Increasing the oscillation frequency of strong magnetic fields above 101 kHz significantly raises peripheral nerve excitation thresholds.

PURPOSE: A time-varying magnetic field can cause unpleasant peripheral nerve stimulation (PNS) when the maximum excursion of the magnetic field (ΔB) is above a frequency-dependent threshold level [P. Mansfield and P. R. Harvey, Magn. Reson. Med. 29, 746-758 (1993)]. Clinical and research magnetic resonance imaging (MRI) gradient systems have been designed to avoid such bioeffects by adhering to regulations and guidelines established on the basis of clinical trials. Those trials, generally employing sinusoidal waveforms, tested human responses to magnetic fields at frequencies between 0.5 and 10 kHz [W. Irnich and F. Schmitt, Magn. Reson. Med. 33, 619-623 (1995), T. F. Budinger et al., J. Comput. Assist. Tomogr. 15, 909-914 (1991), and D. J. Schaefer et al., J. Magn. Reson. Imaging 12, 20-29 (2000)]. PNS thresholds for frequencies higher than 10 kHz had been extrapolated, using physiological models [J. P. Reilly et al., IEEE Trans. Biomed. Eng. BME-32(12), 1001-1011 (1985)]. The present study provides experimental data on human PNS thresholds to oscillating magnetic field stimulation from 2 to 183 kHz. Sinusoidal waveforms were employed for several reasons: (1) to facilitate comparison with earlier reports that used sine waves, (2) because prior designers of fast gradient hardware for generalized waveforms (e.g., including trapezoidal pulses) have employed quarter-sine-wave resonant circuits to reduce the rise- and fall-times of pulse waveforms, and (3) because sinusoids are often used in fast pulse sequences (e.g., spiral scans) [S. Nowak, U.S. patent 5,245,287 (14 September 1993) and K. F. King and D. J. Schaefer, J. Magn. Reson. Imaging 12, 164-170 (2000)]. METHODS: An IRB-approved prospective clinical trial was performed, involving 26 adults, in which one wrist was exposed to decaying sinusoidal magnetic field pulses at frequencies from 2 to 183 kHz and amplitudes up to 0.4 T. Sham exposures (i.e., with no magnetic fields) were applied to all subjects. RESULTS: For 0.4 T pulses at 2, 25, 59, 101, and 183 kHz, stimulation was reported by 22 (84.6%), 24 (92.3%), 15 (57.7%), 2 (7.7%), and 1 (3.8%) subjects, respectively. CONCLUSIONS: The probability of PNS due to brief biphasic time-varying sinusoidal magnetic fields with magnetic excursions up to 0.4 T is shown to decrease significantly at and above 101 kHz. This phenomenon may have particular uses in dynamic scenarios (e.g., cardiac imaging) and in studying processes with short decay times (e.g., electron paramagnetic resonance imaging, bone and solids imaging). The study suggests the possibility of new designs for human and preclinical MRI systems that may be useful in clinical practice and scientific research.

Electromuscular incapacitation results from stimulation of spinal reflexes.

Electronic stun devices (ESD) often used in law enforcement, military action or self defense can induce total body uncoordinated muscular activity, also known as electromuscular incapacitation (EMI). During EMI the subject is unable to perform purposeful or coordinated movements. The mechanism of EMI induction has not been reported, but has been generally thought to be direct muscle and nerve excitation from the fields generated by ESDs. To determine the neuromuscular mechanisms linking ESD to induction of EMI, we investigated EMI responses using an anesthetized pig model. We found that EMI responses to ESD application can best be simulated by simultaneous stimulation of motor and sensory peripheral nerves. We also found that application of local anesthetics limited the response of ESD to local muscle stimulation and abolished the total body EMI response. Stimulation of the pure sensory peripheral nerves or nerves that are primarily motor nerves induced muscle responses that are consistent with well defined spinal reflexes. These findings suggest that the mechanism of ESD-induced EMI is mediated by excitation of multiple simultaneous spinal reflexes. Although direct motor-neuron stimulation in the region of ESD contact may significantly add to motor reactions from ESD stimulation, multiple spinal reflexes appear to be a major, and probably the dominant mechanism in observed motor response.

Mechanisms and Effects of Transcranial Direct Current Stimulation.

The US Air Force Office of Scientific Research convened a meeting of researchers in the fields of neuroscience, psychology, engineering, and medicine to discuss most pressing issues facing ongoing research in the field of transcranial direct current stimulation (tDCS) and related techniques. In this study, we present opinions prepared by participants of the meeting, focusing on the most promising areas of research, immediate and future goals for the field, and the potential for hormesis theory to inform tDCS research. Scientific, medical, and ethical considerations support the ongoing testing of tDCS in healthy and clinical populations, provided best protocols are used to maximize safety. Notwithstanding the need for ongoing research, promising applications include enhancing vigilance/attention in healthy volunteers, which can accelerate training and support learning. Commonly, tDCS is used as an adjunct to training/rehabilitation tasks with the goal of leftward shift in the learning/treatment effect curves. Although trials are encouraging, elucidating the basic mechanisms of tDCS will accelerate validation and adoption. To this end, biomarkers (eg, clinical neuroimaging and findings from animal models) can support hypotheses linking neurobiological mechanisms and behavioral effects. Dosage can be optimized using computational models of current flow and understanding dose-response. Both biomarkers and dosimetry should guide individualized interventions with the goal of reducing variability. Insights from other applied energy domains, including ionizing radiation, transcranial magnetic stimulation, and low-level laser (light) therapy, can be prudently leveraged.

Survey of numerical electrostimulation models.

