HIGH FREQUENCY AMMETER AND PERSONAL DOSIMETER

Abstract: A detector (14, 15, 22, 60 or 90) for detecting high frequency current (I) includes a high resistivity coil (26) wound around a nonferromagnetic core (34). The ratio of the coil (26) spacing to the cross sectional area of the coil (26) is maintained constant over the length of the coil (26). A high resistivity material shield (36) surrounds and is spaced from the coil (26) and has a gap (38) oriented along an azimuth of the closed loop winding and directed orthogonal to the net current direction of the current induced in the coil (26). The ends of the coil (26) are coupled to a high impedance voltage detector (24) through high resistivity leads (46 and 48) and a resistor (52) is coupled between the leads (46 and 48) to reduce the quality factor. The current detector (14, 15, 22, 60 or 90) may be used to detect current (I) flowing in a human body (10) due to the absorption of high frequency incident radiation as an indication of the specific absorption rate of such radiation.

Description: HISH FREQUENCY AMMETER AND PERSONAL DOSIMETER This invention relates to apparatus for detecting a high frequency current in a conductor. and more par¬ ticularly to such apparatus which is placed around the con¬ ductor and uses the magnetic field produced by the high frequency current to measure the current without perturbing the ambient electric and magnetic fields. There is increasing concern regarding the hazards which may result from exposure of humans to electromagnetic energy. A large number of electromagnetic radiators, in— eluding radio and television stations, radar transmitters, microwave ovens, and communications equipment, contribute to the total exposure of humans to non-ionizing radiation. In addition, some industrial workers are exposed to the fields from such devices as radio frequency heat sealers and inductive heaters. Exposure to the field of EHV lines used for power transmission is also becoming more common. The National Institute for Occupational Safety and Health <NIQSH) and the American National Standards Institute <ANSI) have both been involved in attempting to define safety criteria. Considerable effort is being spent in both experi¬ ments with animal models and theoretical analysis to exam¬ ine the biological effects of non-ionizing electromagnetic energy. An important part of this work is dosimetry, that is determining the amount of energy absorbed under various conditions. Dosimetry is much more difficult for nonioniz— ing electromagnetic energy than it is for ionizing radia¬ tion because, unlike ionizing radiation, the absorbed dose is not simply related to the incident flux. The absorption of electromagnetic energy is dependent upon the dimensions, composition and posture of the body, as well as the fre¬ quency, polarization, and other properties of the radia¬ tions. Generally the most intense exposure occurs under near—field conditions, where the electric and magnetic fields are not simply related and where it is often diffi¬ cult to accurately characterize the source. There is a strong need for a non-invasive personal dosimeter for electromagnetic energy corresponding to the devices that are in general use with ionizing radiation; however, no suitable non—invasive device has thus far been suggested. Dosimetry for electromagnetic energy has been limited to invasive measurements of the temperature or fields within laboratory animals and models of man, as well as to computer simulations. The fields external to the body are a superposition of incident and scattered waves, so that measurements made close to the body generally fail to permit accurate characterization of either the external fields or those of the incident wave. For this reason, it has generally been impossible to make meaningful predic¬ tions of the absorbed dose from non—invasive measurements. More specifically, there is also considerable need for a device which would quantify the dose received by a patient when electromagnetic energy is used for therapeutic purposes, such as diathermy and the hyperther ic treatment of cancer. Hyperthermia has shown considerable promise for the adjuvant treatment of cancer, but there has been diffi¬ culty in treating deep—seated tumors with the required de— gree of precision. Computer simulations and clinical ob¬ servations suggest that considerable aberrant heating may occur in hyperthermia due to deposition of energy outside the region intended for treatment. At present, it is only possible to monitor the temperature at a few intracavity or interstitial locations, so that the physician must rely on the complaints of pain from the patient for guidance during treatment. However, heating at locations in the core of the body is often perceived as a dull pressure, so patient complaints a.re difficult to interpret, but significant dam— age can still occur. Ampere's Law states that the line integral of the maqnetic field intensity around the closed path is equal to the total electric current passing through the region enclosed by the path. A ferromagnetic core having cross-sectional area A and permeability H, may be formed to make a closed loop of length L. If the core has a high permeability, the flux within the core at any part of the loop is related to the 10 current passing through the aperture enclosed by the loop, according to the following equation: § = μ AI/L ' (2) A coil consisting of N turns may be wound around It. the core at any location on the loop and the potential in¬ duced on the coil is given by the equation: V =-jwμNAI/L (3) where time dependence of e* is assumed. 