Magnetophosphenes: a quantitative analysis of thresholds

Low-frequency and transient magnetic fields of moderate flux densities are known to generate visual phenomena, so-called magnetophosphenes. In the present study, time-variable very low frequency (10–50 Hz) electromagnetic fields of moderate flux density (0–40 mT) were used to induce magnetophosphenes. The threshold values for these phosphenes were determined as a function of the frequency of the magnetic field both in normal subjects and colour defective ones. Maximum sensitivity occurred at a frequency of approximately 20–30 Hz, and with broad-spectrum light the threshold flux density was 10–12 mT. The threshola values were found to be dependent upon the intensity and the spectral distribution of the background light. Sensitivity decreased during dark adaptation. In certain respects deutans differed from subjects with normal colour vision. Possible mechanisms for generation of magnetophosphenes are discussed. The present magnetic threshold curves show a close resemblance to corresponding curves obtained by electric stimulation at various frequencies provided the electric thresholds are divided by the a.c. frequency. These problems are under current investigation in our laboratory. This is in full agreement with the assumption that the fluctuating magnetic field affects retinal neurons by inducing currents which polarise synaptic terminals.     

Magneto- and electrophosphenes: A comparative study

The purpose of this study was to compare the threshold values for magnetophosphenes and electrophosphenes under identical experimental conditions. Such comparisons between the phosphene types would increase our knowledge of the mechanism of the interaction between magnetic fields and electric current, respectively, and excitable tissue. The phosphenes were generated in the frequency range 10–45 Hz at moderate magnetic flux densities [up to 40 mT (400 G)] and electric currents up to 0.3 mA, respectively. The first part of the study was devoted to the problem of how electrode location and consequent current directions influence the threshold values of electrophosphenes. In the second part a comparison was made of the threshold values for electrophosphenes and magnetophosphenes under identical experimental conditions apart from the stimulation method. With electric-current stimulation in different directions no great differences were obtained with regard to the mean value for the threshold values within the frequency range 10–30 Hz. However, from 30 Hz upwards a significant difference developed between the threshold values for some of the curves. When generating electrophosphenes and magnetophosphenes we found significant differences in the threshold values between approximately 25 and 45 Hz. Both types of phosphenes had a concurring sensitivity maximum at 20 Hz. The deviations between the curves may be due, among other factors, to the generation of different current paths in electrical and magnetic stimulation, respectively.     

Children and adults exposed to low-frequency magnetic fields at the ICNIRP reference levels: theoretical assessment of the induced electric fields

To avoid potentially adverse health effects, the International Commission on Non-Ionizing Radiation Protection (ICNIRP) has defined reference levels for time varying magnetic fields. Restrictions on the electric fields induced in the human body are provided based on biological response data for peripheral nerve stimulation and the induction of phosphenes. Numerical modeling is commonly used to assess the induced electric fields for various exposure configurations. The objective of this study was to assess the variations of the electric fields induced in children and adults and to compare the exposure at reference levels with the basic restrictions as function of anatomy. We used the scalar potential finite element method to calculate the induced electric fields in six children and two adults when exposed to uniform magnetic fields polarized in three orthogonal directions. We found that the induced electric fields are within the ICNIRP basic restrictions in nearly all cases. In PNS tissues, we found electric fields up to 95% (upper uncertainty limit due to discretization errors, k = 2) of the ICNIRP basic restrictions for exposures at the general public reference levels. For occupational reference levels, we found an over-exposure of maximum 79% (k = 2) in PNS tissues. We further found that the ICNIRP recommendations on spatial averaging in 2?×?2?×?2 mm3 contiguous tissue volumes and removal of peak values by the 99th percentile cause the results to depend strongly on the grid discretization step (i.e. an uncertainty of more than 50% at 2 mm) and the number of distinguished tissues in the anatomical models. The computational results obtained by various research institutes should be robust for different discretization settings and various anatomical models. Therefore, we recommend considering alternative routines for small anatomical structures such as non-contiguous averaging without taking the 99th percentile in future guidelines leading to consistent suppression of peak values amongst different simulation settings and anatomical models. The peak electric fields depend on the local tissue distribution in the various anatomical models, and we could not find a correlation with the size of the anatomy. Therefore, we recommend extending the evaluation using a sufficient set of anatomies including other than standing postures to assess the worst-case exposure setting and correspondence to the basic restrictions.     

