residual capacity (MEP-FRC), expiratory reserve volume (ERV), and the forced expiratory flow rate at TLC (FEF-TLC) and at FRC (FEF-FRC) by subjects' voluntary maximal efforts. Results: After 4 weeks of conditioning, the mean ± standard error of the mean values were: MEP-TLC, 55.3 ± 8.6cmH2O; MEP-FRC, 29.6 ± 5.6cmH2O; ERV,.57 ±.08L; FEF-TLC, 4.3 ± 0.5L/s; and FEF-FRC, 1.9 ± 0.2L/s. These values correspond to, respectively, 129%, 137%, 162%, 109%, and 127% of pre-FMS conditioning values. When FMS was discontinued for 2 weeks, the MEP-TLC returned to its pre-FMS training value. Conclusion: A 4-week protocol of FMS of the expiratory muscles improves voluntary expiratory muscle strength significantly, indicating that FMS can be a noninvasive therapeutic technology in respiratory muscle training for persons with tetraplegia.

Functional magnetic stimulation of the colon in persons with spinal cord injury

Lin VW, Nino-Murcia M, Frost F, Wolfe V, Perkash I. Functional magnetic stimulation of the colon in persons with spinal cord injury. Arch Phys Med Rehabil 2001;82:167-73. Objective: To evaluate the usefulness of functional magnetic stimulation (FMS) as a noninvasive method to stimulate the colon in individuals with spinal cord injury (SCI). Design: A prospective before-after trial consisting of 2 protocols. Setting: FMS laboratories of 2 SCI centers. Participants: Two able-bodied men and 13 men with SCI levels ranging from C3 to L1. Protocol 1 consisted of 9 subjects, 2 of whom were excluded from the analysis. Protocol 2 consisted of 4 subjects. Intervention: Commercially available magnetic stimulators with round magnetic coils (MCs) were used. Protocol 1 measured the effects of FMS on rectal pressure by placing the MC on the transabdominal and lumbosacral regions. Protocol 2 consisted of a 5-week stimulation period to investigate the effects of FMS on total and segmental colonic transit times (CTTs). Main Outcome Measure: An increase in rectal pressure and a decrease in CTT by magnetic stimulation. Results: Data were averaged and the standard error of the mean was calculated. Statistically significant changes in rectal pressure and CTT were also measured. Rectal pressures increased from 26.7 ± 7.44cmH2O to 48.0 ± 9.91cmH2O, p =.0037, with lumbosacral stimulation, and from 30.0 ± 6.35cmH2O to 42.7 ± 7.95cmH2O, p =.0015, with transabdominal stimulation. With FMS, the mean CTT decreased from 105.2 to 89.4 hours, p =.02. Conclusion: FMS is able to stimulate the colon and reduce CTT. FMS is a noninvasive, technological advancement for managing neurogenic bowel in patients with SCI.

Transcranial magnetic stimulation of the central and peripheral motor pathways to the lingual muscles in cats

Objective: We have assessed a technique to stimulate the intracranial hypoglossal nerve with ease and reproducibility by using magnetic coils (MCs) and to detect a reliable site of excitation in animal experiments in order to establish a method to evaluate the motor pathway to lingual muscles.Methods: We recorded the motor responses from the lingual muscles of 5 adult cats under general anesthesia by magnetic and electrical stimulation of the intracranial hypoglossal nerves. Figure of 8 and round MCs were used to investigate the optimal position and direction to evoke the motor responses.Results: The round MC was useful for cortical stimulation. The figure of 8 coil, positioned in the back of the head of the examined side, parallel to the cervical spine, was essential for stimulation of the intracranial hypoglossal nerve. Analysis of the latencies, and the observation that the motor responses disappeared after transection of the nerves at the exit of the hypoglossal canal, demonstrated that the site of the excitation is at the exit of the hypoglossal canal.Conclusion: Magnetic stimulation using a figure of 8 coil can elicit tongue motor responses with ease and reliable reproducibility, stimulating the hypoglossal nerve at the exit of the hypoglossal canal.

Functional magnetic stimulation of the respiratory muscles in dogs.