0 Equation 3 can be used to determine the current I from the potential V measured across the coil. This prin¬ ciple has been used in clamp-on ammeter instruments for many years. These devices allow the measurement of a . c . current in power circuits without cutting the lines and b interrupting service. Current probes are also commer¬ cially available, which serve as transducers to be used with separate meters, oscilloscopes, or other measuring instruments. These probes have either a clamp-on design, which opens for placement around the conductor, or a fixed 0 configuration which requires that the conductor be passed through the central aperture. At the present time, no device is available for use as a non-invasive dosimeter for electromaqnetic energy. In prior studies, measurements were made of the fields exter- tD nal to the body and it was concluded that such measurements were not suitable for predicting the internal fields. Oth¬ er studies have shown that electric field probes located near the body have a response that is strongly dependent on a number of factors, including the distance between the probe and the body. For example, see Misra et al "Response of Electric Field Probes Near A Cylindrical Model of the Human Body" , IEEE Transactions on Microwave Theory and Techniques, Vol. MTT 33, pages ^ή-7- -52 (June 1985). These studies confirmed that simple field measurements made ex¬ ternal to the body have little or no value in dosimetry. Others have recommended that, when the BSD Annular O Phased Array manufactured by BSD Medical Corp. of Salt Lake City, Utah is used to induce hyperthermia, non—invasive electric field probes be used to measure the axial electric field at several locations near the surface of the body, particularly beneath the applicator. Measurements made with 5 these probes are used in adjusting the applicator, and it is inferred that such measurements relate to the fields within the body. Since the Annular Phased Array produces an electric field that is predominantly axial in orient¬ ation at the locations where the measurements are made, ϋ boundary relations suggest that the electric field inten¬ sity immediately inside the body would be approximately equal to that measured near the surface. However, such measurements do not accurately reflect the total (axial) current distribution over the cross section of the body; 5 rather they are limited to currents near the surface. Active microwave imaging methods have also been used to obtain approximate images of inho ogeneαus dielec¬ tric objects. For example, see Pichot et al "Active Micro¬ wave Imaging of Inhomogeneαus Bodies" , IEEE Transactions on Antennas and Propagation, Vol. AP-33 pages - 16-425 (April 1985) . Related methods could be used to determine the dis¬ tribution of current within the body from non—invasive mea¬ surements. Other procedures based on moment method calcu¬ lations, have also been considered for use in non—invasive 5 measurements of the human body, and results αf mathematical modeling suggest that it would be possible to use them to determine the specific absorption rate (SAR) of energy dis— position. Each of these methods is quite complex, requir ¬ ing computations based upon accurate measurements with spe ¬ cial purpose probes. They do not appear suitable for use as a personal dosimeter at the present time. In the patent literature, others have attempted to use various techniques to measure the interaction of radia¬ tion with the human body. For example, in U.S. Patent 3,639,941 to Richardson, a circuit was provided for mea¬ suring the field strength near the body. However, such 10 measurements are not suitable for predicting the currents induced within the body. Other patents in which radiation has been measured include U.S. Patent 3,277,300 to Kerr et al in which a photovoltaic cell is used to detect infrared radiation; U.S. Patent 3,878,496 to Erickson, in which a 2^ Geiger-Mueller tube is used to detect ionizing radiation; and U.S. Patent 4,196,425 to Williams Jr. et al, in which the activity of the patient is monitored by detecting a interruption of an infrared beam passing above the area of confinement. Other patents in which radiation is detected g0 include; U.S. Patent 4,199,716 to Reindel for a hand held receiver and antenna used to indicate the presence of mi¬ crowave radiation from microwave ovens or other microwave sources; U. S. Patent 4,229,733 to Tulenko et al using a solar cell to detect ultra-violet radiation; and U. S. g;-, Patent 4,338,595 to Newman describing a device suitable for detection of microwave energy emitted