Fetal exposure to low frequency electric and magnetic fields

To investigate the interaction of low frequency electric and magnetic fields with pregnant women and in particular with the fetus, an anatomical voxel model of an 89 kg woman at week 30 of pregnancy was developed. Intracorporal electric current density distributions due to exposure to homogeneous 50 Hz electric and magnetic fields were calculated and results were compared with basic restrictions recommended by ICNIRP guidelines. It could be shown that the basic restriction is met within the central nervous system (CNS) of the mother at exposure to reference level of either electric or magnetic fields. However, within the fetus the basic restriction is considerably exceeded. Revision of reference levels might be necessary.     

Quandaries in the application of the ICNIRP low frequency basic restriction on current density

This paper identifies uncertainties and problems in the practical application of the ICNIRP low frequency basic restriction on current density. This quantity should be averaged over a cross-section of 1 cm(2) perpendicular to the current direction. The rationale and the sensitivity of the current density average are investigated. There are difficulties in finding a square centimetre of spinal cord over which to average. The consequences of including neighbouring tissues in the averages are investigated in the male and female voxel models NORMAN and NAOMI for applied uniform electric and magnetic fields at 50 Hz. Also the case of the non-uniform magnetic field from a horizontal current carrying conductor adjacent to the back of the body is investigated. The maximum and 99th percentile current density values are compared with the 1 cm(2) average in the derivation of external field reference levels. For more information on this article, see medicalphysicsweb.org.     

Pre- and post-natal exposure of children to EMF generated by domestic induction cookers

Induction cookers are a type of cooking appliance that uses an intermediate-frequency magnetic field to heat the cooking vessel. The magnetic flux density produced by an induction cooker during operation was measured according to the EN 62233 standard, and the measured values were below the limits set in the standard. The measurements were used to validate a numerical model consisting of three vertically displaced coaxial current loops at 35 kHz. The numerical model was then used to compute the electric field (E) and induced current (J) in 26 and 30 weeks pregnant women and 6 and 11 year old children. Both E and J were found to be below the basic restrictions of the 2010 low-frequency and 1998 ICNRIP guidelines. The maximum computed E fields in the whole body were 0.11 and 0.66 V m(-1) in the 26 and 30 weeks pregnant women and 0.28 and 2.28 V m(-1) in the 6 and 11 year old children (ICNIRP basic restriction 4.25 V m(-1)). The maximum computed J fields in the whole body were 46 and 42 mA m(-2) in the 26 and 30 weeks pregnant women and 27 and 16 mA m(-2) in the 6 and 11 year old children (ICNIRP basic restriction 70 mA m(-2)).     

Possible overexposure of pregnant women to emissions from a walk through metal detector

This paper presents a systematic procedure to evaluate the induced current densities and electric fields due to walk-through metal detector (WTMD) exposure. This procedure is then used to assess the exposure of nine pregnant women models exposed to one WTMD model. First, we measured the magnetic field generated by the WTMD, then we extracted the equivalent current source to represent the WTMD emissions and finally we calculated the induced current densities and electric fields using the impedance method. The WTMD emissions and the induced fields in the pregnant women and fetus models are then compared to the ICNIRP Guidelines and the IEEE C95.6 exposure safety standard. The results prove the consistency between maximum permissible exposure (MPE) levels and basic restrictions for the ICNIRP Guidelines and IEEE C95.6. We also found that this particular WTMD complies with the ICNIRP basic restrictions for month 1-5 models, but leads to both fetus and pregnant women overexposure for month 6-9 models. The IEEE C95.6 restrictions (MPEs and basic restrictions) are not exceeded. The fetus overexposure of this particular WTMD calls for carefully conducted safety evaluations of security systems before they are deployed.     

Current densities in a pregnant woman model induced by simultaneous ELF electric and magnetic field exposure

The pregnant woman model SILVY was studied to ascertain to what extent the electric current densities induced by 50 Hz homogeneous electric and magnetic fields increase in the case of simultaneous exposure. By vectorial addition of the electric current densities, it could be shown that under worst case conditions the basic restrictions recommended by ICNIRP (International Commission on Non-Ionizing Radiation Protection) guidelines are exceeded within the central nervous system (CNS) of the mother, whereas in sole field exposure they are not. However, within the foetus the induced current densities do not comply with basic restrictions, either from single reference-level electric fields or from simultaneous exposure to electric and magnetic fields. Basic limits were considerably exceeded.     