This study assessed the ability of functional magnetic stimulation (FMS) to activate the respiratory muscles in dogs. With the animal supine, FMS of the phrenic nerves using a high-speed magnetic stimulator was performed by placing a round magnetic coil (MC) at the carotid triangle. Following hyperventilation-induced apnea, changes in volume (deltaV) and airway pressure (deltaP) against an occluded airway were determined. FMS of the phrenic nerves produced substantial inspired function (deltaV = 373 +/- 20.5 mL and deltaP = -20 +/- 2.0 cm H2O). After bilateral phrenectomies, maximal inspired deltaV (219 +/- 12.2 mL) and deltaP (-10 +/- 1.0 cm H2O) were produced when the MC was placed near the C6-C7 spinous processes, while maximal expired deltaV (-199 +/- 22.5 mL) and deltaP (11 +/- 2.3 cm H2O) were produced following stimulation near the T9-T10 spinous processes. We conclude: (1) FMS of either the phrenic or upper intercostal nerves results in inspired volume production; (2) FMS of the lower intercostal nerves generates expired volume production; and (3) FMS of the respiratory muscles may be a useful noninvasive tool for artificial ventilation and assisted cough in patients with spinal cord injuries or other neurological disorders.

Functional magnetic stimulation for restoring cough in patients with tetraplegia

Objective: To evaluate the usefulness of functional magnetic stimulation (FMS) as a noninvasive method for assisting cough in patients with tetraplegia. Design: A prospective before-after trial. Setting: The functional magnetic stimulation laboratory of a spinal cord injury (SCI) service. Participants: Thirteen male SCI patients, with injury levels between C4 and C7. Intervention: A commercially available magnetic stimulator with a round magnetic coil (MC) was used. Expiratory muscle activation was achieved by placing the MC along the lower thoracic spine. Main Outcome Measure: The planned major outcome measures were the maximal expired pressure (MEP), expiratory reserve volume (ERV), and forced expiratory flow rate (FEF) by FMS compared with voluntary maximal efforts. Another outcome was the optimal MC placement and stimulation intensity that would result in highest expired pressure. Results: The mean (±SEM) MEP, ERV, and FEF generated by FMS were 66.40 ± 6.69cmH 20, .77 ± .14L, and 5.28 ± .42L/sec, respectively. They were 118%, 169%, and 110% of voluntary maximum efforts. MC placement at the T10 to T11 spinous process and stimulation intensity at 80% produced the highest MEP and FEE Conclusion: FMS of the expiratory muscles produced significant expired pressures, volumes, and flow rates when compared with voluntary maximum efforts; therefore, FMS can be used as an effective method to restore cough in tetraplegic patients.

Modelling magnetic coil excitation of human cerebral cortex with a peripheral nerve immersed in a brain-shaped volume conductor: the significance of fiber bending in excitation

To help elucidate some basic principles of magnetic coil (MC) excitation of cerebral cortex, a model system was devised in which mammalian phrenic nerve, or amphibian sciatic nerve with its branches was suspended in appropriate Ringer's solution in a human brain-shaped volume conductor, an inverted plastic skull. The nerve was recorded monophasically out of the volume conductor. The site of nerve excitation by the MC was identified by finding where along the nerve a bipolar electrical stimulus yielded a similar action potential latency. MC excitation of hand-related corticospinal (CT) neurons was modelled by giving the distal end of nerve attached to the lateral skull an initial radial (perpendicular) trajectory, with subsequent bends towards the base and posterior part of the skull; this nerve was optimally excited by a laterally placed figure 8 or round MC when the induced electric field led to outward membrane current at the initial bend. By contrast, nerve given a trajectory modelling CT neurons related to the foot was optimally excited when the coil windings were across the midline, but again when membrane current flowed outward at the first bend. Corticocortical fibers were modelled by placing the nerve in the anteroposterior axis lateral to the midline; with the round MC vertex-tangentially oriented, optimal excitation occurred at the bend nearest the interaural line, i.e., near the peak electric field. The findings emphasize the importance of orientation and direction of current in the MC and fiber bends in determining nerve excitation. The findings in the peripheral nerve-skull model help explain (1) why lateral and vertex-tangentially oriented MCs preferentially excite arm-related CT neurons directly and indirectly (through corticocortical fibers), respectively, and (2) why the MC orientations for optimally exciting directly arm and leg-related CT neurons differ.