by microwave ovens or other microwave devices. These last three patents, in par¬ ticular, have virtually no use in measuring the amount of energy absorbed by a object, such as a human body which is Q a lossey dielectric object. It is also well known that current can be deter¬ mined from measurements of the magnetic field surrounding the object carrying the current. Well known examples of devices for determining current based upon a magnetic > 3 field measurement include clamp-on ammeters, which typi¬ cally include an iron core which is adapted to being placed around the conductor. The maαnetic field surroundinα the conductor will then flow through the magnetic core of the clamp-on ammeter and can be detected by placing a winding at a convenient location around the core. Patents litera¬ ture illustrating this techniques include U.S. Patents 2,802,182 to Godshalk et al 3,984,798 to Bussen, 4,456,873 to Schweitzer and U.S. Patent 3,323,056 to Haley. As will be discussed hereafter, it is preferable, when measuring high frequency currents through a conductor, to utilize a non—ferromagnetic core with high resistivity; air is an excellent example of such a core. Many prior patents have discussed utilizing non-magnetic cores, in¬ cluding air cores, for various purposes. They include U.S. Patents 3,434,052 to Fechant; 3,652,935 to Shaw; 4,348,638 to Boldridge; 4,581,598 to York; and 4,621,231 to Heinrich et al. However, none of the structures described in the aforementioned patents are satisfactory for measuring the current flowing through the conductors, such as an animal body because they perturb the surrounding fields due to the low resistance coil. In accordance with one aspect of this invention, there is provided a current detector for detecting high frequency currents flowing through an object. The detector includes a conductor wound in multiple turns around a sub¬ stantially closed loop core and has a loop having an aper— ture greater than the corresponding dimensions of the ob¬ ject to be measured. The detector is characterized by the conductor being a high resistivity material, by the core being non-ferromagnetic and by the ratio of the turns spac¬ ing to the core cross-sectional area being constant at any given point along the loop. In accordance with another aspect of this inven¬ tion, there is provided a detector for non-invasively de¬ termining the specific absorption rate of high frequency electromagnetic energy in an animal body subjected to such radiation by measuring the electric current flowing through the body as a manifestation of the specific absorption rate. The detector is characterized by current detector means, including a helical coil of electrical conductor material having two ends and a lead extending from each end, by flexible means for containing the current detector means and for being affixed around a portion of the body so that the two ends are substantially juxtaposed to one an ¬ other to form a closed loop coil around the body portion, and by means coupled to the leads for detecting the voltage induced in the coil by the current flowing through the body. In accordance with another aspect of this inven¬ 10 tion, there is provided a method for detecting aberrant heating in an animal body undergoing an application of electromagnetic energy. The method is characterized by the steps of placing a current detecting probe, which does not significantly perturb the electromagnetic energy, around at l b least one portion of the body, by monitoring the probe for an indication of voltage induced in the probe in excess of a certain threshold as manifestation of aberrant heating, and by affecting the application of the energy to reduce the aberrant heating, in response to the indication. 0 One preferred embodiment of the subject invention is hereafter described,. with specific reference being made to the following Figures, in which: Figure 1 shows a patient undergoing a hyperthermic treatment for cancer; b Figure 2 shows a personal dosimeter device useful for measuring the current through the patient shown in Fig¬ ure 1 ; Figure 2A is a cross-section taken across lines 2A-2A of Figure 2; 0 Figure 3 shows another embodiment of the personal dosimeter device of the subject invention. Figure 4 shows a general manner in which the per¬ sonal dosimeter device, shown in Figure 3, can be affixed b around an object for measuring high frequency current flowinq therein; and Fiαure 5 shows yet another embodiment of the per— sonal dosimeter device of the subject invention. As previously discussed, it has been long known that it is possible to measure the current through a con— ductor by surrounding the conductor with a magnetic core having windings around the core. It also is possible to determine the current from non-invasive measurements with¬ out using a ferromagnetic core. A solenoidal winding hav¬ ing N uniformly spaced turns with crass—sectional area A may be formed into a closed loop of length L . If the coil is formed around a non-ferromagnetic core, such as air (permeability H σ ) » then the flux within the core at any point along