Abstract:
An induced body current meter uses measurements of extremely low frequency magnetic fields in three spatial axes to determine the maximum current density induced in the brain or other body organ from exposure to the magnetic fields. The method extrapolates from a detailed dosimetry of induced current from magnetic field exposure for a reference body in a reference magnetic field. The meter can be carried or worn to monitor magnetic field exposure of its user. The meter&#39;s induced current measurement can be directly compared to induced current health hazard guidelines for health regulation compliance assessment.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 60/282,352, filed Apr. 5, 2001. 
     
    
     
       TECHNICAL FIELD  
         [0002]    This invention relates generally to measurement of extremely low frequency (ELF, =˜3-3000 Hz) magnetic fields in the environment including workplaces, such as for purposes of assessing compliance with magnetic field health hazard standards. The invention more particularly relates to determine induced body currents caused from exposure to such magnetic fields.  
         BACKGROUND AND SUMMARY  
         [0003]    Based on scientific studies linking exposure to high strength magnetic fields with neurologic and cardiac problems due to induced body currents, an international non-governmental scientific body titled the International Commission for Non-Ionizing Radiation Protection (ICNIRP) has established a magnetic field exposure guideline to protect against these acute health hazards. The ICNIRP guideline recommends against magnetic field exposure that would produce induced body currents of more than 10 mA/m 2  in the brain and central nervous system.  
           [0004]    This ICNIRP induced current guideline has been adopted for purposes of establishing magnetic field health standards by government agencies in many countries, including the European Union, Great Britain, and Germany, among others. In the U.S., the American Conference for Governmental Industrial Hygienists (ACGIH) has adopted the ICNIRP induced current guideline as the basis for establishing a Threshold Limit Value.(TLV) for ELF magnetic fields. The U.S. Occupational Safety and Health Administration (OSHA) uses the TLV to regulate. ELF magnetic field exposures. Accordingly, in these countries, the ICNIRP induced. current guideline determines when the magnetic field environment poses a health risk, and when controls on exposure to such fields are needed. Further, in some of these countries, standards based on the ICNIRP induced current guideline carry the force of law, such that fines or other penalties may be imposed for failure to comply with the standards.  
           [0005]    Due to the impracticality of measuring induced current within living persons, ICNIRP and ACGIH have set reference levels for environmental and occupational magnetic fields at which induced body currents may be expected to exceed the ICNIRP induced body current guideline. These reference levels are based on a calculation of the induced current density in a simplified dosimetric model of the human body from a given magnetic field. The ICNIRP calculation is based on using a coil of wire the radius of a person&#39;s waist. The ACGIH&#39;s calculation uses a homogenous “egg” (oblate spheroid) having a person&#39;s height and width. Both agencies use a simplified magnetic field having a single frequency and a linear vertical orientation (or “polarization”). From these simplified models, the agencies derived formulas for the magnetic field reference levels, which depend upon the frequencies present in the magnetic field and the magnetic field magnitudes at those frequencies (i.e., the magnetic field frequency spectrum).  
           [0006]    Use of these magnetic field reference levels to assess violation of magnetic field exposure guidelines has several drawbacks. Because the formulas derived for the reference levels depend on the magnetic field magnitude at multiple frequencies, accurate measurement requires an instrument that can measure the magnetic field frequency spectrum. These instruments are costly (e.g., $3,000 and up). Further, accurately measuring the magnetic field frequency spectrum requires extensive training, and a multiple step procedure. The instruments&#39; frequency spectrum measurements are based on digital Fourier transforms, a sophisticated mathematical procedure whose improper use can give erroneous results. In cases where the frequency spectrum measurement indicates only a single frequency is present in the environment, a simpler “gauss meter” costing less than $1,000 can be used to measure the magnetic field magnitude. However, most workplaces have magnetic fields with multiple frequencies.  
           [0007]    A further problem is that the ICNIRP and ACGIH magnetic field reference levels may be insufficiently accurate at estimating compliance with the induced body current guideline. Accurate compliance measurements can have very significant impacts in the workplace. Negative errors in measuring guideline compliance can lead to a false decision that there is no health risk. Positive errors can lead to false decision that controls are needed. In either case, compliance measurement inaccuracies can prove costly.  
