Magnetic Stimulation Thresholds Research Articles

Does Exposure to Diagnostic Ultrasound Modulate Human Nerve Responses to Magnetic Stimulation?

Ultrasound (US) at diagnostic frequency and power is known to alter nerve potentials; however, the precise mechanism of action is unknown. We investigated whether US alters resting nerve potential to lower the threshold for magnetic nerve stimulation. Seventeen healthy subjects were recruited. For each subject, a 1.5 MHz US imaging probe was placed onto the elbow with the beam directed at the ulnar nerve. The probe was coupled to the skin using standard acoustic coupling gel as would be done for a routine clinical US scan. Ulnar nerve stimulation was performed simultaneously with magnetic stimulation (MS). Successful magnetic stimulation of the ulnar nerve was confirmed with nerve potentials measured by electromyography. There was no significant change in electromyography signal when MS was performed during US exposure. US at the diagnostic frequency and power tested does not alter nerve thresholds with MS. Testing at other frequencies is required, however, before US is negated as a technique to modify MS thresholds.

Experimental thresholds of magnetically induced currents via a figure-of-eight coil up to 25 kHz

Magnetic particle imaging, uses magnetic fields of frequencies in the kilo-hertz range. Little research has been carried out upon effects on excitable tissue caused by time-varying magnetic fields in that frequency range. To learn about magnetic stimulation thresholds, a system that is capable of generating a very focused magnetic field strong enough to induce a muscular reaction was built. Stimulation thresholds at 7.38, 12.04, 16.5, and 25.27 kHz have been recorded in 31 trial participants. It has been proved that magnetic stimulation is possible up to even 25 kHz, and respective thresholds have been determined. This article presents the trial setup, procedure, and evaluation of field quantities.

The discrepancy between human peripheral nerve chronaxie times as measured using magnetic and electric field stimuli: the relevance to MRI gradient coil safety

Peripheral nerve stimulation (PNS) resulting from electric fields induced from the rapidly changing magnetic fields of gradient coils is a concern in MRI. Nerves exposed to either electric fields or changing magnetic fields would be expected to display consistent threshold characteristics, motivating the direct application of electric field exposure criteria from the literature to guide the development of gradient magnetic field exposure criteria for MRI. The consistency of electric and magnetic field exposures was tested by comparing chronaxie times for electric and magnetic PNS curves for 22 healthy human subjects. Electric and magnetic stimulation thresholds were measured for exposure of the forearm using both surface electrodes and a figure-eight magnetic coil, respectively. The average chronaxie times for the electric and magnetic field conditions were 109 ± 11 µs and 651 ± 53 µs (±SE), respectively. We do not propose that these results call into question the basic mechanism, namely that rapidly switched gradient magnetic fields induce electric fields in human tissues, resulting in PNS. However, this result does motivate us to suggest that special care must be taken when using electric field exposure data from the literature to set gradient coil PNS safety standards in MRI.

Comparison of rectangular and damped sinusoidal dB/dt waveforms in magnetic stimulation

In magnetic (eddy-current) stimulation, sensory and motor nerve are stimulated by a time-varying magnetic field (dB/dt) produced in an appropriate coil. In this study, magnetic stimulation thresholds for a solenoidal coil encircling the forearm were determined in 14 healthy human volunteers, 11 males and 3 females. Damped sinusoidal dB/dt pulses with durations (onset to zero crossing) ranging from 120 /spl mu/s to 2500 /spl mu/s were produced by discharging a capacitor bank into the coil. Rectangular dB/dt pulses with durations ranging from 50 /spl mu/s to 2500 /spl mu/s were generated by a Gradient Ramp Accelerator Module (GRAM) in series with a linear amplifier. The threshold data were fitted to an equivalent circuit underlying the hyperbolic form of the strength-duration curve. Average electric field rheobases for the sinusoidal and rectangular pulses were 5.87 V/m and 5.36 V/m, respectively; chronaxies for these waveforms were 393 /spl mu/s and 395 /spl mu/s, respectively. This study shows that stimulation thresholds obtained with damped sinusoidal dB/dt pulses can be used to accurately predict the thresholds that would be obtained for rectangular waveforms.

Responses of central neurons to electrical and magnetic stimuli of the ampullae of lorenzini in the Black Sea skate

1. Evoked and neuronal responses in the area acustico-lateralis to electrical and magnetic stimulation of ampullae of Lorenzini were studied in the anaesthetised Black Sea skate,Trygon pastinaca. 2. Evoked responses with marked negative phase corresponded to the excitation of neurons, those with marked positive phase to the inhibition of neurons (Fig. 2). 3. 80% of all studied neurons were spontaneously active (8.2±1.0 imp/sec). The background activity of some neurons was modulated in the rhythm of the respiratory movements. 4. Thresholds for electrical stimulation were in the range of 1×10−10−6× 10−9 A/mm2. 5. The character of neuronal responses depended on the stimulus polarity (Figs. 2, 3). 6. Relations between the parameters of stimulation and the latencies of “on” -and “off”-responses, the spike frequency, the duration of “contrast interval” as well as the thresholds of reactions were analysed (Figs. 4, 5, 7). 7. Four main patterns of responses to electrical stimulation were observed: phasic-tonic, tonic, sustained and bursting (Fig. 6). 8. Neurons responded to a changing magnetic field penetrating the fish. The character of the responses depended on the direction of the magnetic field, and the intensity depended on the rate of magnetic field change. A constant magnetic field failed to influence single unit activity (Figs. 8–11). 9. The threshold for magnetic stimulation was 2 Gs/sec. Thus perception of the magnetic field of the earth by the fish is possible.