the coil is given by φ = μ H-1 * (4) The potential induced in this coil is then V = <. μ o L ^H " -?l (5) Using Ampere's law, the potential is V =-jw^NAI/L (6) Equation (6) may be use to determine the current I from the potential V measured across the coil. The differ¬ ence between this procedure and the procedure described above with respect to equation (3) is that a ferromaqnetic core is not required. However, the ratio of the wire εpac— inq between turns to the core cross sectioned 1 area must be constant over the length of the coil when equation (6) is used. Conventional clamp—on ammeter instruments utilized a ferromaqnetic core, and low resistance windinqε, thereby disturbing the ambient electric and magnetic fields. Where one is measuring a current induced in an object by an external field, the disturbance of the field effects the current being measured, thereby makinq the results in¬ correct. In the present invention, the use of non- ferromagnetic core, together with hiqh resistance windings, dose not disturb the ambient field, and thus, the present invention is useful to measure a current induced by an electromagnetic field. Since the electromaqnetic fields of interest are time dependent, personal dosimeter devices based on either equation (3) or equation (6) must respond to displacement current, as well as conduction current. Thus, it is ex- 5 pected that there will be a reading due to the surroundinq environment, even without the body or other object being inserted in the device aperture. When the body or other object is present and the fields are axially polarized, a sizeable space may still exist in the aperture between the 10 coil and the body without introducing appreciable errors due to displacement currents in the aperture. Since the magnitudes of the electric field intensity inside a d out¬ side the body are comparable, and the magnitude of the rel¬ ative permittivity of muscle and other body tissues having Itj a high water content is larqe, the current density within the body is much greater than the displacement current den¬ sity in air. Futhermore, since the imaginary component of the relative permittivity of the body tissue is dominant, the measured value of the current is directly related to 0 * rie energy deposited in the tissues. Low- requency approximations are implicit in both of the derivations presented in this section. If the length of the loop (L) is not much smaller than a wave- lenqth within the contained medium, it is necessary to cor- b rect for both retarded times and attenuation during propa¬ gation to the coil. These effects cause a decrease in the output from the coil. Table I shows calculated values illustrating the effect of propagation on the output of a torroid surrounding a cylinder of muscle having an axial 0 current that is assumed constant over the cross section. TABLE I Radii of Muscle Cylinders (cm) for Specified Deviation of Coil Output from Equation (6) 5 1 10 27.12 50 80 150 (Frequency in MHz) ιυ 1MHz), the value from equation (6) have errors less than 10 percent for cylinders havinq radii as larqe as the human 15 torso (24 cm radius). At 27.12 MHz, commonly used with RF heat sealers and other commercial devices, errors of less than 20 percent, would be found with cylinders having radii as large as the thiqh (9 cm). At the hiqher frequencies (>80MHz) used in hyperthermia, the deviation would be about 20 30 percent with a cylinder having a radius as larqe as the neck or limb. Smaller errors are expected with measurements of the human body than with muscle cylinders havinq similar cross sections, since the attenuation in bone and fat is b substantially less than that in muscle. The effects of propaqation, which limit the accuracy of equation (6), also limit the depth of deposition of electromaqnetic energy in tissue. For this reason, the conditions under which size¬ able errors would be found with equation (6) are those in 30 which it would be unlikely that electromagnetic energy could cause significant heating. The American National Standards Institute (ANSI) guide recommends that, for human exposure, the incident electric field (rms) should be limited as follows: 35 E = 632 V/m from a Frequency F = 0.3 MHz to 3.0 MHz E = 1897/F V/m for F = 3.0 MHz to 30 MHz E = 61.4 V/m for F = 30 MHz to 300 MHz The ANSI committee used the results from experi¬ ments with laboratory animals and models of man, as well as computer simulations, to determine these limits for incident fields. The two expressions for frequencies above tJ 3 MHz were designed to compensate for the frequency depen¬ dence of absorption, so that the rate of whole body energy absorption under various exposure conditions will not ex¬ ceed a limit which they chose. At frequencies below 3 MHz, the limit was set primarily due to consideration of hazards 10 from shock and burns. Since a variety of exposure conditions is possible, it appears that the limits recommended by ANSI may be over¬ ly protective in some cases, but insufficiently strict in others. Uncertainty reqardinq the conditions of exposure 15 required the ANSI committee to make