           [0008]    Analysis by the inventor has shown that the ICNIRP and ACGIH magnetic field reference levels may be off by 100% or more at determining compliance with the induced current guideline. The analysis used detailed dosimetry models developed by researchers based on magnetic resonance imaging (MRI) images of an average male body. The MRI-based dosimetry provides induced current data from “standard” magnetic fields, i.e., linearly polarized fields of 60 Hz frequency and 1 μT (microtesla) magnitude aligned along each of the body&#39;s three axes (vertical, transverse and lateral). Measurements in a workplace with a “wave capture” instrument (called the Multiwave II® of Electrical Research and Management, State College, Pa.) were used to determine the frequency spectrum, and calculate compliance with the TLV and ICNIRP magnetic field reference levels. For comparison, the induced body current also was estimated using a methodology developed by the inventor from the Multiwave data and the MRI-based dosimetry. In 59 measurements taken in six factories, the ICNIRP reference levels had errors relative to the induced current estimates ranging from −97% to +1%, and the ACGIH&#39;s TLV had errors from −10% to +1470%. Although the induced current estimation methodology uses a few mathematical approximations, the induced current estimates are expected to be accurate within ±10%.  
           [0009]    The errors in the ICNIRP and ACGIH magnetic field reference levels can result from: (1) multiple magnetic field frequencies, (2) variable spatial orientation of the field, (3) non-linear polarization, (4) variable magnitudes across the body, (5) variable conductivities within the body, and (6) the shape of bodily organs.  
           [0010]    The present invention provides a more practical and accurate measurement of compliance with magnetic field health guidelines. An induced body current meter according to the invention provides a reading of the induced body current produced from exposure to an environment&#39;s magnetic field. The reading can be compared directly to established body current guidelines. In one implementation of the invention, the induced body current meter includes a multi-axis magnetic field sensor (e.g., a 3-axis gaussmeter), and a programmed processor. The processor performs a calculation estimating the induced body current resulting from exposure to the sensed magnetic field, based on a dosimetry model derived from MRI imaging data of a reference human body. In a representative implementation, the calculation extrapolates the body current induced by the sensed magnetic field in the brain from a multiple-axis numerical dosimetry model of an average adult male body in a 60 Hz magnetic field. Alternative implementations of the meter can be adapted to determine induced body currents in other organs of the body.  
           [0011]    In at least some implementations of the invention, the induced body current meter is designed as a mobile instrument that can be worn on the person (e.g., by a work or health inspector) into a monitored environment. For an implementation for measuring induced body current in the head for example, the mobile instrument preferably is carried on the person&#39;s head (e.g., with a head band or hard-hat mounting) with gauss-meter induction coils aligned with the dosimetry model axes (e.g., with coils aligned on the vertical, transverse and lateral axes of the body as in the dosimetry model). Alternative implementations for other organs may be carried on the person near the organ in question. Although integrated in single portable unit housing in some implementations, the magnetic field sensor and programmed processor portions of the instrument can be housed separately in other implementations (e.g., separate portable magnetic field sensor unit and programmed computer for the induced body current calculation).  
           [0012]    Implementations of the meter can provide a read-out of the induced body current measurement on a display, or log measurement data to a recording media (e.g., in an electronic memory, magnetic recording medium, printed medium, or other recording media) for later analysis. Some implementations can provide a peak hold feature for obtaining an upper bound of the exposure measurement.  
           [0013]    Additional features and advantages will be made apparent from the following detailed description of the illustrated embodiment which proceeds with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a flow diagram of an induced body current meter instrument in accordance with one implementation of the invention.  
     
    
     DETAILED DESCRIPTION  
       [0015]    In the following detailed description, one implementation of measuring induced body current exposure from magnetic fields according to the invention is embodied in an induced body current meter  100 . The meter  100  utilizes a multiple axis sensing of the magnetic field of an environment, and extrapolates the induced body current from exposure to that field based upon a detailed, empirically-derived dosimetric model of the human body. This yields an induced body current value for ready comparison against established induced body current guidelines for health hazard compliance assessment. The induced body current determination also is expected to have higher accuracy comparative to prior magnetic field health hazard assessment methodologies.  