射频美容电极在人脑中电磁场和比吸收率分布研究

研究在频率40.68 MHz下,使用多极射频电极头对人进行射频美容治疗时在人头部所产生磁场强度、电场强度和比吸收率(Specific Absorption Ratio,SAR)的分布,并将得出的结果与国际非电离辐射防护委员会(ICNIRP)所指定的安全限值进行对比。结果表明,脑组织中磁场强度最大值为0.06 A/m,约为ICNIRP限值的82.19%;电场强度最大值为5.5 V/m,约为INCIRP安全限值的19.64%;SAR最大约为0.03 W/kg,远小于ICNIRP标准的2 W/kg。所有结果均处于ICNIRP安全限值以内,说明射频美容电极所产生的电磁暴露不会对人体造成威胁。     

Current densities in a 2 mm resolution anatomically realistic model of the body induced by low frequency electric fields

Current density distributions in a fine resolution (2 mm) anatomically realistic voxel model of the human body have been calculated for uniform, low frequency vertically aligned electric fields for a body grounded and isolated from 50 Hz to 10 MHz. The voxel phantom NORMAN is used which has a height of 1.76 m and a mass of 73 kg. There are 8.3 million voxels in the body differentiated into 37 tissue types. Both finite-difference potential and time-domain methods were used. Results are presented for the current density averaged over 1 cm2 in muscle, heart, brain and retina. Electric field values required to reach the NRPB and ICNIRP basic restrictions on current density are derived and compared with the external field guidelines from these standards.     

Influence on frog retina of alternating magnetic fields with special reference to ganglion cell activity

The effects of extremely low frequency (e.l.f.) electromagnetic fields on basal systems were studied. Frog retinas were exposed to magnetic fields with frequencies and flux densities that have been shown to induce magnetophosphenes in volunteers. The electrical activity in the retina induced by the field was recorded from the ganglion cell layer with a microelectrode technique. A threshold value was obtained at approximately 20 mT, and a sensitivity maximum at 20 Hz. A significant prolongation (4 ms) of the latency from light stimulus to response in the ganglion cell layer was obtained, if the preparation was simultaneously and continuously exposed to a magnetic field. A study of the reaction of the ganglion cells to light and to magnetic fields showed that those cells which were on-cells during light stimulation became off-cells during magnetic stimulation andvice versa. The magnetic field response occurred within approximately 5 ms. while the light stimulus response occurred only after an average of approximately 85 ms. Addition of Na-aspartate or CoCl₂ extinguished simultaneously the response both to light and to magnetic field stimuli.     

Occupational exposure assessment of magnetic fields generated by induction heating equipment-the role of spatial averaging

Induction heating equipment is a source of strong and nonhomogeneous magnetic fields, which can exceed occupational reference levels. We investigated a case of an induction tempering tunnel furnace. Measurements of the emitted magnetic flux density (B) were performed during its operation and used to validate a numerical model of the furnace. This model was used to compute the values of B and the induced in situ electric field (E) for 15 different body positions relative to the source. For each body position, the computed B values were used to determine their maximum and average values, using six spatial averaging schemes (9-285 averaging points) and two averaging algorithms (arithmetic mean and quadratic mean). Maximum and average B values were compared to the ICNIRP reference level, and E values to the ICNIRP basic restriction. Our results show that in nonhomogeneous fields, the maximum B is an overly conservative predictor of overexposure, as it yields many false positives. The average B yielded fewer false positives, but as the number of averaging points increased, false negatives emerged. The most reliable averaging schemes were obtained for averaging over the torso with quadratic averaging, with no false negatives even for the maximum number of averaging points investigated.     

Calculation of induced current densities for humans by magnetic fields from electronic article surveillance devices.