the additional recom¬ mendations that the peak rate of enerqy deposition (spe¬ cific absorption rate, or SAR, in watts per unit mass) should not exceed 8 W/kq for any one qram of tissue in the body. The equipment that is presently available would re- 20 quire the use of invasive measurements for enforcement of the latter recommendation. The ANSI committee has not yet made recommendations for frequencies below 0.3 MHz, but it appears that the haz¬ ard from shock or burns at the lower frequencies is di- et:ι rectly related to the electric current, rather than the SAR. The present invention appears especially appropriate for use at these lower frequencies because the measured parameter is electric current. Another possible applica¬ tion, of the present invention would be a personal dosime- 0 ter device that could be fabricated as a part of a belt to be affixed over the clothing of utility workers, in order to quantify the intensity of possible electrical shock. Values of the "let-qo" current have been specified as safe¬ ty criteria for use at power-line frequencies. Values of - • - ■ the SAR can be calculated from non-invasive measurements of current and published values for the dielectric proper- ties of tissue. The ANSI recommended limit of peak SAR could be enforced if local values were approximated in this manne . Referring now to Fiqure 1, both modelinq experi- ents and computer simulations have show that, when a pet son 10 is exposed to electromaqnetic energy, such as from a high frequency applicator device 12, the energy absorbed by the body is qreatest when the electric field vector is par¬ allel to the lenqth of the body", this is known as E- polarization. Experiments with mannequins and computer simulations have shown that with E-polarization, there is an electric current I that follows vertically throuqh the body such that the current at one location is related to the current at other locations. The dosimeter 14 of the present invention may be used to determine the current I at a convenient location, such as a leq or an ankle. A larqer dosimeter 15 may be attached around the area of a person 10 receivinq treatment, such as the abdomen. Published values of the dielectric properties of tissue can be used with these measurements to determine the approximate SAR in the treated area, as well as other parts of the body. Both computer simulation and odelinq experiments have shown that an applicator 12 providinq E—polarization, such as an Annular Phased Array or a helical coil, is more suitable for treatinq deep—seated tumors than are applica¬ tors havinq other polarizations. Computer simulations fur¬ ther suqgest that the aberrant heatinq is most severe with E-polarization. Experiments usinq models of man have shown that the local SAR in the neck may be twice that in the abdomen when a Annular Phased Array applicator 12 is posi¬ tioned for abdomial heatinq, as shown in Fiqure 1. Siqnif— icant aberrant heatinq has also been observed in experi¬ ments with the helical coil applicators. At the present time, clinicians have no tool to measure aberrant deposition non-invasively. The dosimeter 14 of the present invention may be effectively used to determine the lαnqitudinal flow of current I throuqh vari- 13 ous parts of the b o d y, since it is insensitive to azi uthal or other circulating current. Since the aberrant heating during hyperthermia treatment is qenerally due to the axial flow of current in various narrow bodily reqions ( e.q. the neck, arms, and leqs), dosimeter 14 is useful as a tool for b indicatinq such aberrant heatinq. Certain industrial workers may be repeatedly ex ¬ posed to hazardous electromaqnetic fields as part of their duties. A personal dosimeter, such as dosimeter 14 in Fiq¬ ure 1, may be used to determine and store the currents in¬ 10 duced as a result of the exposure. Later the stored values may be retrieved and used to approximate the SAR throuqhout the body for that worker. If the approximate SAR exceeds a threshold, corrective action will be required. Local values of SAR determined for the leq or ankle are particularly of lb interest, since the peak value often occurs in these re¬ qions. Alternatively, several personal dosimeter devices 14 may be used at different parts of the body. The principle of the present invention is related to that used in commercial clamp-on a . c . ammeters and cur¬ 20 rent probes. However, these instruments have not been used in dosimetry and it appears that they do not satisfy some of the requirements which are essential for proper imple¬ mentations of the present invention. The most siqnificant difference between the present invention and the available b devices is that the available devices all perturb the field which induce the current beinq measured, thereby ren- derinq inaccurate any readinq obtained. Another difference between the present invention end the prior art is that the present invention permits a significantly larger aper¬ 0 tures so that the device can surround the body at the waist, or another location, instead of only enclosinq a wire. Further, and particularly at