Measurement of the electric field induced into inhomogeneous volume conductors by magnetic coils: application to human spinal neurogeometry

We measured the elctric fields in duced by round and figure “8” magnetic coils (MCs) in homogeneous and inhomogeneous volume conductors. In homogeneous media, the round MC held tangential (i.e., flat) to the volume conductor induced an annular electric field. When the round MC was held on-edge (i.e. orthogonal) to the volume conductor, the induced electric field consisted of two loops mainly parallel to the surface of the volume conductor and which approximated each directly under the contracting edge of the MC. The tangentially oriented figure “8” MC similarly induced in two electric field loops which approximated one another maximally under the region of the junction in its long axis. In a complex inhomogeneous volume conductor, such as a segment of human cervical-thoracic vertebral spine located eccentrically within a large cylindrical tank and submerged in isotonic saline, the direction of electric fields within the spinal canal and across the vertebral neuroforamina was similar to that observed in the homogeneous volume conductor. However, in and near a single neuroforamen, the electric field and especially its first spatial derivative were markedly elevated compared to that recorded within the long central axis of the vertebral canal. Motor unit and compound muscle action potentials elicited in limb muscles by MC stimulation of human cervical spine confirmed prediction derived from the physical model. The predictions included: (1) absence of spinal cord stimulation compared to relative ease of nerve root stimulation by current that is most likely concentrated at the neuroformina. When stimlulating current is directed towards theperiphery, the most likely low threshold site of stimulation is inferred to be just distal to the neuroforamina. It is emphasized that with supramaximal stimulation, more distal sites of excitation may occur; (2) invariant latency shifts at threshold intensities when moving the MC along the rostrocaudal axis of the cervical vertebral column; (3) significant effect ( on motor unit activation thresholds) of the direction of induced current flow across the neuroforamina; (4) reduced stimulation when the targeted nerve roots are close to the null point of the electric field, i.e., between locations of high electric field intensity, of opposite polarity; and (5) relatively focal nerve root stimulation by the junction of a transversely oriented figure “8” MC, i.e., parallel to the nerve roots.

PARAESTHESIAS ARE ELICITED BY SINGLE PULSE, MAGNETIC COIL STIMULATION OF MOTOR CORTEX IN SUSCEPTIBLE HUMANS

A minority of normal humans experience paraesthesias (usually tingling) projected to the contralateral hand in response to individual transcranial magnetic coil (MC) pulses. The cortical source of the paraesthesias was sought by comparing their incidence with that of muscle responses to focal MC stimulation with either a figure 8 MC or with edge stimulation of a tilted round MC in 4 susceptible subjects. In all 4, paraesthesias were best felt with MC stimulation either at, or anterior to sites yielding movement, implying an initial source in precentral gyrus (and possible premotor cortex), rather than parietal cortex. In the two subjects exhibiting the strongest paraesthesias, the threshold for the paraesthesias was less than that for movement in the relaxed arm. The optimal site of the paraesthesias within the hand was usually in the digits, but differed among subjects. Motor responses and paraesthesias following a given stimulus occurred at different sites in the hand, implying that excitation of differing sets of motor cortical neurons subserved sensory and motor responses. In only one subject were the paraesthesias sufficiently reproducible to warrant interacting electrical digital and transcranial MC pulses. The data suggested that central processing of the response to the MC pulse is slowed by an antecedent digital stimulus, but the delay for perception of each type of stimulus does not greatly differ. The central sense of movement (Amassian et al., 1989a) elicited by MC stimulation of motor cortex is compared with the paraesthesias. Both are attributed to brief, high frequency discharge by motor cortical neurons accessing the perceptual system more readily than after excitation of post-central gyrus, which requires prolonged repetitive stimulation (Libet et al., 1964). Given also the normal pattern of muscle responses in the 4 subjects, their paraesthesias are best explained by a heightened sensitivity of the perceptual system to the motor cortical response to MC stimulation.