This paper illustrates the use of the impedance method to calculate the electric fields and current densities induced in millimetre resolution anatomic models of the human body, namely an adult and 10- and 5-year-old children, for exposure to nonuniform magnetic fields typical of two assumed but representative electronic article surveillance (EAS) devices at 1 and 30 kHz, respectively. The devices assumed for the calculations are a solenoid type magnetic deactivator used at store checkouts and a pass-by panel-type EAS system consisting of two overlapping rectangular current-carrying coils used at entry and exit from a store. The impedance method code is modified to obtain induced current densities averaged over a cross section of 1 cm2 perpendicular to the direction of induced currents. This is done to compare the peak current densities with the limits or the basic restrictions given in the ICNIRP safety guidelines. Because of the stronger magnetic fields at lower heights for both the assumed devices, the peak 1 cm2 area-averaged current densities for the CNS tissues such as the brain and the spinal cord are increasingly larger for smaller models and are the highest for the model of the 5-year-old child. For both the EAS devices, the maximum 1 cm2 area-averaged current densities for the brain of the model of the adult are lower than the ICNIRP safety guideline, but may approach or exceed the ICNIRP basic restrictions for models of 10- and 5-year-old children if sufficiently strong magnetic fields are used.     

Electrical phosphenes: on the influence of conductivity inhomogeneities and small-scale structures of the orbita on the current density threshold of excitation

Electrical and magnetic phosphenes, perceptions of light as a result of non-adequate stimulation of the eye by electrical current or magnetic induction, respectively, are one of the cornerstones to justify limit values for extreme low-frequency fields specified by statutory regulations. However, the mechanism and place of action, as well as the excitation threshold, remain unknown until now. We suggest that the origin of phosphene excitation is the synaptic layer of the eye. The current density threshold value for electrical phosphene excitation was numerically quantified for this area on the basis of a detailed geometrical model in original submillimetre resolution and specifically measured conductivities in the LF range. The threshold values found were 1.8 Am⁻² at 60 Hz and 0.3 Am⁻² at 25 Hz. These values are comparable with values of other excitable tissues. It has been shown that the current density threshold for phosphene generation depends on small-scale structures not taken into account by previous models.     

An electric field induced in the retina and brain at threshold magnetic flux density causing magnetophosphenes

For magnetic field exposures at extremely low frequencies, the electrostimulatory response with the lowest threshold is the magnetophosphene, a response that corresponds to an adult exposed to a 20 Hz magnetic field of nominally 8.14 mT. In the IEEE standard C95.6 (2002), the corresponding in situ field in the retinal locus of an adult-sized ellipsoidal was calculated to be 53 mV m(-1). However, the associated dose in the retina and brain at a high level of resolution in anatomically correct human models is incompletely characterized. Furthermore, the dose maxima in tissue computed with voxel human models are prone to staircasing errors, particularly for the low-frequency dosimetry. In the analyses presented in this paper, analytical and quasi-static finite-difference time-domain (FDTD) solutions were first compared for a three-layer sphere exposed to a uniform 50 Hz magnetic field. Staircasing errors in the FDTD results were observed at the tissue interface, and were greatest at the skin-air boundary. The 99th percentile value was within 3% of the analytic maximum, depending on model resolution, and thus may be considered a close approximation of the analytic maximum. For the adult anatomical model, TARO, exposed to a uniform magnetic field, the differences in the 99th percentile value of in situ electric fields for 2 mm and 1 mm voxel models were at most several per cent. For various human models exposed at the magnetophosphene threshold at three orthogonal field orientations, the in situ electric field in the brain was between 10% and 70% greater than the analytical IEEE threshold of 53 mV m(-1), and in the retina was lower by roughly 50% for two horizontal orientations (anterior-posterior and lateral), and greater by about 15% for a vertically oriented field. Considering a reduction factor or safety factors of several folds applied to electrostimulatory thresholds, the 99th percentile dose to a tissue calculated with voxel human models may be used as an estimate of the tissue's maximum dose.     

The site of saccadic suppression

During rapid eye movements, or saccades, stable vision is maintained by active reduction of visual sensitivity. The site of this saccadic suppression remains uncertain. Here we show that phosphenes--small illusory visual perceptions--induced by transcranial magnetic stimulation (TMS) to the human occipital cortex are immune to saccadic suppression, whereas phosphenes induced by retinal stimulation are not, thus providing direct physiological evidence that saccadic suppression occurs between the retina and the occipital visual cortex.     

A model of the electrical volume conductor in the region of the eye in the ELF range.