hiqh frequencies, the larqe size required for the winding of a conventional clamp-on ammeter can cause increased problems from self b resonance of the device. These problems have been over¬ come in dosimeters 14 and 15 by reducing the Q-factor throuqh loadinq. Another difference is that existinq clamp-on ammeters require a ferromaqnetic core, which should be avoided in order to reduce the weiqht of the do¬ simeter, as well as to avoid the errors caused by satura- tion and dependence of permeability on frequency. The use of erromaqnetic cores must further be limited to use with low frequencies, where the added qain from larqe relative permeability is desirable. Even in these applications, such as a shock-hazard monitor for utility workers, a do- simeter device should have low weight and be flexible and adjustable so that it can be incorporated and easily af¬ fixed around the body without undue discomfort. Referring now to Fiqure 2, a personal dosimeter 20 includinq in a transducer 22 and a detector 24, is shown, which can measure the current I, shown in Fiqure 1. The detector 24 may be any conventional high impedance voltaqe detector, and any diqital readout or storaqe devices desit— ably attached thereto. Transducer 22 includes a coil 26 surroundinq *a central aperture 28 through which an abject 30 passes. Object 30 may be a body limb or other conductor through which current passes. Coil 26 is formed by winding a high resistance con¬ ductor 32 around a non—ferromagnetic material, such as air core 34. Because the air core 34 has a low permeability, at each point of coil 26 the ratio of the turn spacing between adjacent turns of conductor 32 and the cross- sectional area of air core 34, at that point, should be constant. This constant ratio of turn spacinq to cross sectional area is desirably maintained over the entire length of coil 26. The reason for this is to permit equal sensitivity at any, position within aperture 28. In other words, object 30 may be positioned anywhere in aperture 28 and the same results will be obtained. The requirement for a constant turns spacinq to area ratio is not necessary if a ferromaqnetic core is used, because a constant flux is maintained at all points in the ferromaqnetic core. The core 34 must have a low permeability, such as air, in order to allow measurement of the induced hiqh fre¬ quency currants normally associated with diathermy and hyperthermic treatments, so as not to perturb the inducinq field. An air, or other flexible low permeability mate¬ rial, core further has the advantaqe that the coil 26 is flexible and can fit around the conductor 30 in an non- circular fashion. For example, where the person 10 in Fiq¬ ure 1 is positioned on a examininq table and the personal dosimeter 14 of Fiqure 1 corresponds to dosimeter 20 of Figure 2, the dosimeter 20 is positioned around the ankle area of the person. In this position, the coil 26 would tend to be more elliptical than circular. This is particu¬ larly advantaqeous so that the lenqth of the coil 26 can be made qreater than the ircumference around the ankle, or other location of measurement, which vary substantially from patient to patient. Hiqh resistivity conductor 32 should have a volume resistivity such that the ratio of the volume resistivity to the thickness or diameter of conductor is in the ranqe of 1000 ohms to 1,000,000 ohms and preferable about 100,000 ohms. The material of conductor 32 may be, for example, carbon loaded Teflon or conductive rubber materials. The purpose for usinq a hiqh resistivity conductor 32 is to prevent or inhibit the perturbance of any existinq electric fields which are in the area. Ideally, the windinqs 32 over the air core 34 will only respond to the maqnetic field from the conductor 30 in the aperture 28. This maq¬ netic field will be qenerated due to the current I flowinq throuqh conductor 30, and as previously discussed, relates to the SAR from applicator 12. The hiqh resistivity of the windinqs formed by conductor 32 also prevents false readings due to perturbance of the current inducinq fields. In order to prevent capacitive couplinq between transducer 22 and object 30, an electrostatic a shield 36 of a hiqh resistivity material is wrapped around the coil 26. A gap 38 in shield 36 extends entirely alonq the closed loop of coil 26. The purpose of qap 38 is to permit o ly maqnetic couplinq throuqh the shield. It may be one millimeter in width. Aqain, shield 36 may be made of carbon loaded Teflon material or conductive rubber material. The ratio of the volume resistively to the thickness of the material of shield 36 ideally should be 100,000 ohms, but may be within the ranqe of 1,000 ohms to 1,000,000 ohms. The presence of shield 36 as well as the constant ratio of turns spacinq to cross sectional area, previously dis¬ cussed, permits object 30 to be positioned at any point within the aperture 28 of coil 26 and still obtain substan¬ tially the same readinq. In order for shield 36 to properly operate, a space 40 is left between the inner surface of shield 36 and the outer surface of the conductors 32 forming coil 