A comparison of corticospinal activation by magnetic coil and electrical stimulation of monkey motor cortex

The effects of different orientations of a Cadwell round magnetic coil (MC) were compared with each other and with surface electrical stimulation of motor cortex in monkeys anesthetized with pentobarbital or urethane. Recordings were made from within the lateral corticospinal tract, either from axonal populations or with a microelectrode from individual axons. A lateral-sagittally orientated MC directly excited corticospinal neurons at lower stimulus intensity than was required for indirect, i.e., transsynaptic excitation via inputs to corticospinal neurons. By contrast, in 2 out of 3 macaques tested, a vertex-tangential orientation could excite corticospinal neurons indirectly at lower intensities than were required for direct excitation; at higher intensities, direct excitation also occurred. The site of direct corticospinal excitation by a lateral-sagitally oriented MC was inferred by comparing the response variability and latency to MC and surface electrical stimuli. Cathodal stimuli elicited more variable corticospinal population responses and later individual axonal responses than were obtained with anodal stimuli. The variability in response is attributed to interaction between nearby, on-going synaptic bombardment and the stimulus, implying that surface cathodal stimuli directly activate corticospinal neurons at the spike trigger zone (presumably the initial segment). By contrast, the consistency and reduced latency of the corticospinal responses to surface anodal stimuli are attributed to the direct excitation of corticospinal fibers within the white matter. When the stimulus intensity is clearly above threshold, surface anodal and cathodal stimuli can activate corticospinal neurons both directly and indirectly. Direct corticospinal excitation by the MC can resemble the effects of either surface anodal or surface cathodal stimuli. We conclude that the MC can activate corticospinal neurons at the spike trigger zone or their fibers deeper in white matter. The findings in the monkey are used to interpret the effects of different MC orientations in the human.

Spatial distribution of the electric field induced in volume by round and figure ‘8’ magnetic coils: relevance to activation of sensory nerve fibers

The electric fields induced in finite homogeneous volume conductors by a round and a figure ‘8’ magnetic coil (MC) were measured and related to MC stimulation of the median nerve. The volume conductors, filled with isotonic saline, consisted of a large rectangular trough (‘unrestricted’) and a smaller trough, whose dimensions approximated human forearm (‘restricted’). Various MC orientations were applied to the volume conductor. Bipolar recordings were obtained with a coaxial electrode, which measured the voltage gradient between the exposed edge of the cable shield and the central wire at its tip, 1 cm distant (a linear probe). The probe was moved in 3 dimensions, allowing computer reconstruction of the electric field as a function of the 3 spatial axes. When the probe was parallel to the plane of the round MC and tangential to the direction of current in its windings, the induced electric field was maximal; it tended towards zero when the probe was over the center of the MC, or when the probe, remaining parallel to the plane of the MC, was radial (i.e., perpendicular) to the direction of the current in the windings. For a variety of MC orientations, the electric field was consistently increased when the probe was adjacent and parallel to the edge of the trough, indicating the important effect of boundaries. The electric field was greatly increased focally when the round MC was applied orthogonally to the volume conductor, or when the figure ‘8’ MC was applied tangentially (i.e., flat) to the volume conductor. With the figure ‘8’ MC, a sharp central peak parallel to the long axis was bounded on each side by smaller (less than half amplitude) peaks. The findings from physical modeling led to correct predictions as to the most effective orientations of round and figure ‘8’ MCs for eliciting sensory nerve action potentials (SNAPs) from the median nerve.