Electrical and magnetic phosphenes are irritations of the eye caused by electric currents or magnetic fields. These are well known effects initially investigated in the early 1900s. Available estimations of the current densities in the eye, based on the assumption of a homogeneous volume conductor, show low thresholds. These outdated thresholds are still an important cornerstone when justifying today's limit values for extremely low-frequency (ELF) fields specified by statutory regulations. In vitro measurements of the complex conductivity of cattle eye are carried out for the ELF range (5-2000 Hz) separated for the different tissues of the eyeball. They do not show peculiarities at 20 Hz which is the threshold minimum for the phosphene generation. The reported conductivity data of the eye region show variations of two orders of magnitude regarding the electrical conductivity of the individual tissue layers. Starting with these new data, a model of the orbita is introduced describing the eye and its periphery as an electrically inhomogeneous volume conductor. This model contains small-scale structures which are expected to behave as good electrical conductors yielding regions of higher field values within the eye. Therefore, earlier models assuming a homogeneous volume conductor can be regarded as oversimplistic.     

Effect of time-varying magnetic fields on the action potential in lobster giant axon

The possible influence of time-varying magnetic fields on action potential in the lobster giant axon was studied. The axon membrane was excited by galvanic stimulation and the action potential was recorded intracellularly with microelectrodes. During the propagation of the action potential along the axon, alternating or pulsed magnetic fields were applied across the middle part of the axon to study whether or not magnetic fields have any effect on parameters such as conduction velocity and refractory period of the nerve fibre and amplitude, duration and shape of the action potentials. No effect on these parameters was observed under different flux densities and frequencies of the magnetic fields. When simulating the conductive properties of tissue surrounding the nerve with the aid of an external conducting loop with a load resistance, action potentials were generated which made it possible to study the threshold value of the induced eddy current for nerve excitation. Based on the results of the experiment, the influence of magnetic flux density, frequency, conductivity, induced EMF and induced eddy current density is discussed, and a method is proposed for estimating the threshold values of magnetic flux density for nerve excitation.     

Electromagnetic fields in the human body due to switched transverse gradient coils in MRI

Magnetic resonance imaging scans impose large gradient magnetic fields on the patient. Modern imaging techniques require this magnetic field to be switched rapidly for good resolution. However, it is believed that this can also lead to the unwanted side effect of peripheral nerve stimulation, which proves to be a limiting factor to the advancement of MRI technology. This paper establishes an analytical model for the fields produced within an MRI scanner by transverse gradient coils of known current density. Expressions are obtained for the magnetic induction vector and the electric field vector, as well as for the surface charge and current densities that are induced on the patient's body. The expressions obtained are general enough to allow the study of any combination of gradient coils whose behaviour can be approximated by Fourier series. For a realistic example coil current density and switching function, it is found that spikes of surface charge density are induced on the patient's body as the gradient field is switched, as well as loops of surface current density that mimic the coil current density. For a 10 mT m(-1) gradient field with a rise time of 100 micros, the magnitude of the radial electric field at the body is found to be 10.3 V m(-1). It is also found that there is a finite limit to radial electric field strength as rise time approaches zero.     

Magnetic field exposures for UK live-line workers

Dosimetry is evaluated for live-line workers exposed to 50 Hz non-uniform magnetic fields from typical high-voltage transmission lines in the United Kingdom. The configurations involve twin-, triple- and quadruple-conductor transmission line bundles. Scenarios include three worker postures for the twin and triple bundles, and four postures for the quadruple bundle. The postures are selected to simulate worst case scenarios representative of work practices and result in highest values of dosimetric measures in critical organs. Only single-phase bundles are considered, as adjacent bundles of differing phase result only in a small reduction of the dosimetric measures. Reported data include various measures of the electric field and current density induced in tissues, as well as of the current density averaged over 1 cm2 areas normal to the current flow. A value of this latter quantity of 10 mA m(-2) is suggested as a threshold for neural tissue in the UK and international regulations. Critical tissues considered in this study include the retina, spinal cord, brain and cerebrospinal fluid. Some discussion is devoted to problems associated with the concept of current-density averaging, and two algorithms are considered. For a nominal load of 1 kA per subconductor, averaged current densities exceed the guideline bounds, only for a small number of postures and bundle configurations, in the brain, retina and cerebrospinal fluid. Non-averaged current densities in the cerebrospinal fluid exceed the suggested bound for all scenarios modelled, as well as in the retina for three postures involving a quadruple bundle.