         [0016]    With reference to FIG. 1, a suitable hardware platform for the meter  100  includes a multiple-axis magnetic field sensor  104 , signal processing circuitry  105 - 109  for calculating the induced body current from the sensed magnetic field signals, and an induced body current measurement output  110 . An example of such platform is a programmable three-axis gaussmeter, such as the currently available EMDEX PAL™ from Enertech Consultants of Campbell, Calif., USA. The programmable three-axis gaussmeter is modified via programming to perform processing of the sensed magnetic field signals to yield the induced body current measurement. In alternative implementations of the meter  100 , a purpose-built hardware platform can be used. Further, although the exemplary meter  100  is housed in a single mobile unit, alternative implementations of the meter  100  can have any variety of configurations, such as a handheld, laptop or other mobile computer in communication with a magnetic field sensor peripheral component; or a mobile magnetic field sensor unit for recording or transmitting magnetic field data and operated in combination with a stationary desktop, workstation or server computer that performs the induced body current measurement calculations to provide the induced body current measurements according to the invention.  
         [0017]    A suitable multiple-axis magnetic field sensor  104  for the meter  100  is a three-axis induction coil sensor, which has three induction coils with ferrite cores in an orthogonal axis configuration (with axes denoted x, y and z corresponding to lateral (left-to-right), transverse (front-to-back) and vertical axes of the body, respectively). The induction coils each produce a voltage signal (i.e., V x (t), V y (t), and V z (t)) related to the magnetic field&#39;s derivative in the direction of its respective axis. Alternative implementations of the meter  100  can employ different magnetic field sensor technologies that produce signals indicative of the magnetic field sensed in multiple spatial dimensions, whether in 3 orthogonal axes or other configuration.  
         [0018]    In the signal processing circuitry  105 - 109 , signal calibration and conditioning circuitry  105  first calibrates the sensor voltage signals to ensure proportionality with magnetic field measurement units, i.e., calculates the magnetic field value for each axis as a calibration function (F) of the sensor voltage signals as shown in the following relation:  
           dB   i ( t )/ dt=F[V   i ( t )], for  i=x, y, z.    
         [0019]    The circuitry  105  can include dynamic ranging. In alternative implementations of the meter  100 , the digital signal processor  108  can be programmed to perform some or all of the calibration function (F).  
         [0020]    The meter  100  next includes a motion filter  106 , which filters out the contribution to the magnetic field signal (dB i (t)/dt) due to the meter&#39;s motion through gradients in the earth&#39;s magnetic field, while passing contributions from AC electrical equipment and other higher frequency magnetic field sources in the environment. A suitable motion filter  106  is a high-pass filter set to pass 40 Hz and higher signals. Although the biological effects of induced body currents have not been shown to have any frequency dependency, gaussmeters designed for cancer research commonly include such motion filters as a standard feature. The meter  100  includes the motion filter  106  so that the magnetic field sensor signal is comparable to a standard gaussmeter signal. Some implementations of the meter  100  include a switch to selectively bypass the motion filter  106  (i.e., remove the motion filter from the signal path) in case it is desired to also measure induced body currents resulting from motion.  
         [0021]    Following the motion filter  106 , the meter  100  has an analog-to-digital converter  107  for converting the analog magnetic field sensor signal into digital data for processing by a digital signal processor  108 - 109 . The sampling rate of the converter  107  should be at least twice the upper bound of the instrument&#39;s flat frequency response. Since some existing gaussmeters have an upper bound of 800 Hz for example, this implies a sampling rate of around 1600 Hz. However, because violations of the induced current guideline can occur with induction heaters that operate at much higher frequencies, the meter  100  preferably provides a broader frequency range and higher sampling rate for measuring induced current guideline compliance. The output of the analog-to-digital converter  107  are the three components of the magnetic field derivative vector dB α (t)/dt (where α=x, y, z or lateral, transverse and vertical) as a function of time (t).  
         [0022]    The digital signal processor (DSP) in the meter  100  processes the magnetic field derivative vector data in an induced current density calculation  108  to produce a measurement of the body current induced in the brain from exposure to the sensed magnetic field. The digital signal processor is programmed with data  104  from an MRI-based dosimetry model for use as parameters of the induced body current density calculation. The dosimetry parameters are obtained from magnetic resonance imaging (MRI) of an adult male body. The parameters are the x, y and z vector components of current density  α J calculated for a 1 μT peak, 60 Hz magnetic field applied along the respective α axis (where α=x, y, z).  
         [0023]    Exemplary MRI-based dosimetry parameters. for induced brain currents are listed in the following Table 1. Because the ICNIRP induced current guideline is based on neurological disturbances, induced brain current measurement using the listed parameters is currently preferred for health hazard compliance assessments. However, the meter  100  can be reprogrammed with dosimetry parameters for induced current in other body organs, such as in the case that future research identifies health risks from induced currents in such other organs (e.g., heart, uterus, whole body, etc.).  