This paper evaluates results of a survey of electrostimulation models of myelinated nerve. Participants were asked to determine thresholds of excitation for 18 cases involving different characteristics of the neuron, the stimulation waveform, and the electrode arrangement. Responses were received from 7 investigators using 10 models. Excitation thresholds differed significantly among these models. For example, with a 2 ms monophasic stimulus pulse and an electrode/fiber distance of 1 cm, thresholds from the least to greatest value differed by a factor of 8.3; with a 5 µs pulse, thresholds differed by the factor 3.8. Significant differences in reported simulations point to the need for experimental validation. Additional efforts are needed to develop computational models for unmyelinated C-fibers, A-delta fibers, CNS neurons, and CNS Synapses.

Low-frequency electrical dosimetry: research agenda of the IEEE International Committee on Electromagnetic Safety.

This article treats unsettled issues in the use of numerical models of electrical dosimetry as applied to international limits on human exposure to low-frequency (typically < 100 kHz) electromagnetic fields and contact current. The perspective in this publication is that of Subcommittee 6 of IEEE-ICES (International Committee on Electromagnetic Safety) Technical Committee 95. The paper discusses 25 issues needing attention, fitting into three general categories: induction models; electrostimulation models; and human exposure limits. Of these, 9 were voted as 'high priority' by members of Subcommittee 6. The list is presented as a research agenda for refinements in numerical modeling with applications to human exposure limits. It is likely that such issues are also important in medical and electrical product safety design applications.

An analysis of differences in the low-frequency electric and magnetic field exposure standards of ices and ICNIRP.

Technical factors accounting for differences in low-frequency (0-3 kHz) standards of IEEE C95.6 and ICNIRP are identified. These factors are related to thresholds of adverse reactions, safety factors, probability factors, thresholds for different tissue types, magnetic induction models, and induced current and spark discharges with E-field exposure. This paper summarizes technical factors accounting for differences between the two standards and recommends resolution of those factors.

Spatial relationships in electrostimulation: application to electromagnetic field standards.

Spatial relationships in electrostimulation are examined with a myelinated nerve model. Excitation thresholds are determined for: 1) terminated axon within a locally constant electric field; 2) bent axon within a locally constant field; and 3) a field of finite extent over the affected axon. For application to electromagnetic field standards, the minimum excitation threshold of the in situ electric field applies to a straight, terminated axon; a field measurement averaging distance of 5 mm is recommended.

Neuroelectric mechanisms applied to low frequency electric and magnetic field exposure guidelines--part I: sinusoidal waveforms.

Electric and magnetic field exposure guidelines are developed from established mechanisms of bioelectric interaction. Such mechanisms involve phenomena of electrostimulation-the functional influence of applied electrical forces on nerve and muscle, and, at quasi-static frequencies, on magneto-dynamic mechanisms. The paper develops criteria of human reactions based on theoretical models with parametric values derived from experimental observations. These basic restrictions on electrostimulation effects are referenced to the induced in situ electric fields. Basic limitations are differentiated for induction in the heart, peripheral nerves, the extremities, and the central nervous system. The paper recommends maximum permissible exposure limits which account for (a) adverse reaction criteria, (b) statistical distribution of reaction thresholds, and (c) acceptability factors. From the basic limitations the paper further develops reference levels which apply to environmental electric or magnetic fields.

Neuroelectric mechanisms applied to low frequency electric and magnetic field exposure guidelines--part II: non sinusoidal waveforms.

Standards for human exposure to electromagnetic fields typically express maximum permissible exposure limits as a function of frequency. Often, these limits have been derived from experiments or theoretical models involving sinusoidal waveforms. In many practical situations, however, the relevant waveforms of interest may not be sinusoidal, such as with waveforms having harmonic distortion, or with pulsed waveforms. This paper evaluates methods for applying sinusoidal exposure standards to non-sinusoidal waveforms in the frequency regime below a few MHz where electrostimulation is the dominant mechanism. Waveforms treated include those of a pulsed or mixed frequency variety. We evaluate acceptance criteria for mixed frequency exposure using summation formulae cited by IEEE C95.1, ICNIRP, and NRPB. This is carried out using a Fourier synthesis of various waveshapes. Also evaluated is an acceptance criterion based on the peak of the exposure waveform. Excitation thresholds are evaluated using a myelinated nerve model that accounts for the nonlinear electrodynamics of the neural membrane. It is shown that a method based on the peak and phase duration of the in situ field waveform provides a typically conservative test for compliance with non sinusoidal waveforms. An alternate method, based on amplitude summation of the Fourier components of the applied waveforms, can also provide a meaningful test, albeit a more conservative one.

The relationship between anatomically correct electric and magnetic field dosimetry and publishe delectric and magnetic field exposure limits.

Electric and magnetic field exposure limits published by International Commission for Non-Ionizing Radiation Protection and Institute of Electrical and Electronics Engineers are aimed at protection against adverse electrostimulation, which may occur by direct coupling to excitable tissue and, in the case of electric fields, through indirect means associated with surface charge effects (e.g. hair vibration, skin sensations), spark discharge and contact current. For direct coupling, the basic restriction (BR) specifies the not-to-be-exceeded induced electric field. The key results of anatomically based electric and magnetic field dosimetry studies and the relevant characteristics of excitable tissue were first identified. This permitted us to assess the electric and magnetic field exposure levels that induce dose in tissue equal to the basic restrictions, and the relationships of those exposure levels to the limits now in effect. We identify scenarios in which direct coupling of electric fields to peripheral nerve could be a determining factor for electric field limits.