Brain Stimulation Revisited

The results of the following selected studies using magnetic coil (MC) stimulation are presented: (1) evidence for focality of MC stimulation, (2) MC stimulation of frontal areas related to speech, (3) transcallosal responses evoked by MC stimulation, and (4) suppression of visual perception with MC stimulation over occipital cortex. The authors served as subjects, and in most studies a standard Cadwell stimulator and round MC were used. Using a more vertical, rather than tangential, MC orientation and threshold stimulation, nearly isolated movements of individual digits were elicited implying focal cortical excitation. MC stimulation of frontal areas of either hemisphere elicited electromyography in contralateral laryngeal muscles. The shortest latency responses that were often accompanied by arm movement were thought to be elicited from intermediate areas of precentral gyrus and longer latency responses, from near Broca's area, extreme lateral precentral gyrus, and the supplementary motor area. MC stimulation over the occipital cortex resulted in suppression of visual perception of letters briefly flashed on a screen. The topography of suppression implicated the geniculocalcarine system as the site of MC effect. Focal MC stimulation of posterior frontal cerebral cortex elicited a transcallosal response from contralateral homologous cortex with a latency similar to that obtained with focal anodic stimulation but with considerably less excitation of cranial muscles.

Focal stimulation of human cerebral cortex with the magnetic coil: a comparison with electrical stimulation

Percutaneous stimulation of human motor cortex electrically (focal anode) and with magnetic coils (MCs) of various designs is compared. The theoretical prediction was confirmed that positioning the standard round MC laterally and orientating it more towards the vertical induces an electric field appropriate for directly exciting corticospinal neurons (cf., the conventional tangential orientation at the vertex). Thus, during voluntary contraction, minimal latency compound motor action potentials (CMAPs) in contralateral arm were elicited both by focal anodic and appropriately oriented MC stimulation. Conduction time from motor cortex to motoneuron was estimated by subtracting peripheral conduction time and monosynaptc delay at the motoneuron from the overall CMAP latency, yielding an estimated corticospinal conduction velocity as high as 66 m/sec. Discontinuous latency variations observed in population CMAPs or individual motor units approximated mono- or polysynaptic cortical synaptic delays and, therefore, are attributed to the intervals between direct and early, or late indirect corticospinal discharges. A TV computer system was used to track movements of individual digits and the hand following MC stimulation. An appropriately oriented MC readily elicited movements predominantly of a single digit, implying focal activation of motor cortex. A double square and a small pointed MC proved especially convenient for eliciting reproducibly single digit movements. Stronger stimulation revealed a topographical gradient in the responses of the different digits. Responses to a given MC stimulus a little above threshold were variable in amplitude, which could not be explained by the relationship of stimulus to phase of the cardiac or respiratory cycle. Overall, our findings indicate the importance of appropriately orientating a standard round MC and using a specially designed MC to obtain the various types of motor response to stimulation of cerebral cortex.

Focal stimulation of human peripheral nerve with the magnetic coil: A comparison with electrical stimulation

The hypothenar compound motor action potential (CMAP) response to ulnar nerve stimulation at the elbow was used as a test system in the human to compare excitations by a round magnetic coil (MC), 92 mm in outer diameter, and by electrical bipolar or tripolar stimulation. Optimal focality of excitation was obtained with the MC at 90° to the extended arm and its plane parallel to the arm, i.e., an orthogonal-longitudinal orientation. Tangential orientation of the MC on the arm, i.e., laying it flat on the arm, powerfully excited additional structures. As expected from classical axonology, orthogonal-transverse orientations were the least effective. With orthogonal-longitudinal orientation and submaximal stimulation, the spread of excitation lateral to the median nerve at the wrist was of the order of 10–15 mm, the thickness of the MC being 12 mm. With the same orientation, the site of origin of the distally propagating impulse was estimated by comparing CMAP latencies to bipolar electrical (with cathode distal) and MC stimulation. Tripolar stimulation (with cathode intermediate) had no advantage over bipolar stimulation. The impulse originated 13–22 mm from the midpoint of the contacting edge. Rotating the MC through 180° and thus reversing the field polarity did not significantly change the CMAP latency, indicating that the effective cathode and anode lay within a few mm of each other. Stimulating with a tilted MC resulted in a maximum CMAP when the orthogonal-longitudinal orientation failed to do so. A simple volume conductor model yielded a potential gradient of the right order of magnitude (35 mV/mm) when the MC stimulator output was 25% of maximum, i.e., a little above threshold for exciting ulnar motor axons.