                                                                   TABLE 1                                         α J β  [μA/m 2 ]                Magnetic field axis   β = x   β = y   β = z                            x   −0.0145   0.3611   0.1028           y   −0.3315   0.0018   −0.0100           z   −0.0557   0.0289   0.0003                      
 
         [0024]    In one implementation, the DSP is programmed to perform the calculation  108  in accordance with the following Equation 1 of the induced current density J(t) as a function of time using the dosimetry parameters  104  and magnetic field derivative vector data from the converter  107 .  
               J        (   t   )       =       1     2                 π   ×   1                 µT   ×   60                 Hz                ∑       α   =   x     ,   y   ,   z              (       ∑       β   =   x     ,   y   ,   z              J   β               α                   B   α          (   t   )              t           )     2                   (   1   )                               
 
         [0025]    The meter&#39;s digital signal processor further performs a calculation  109  of the root-mean-square (RMS) value (J RMS ) of the just calculated induced current density. The averaging time of the RMS calculation  109  should depend on the biologic response time. A suitable averaging time for the meter  100  is several cycles of 50/60 Hz AC electricity, or about 0.1 seconds.  
         [0026]    The just described calculations  108 - 109  in this implementation of the meter  100  have the advantage of not involving measurement of the frequency spectrum, and thus does not require calculation of the Fourier or other complex mathematical transform. This can reduce the cost of the meter (e.g., because higher cost/speed components needed to process such transforms are not needed), and can reduce the likelihood of error from improper use.  
         [0027]    The meter  100  finally provides an output  110  of the induced body current measurement. In one implementation, the meter  100  provides a variety of output options, including via a data logger, a direct digital read-out, and read-out with peak hold. With the data logger option, the meter  100  stores a data set in an on-board digital memory  111  containing the J RMS  measurement taken at periodic intervals (e.g., every 1-10 seconds), along with the time of measurement. Via user selection, the meter  100  can be set to display various statistics (produced from a statistical analysis  112 ) from the logged measurements on a meter display  113 , such as average and maximum. Also, the data set can be downloaded from the meter  100  to a computer (e.g., laptop, desktop, handheld, server, etc.) for further numerical analysis and graphing (e.g., using a spreadsheet or like software application).  
         [0028]    The meter  100 , with the direct digital read-out option, displays the J RMS  measurement on the meter&#39;s display  113  (e.g., on a digital LCD display screen). Alternatively, with the peak hold read-out option, the meter  100  captures and displays a peak (maximum) value of the induced current measurement since last reset or other time interval. The peak hold option is particularly appropriate for obtaining the upper bound of the magnetic field-induced current exposure within an environment for health hazard compliance decisions.  
         [0029]    For use in induced current guideline compliance assessment, the meter  100  can be carried or worn on the person, e.g., of a worker in course of work duties or an inspector on a walk-through of a work place or other strong magnetic field environment. Preferably, the meter is carried near the head, such as attached on a head band or mounted on a hard hat. The meter  100  is oriented with the induction coils of the sensor  104  aligned to the vertical, transverse and lateral axes of the person&#39;s body, as in the dosimetry model. In this way, the meter induced body current measurement more closely reflects the person&#39;s actual exposure in the environment. In implementations of the meter adapted for measuring induced current exposure of other body organs (e.g., programmed with dosimetry parameters for the heart, uterus, or other organs), the meter can be carried or worn near such other organ of the person, again with the sensor appropriately aligned with the dosimetry model axes.  
         [0030]    Having described and illustrated the principles of our invention with reference to an illustrated embodiment, it will be recognized that the illustrated embodiment can be modified in arrangement and detail without departing from such principles. It should be understood that the programs, processes, or methods described herein are not related or limited to any particular type of computer, sensor and signal processing hardware platform. Various types of general purpose or specialized hardware may be used with or perform operations in accordance with the teachings described herein. Elements of the illustrated embodiment shown in software may be implemented in hardware and vice versa.  
         [0031]    In view of the many possible embodiments to which the principles of our invention may be applied, it should be recognized that the detailed embodiments are illustrative only and should not be taken as limiting the scope of our invention. Rather, we claim as our invention all such embodiments as may come within the scope and spirit of the following claims and equivalents thereto.