26. The space 40 is reguired to reduce the self-capacitance of the coil 26, thereby increasing the sensitivity and frequency responses of device 22. A distance of between two and three millimeter may be used for space 40. As current flows through object 30, it creates a magnetic field around object 30, including in the core 34 of coil 26. This magnetic field in the core 34 induces a voltaqe on each turn of coil 26. The total voltage across the two ends 42 and 44 of the high resistivity conductor 32 forming coil 26 as measured by high impedance voltaqe de¬ tector 24, in turn, manifests the current flowing in object 30. In order to maintain a uniform spacinq between the turns of coil 26, the ends 42 and 44 of coil 26 should he positioned substantially adjacent to one another, sepa¬ rated only by a spacinq equal to the normal turns spacing. This spacing is necessary in order to minimize the errors due to non—constant turns spacing to coil cross sectional area ratio, as discussed above. However, if care is utilized to maintain the ratio of turns spacing to cross sectional area constant throughout the remainder of coil 26, some error can be permitted at the junction of ends 42 and 44. The leads extendinq from ends 42 and 44 of high resistivity material conductor 32 are coupled to the high impedance voltage detector 24 by high resistivity leads 46 and 48, which are also contained in a shield 50. By making leads 46 and 48 of a high resistivity material, such as carbon loaded Teflon or conductive rubber, and providing a similar high resistivity shield 50, the minimal disturbance of the ambient electromagnetic fields is maintained Device 22 includes inductance due to the winding of coil 26, capacitance due to the coupling of coil 26 to shield 36 and the coil 26 self capacitance and resistance from conductor 32, thereby creating and RLC resonant cir¬ cuit. The sensitivity of device 22, as a result of this resonant circuit, is strongly dependent an frequency, un¬ less the quality factor (Q) is reduced to ten or less. Re¬ duction of Q may be accomplished by adding a register 52, which may be 1000 ohms, in parallel with coil 26. Alterna¬ tively, a resister of typically ten ohms may be added in series with coil 26. In some applications, the high resis¬ tivity material of conductor 32, which is preferably used, may reduce the Q factor sufficiently to permit stable oper- ation without the use of additional resistors. However, for certain application, a further reduction in Q may be re¬ guired and a resister, such as resistor 52, will be re¬ quired. When hiqh resistance leads 46 and 48 and a hiqh frequency (e.q. qreater than 1 MHz for long leads or qreat- er than 10 MHz for short leads) are used, radio frequency attention in the leads 46 and 48 will require an extremely sensitive detector 24. To reduce this attenuation, a diode 53 may be added to rectify the coil 26 output voltaqe. The external circuit capacitances act as a smoothing capacitor, thereby providing a d.c. voltage, which is attenuated much less, to be provided over leads 46 and 48 to detector 24. The siqnal to noise ratio may be improved when it is possi¬ ble to modulate the source of the radiation, and add a fil¬ ter to detector ' 24. Referring now to Figures 3 and 4, a belt dosimeter 60 is shown, in which the structure, described in detail above with respect to Figure 2, is fabricated into a belt arrangement adapted to be strapped around an object, such as the ankle or knee of a person, for use in either Figure 1 type environment or for use as a personal dosimeter by a person workinq in a potentially hazardous electromaqnetic enerqy environment. The belt dosimeter 60 device may also be strapped around any other type conductors, where the current therethrough is desired to be measured. Belt dosimeter 60 includes a high resistivity coil 62 surrounding an air core 64 in the manner described above in respect to Figure 2. Extending from one end of dosime¬ ter 60 is a portion 66 of a connector, such as the hooks side of hook and loop type fastener, such' as Velcro. The other portion 68 of the connector is shown in dash lines as being on the opposite end of coil 62 and may be the loop portion of the hook and loop type fastener. Connector por¬ tion 68 may be positioned above the shield material 70 sur¬ rounding coil 62 and aligned to mate with portion 66 when the two coil 62 ends are fastened together to form a closed coil 62. Extending from each end of coil 62 is respective leads 72 and 74, which are adapted to be inserted along a lead shield 76 when portion 66 is affixed to portion 68 of the fastener. Means (not shown) may be including for at¬ taching a shield 76 to the assembled closed loop forming dosimeter 60 when portion 66 is affixed to portion 68. The two portions of the connector 66 and 68 should be arranged so that when portions 66 is affixed to portions 68, leads 72 and 74 are separated from one another by the same.spac¬ ing as between turns forming coil 62. In order to make coil 62 sufficiently flexible for being formed into a closed loop around a conductor, shield 70 should be a flex¬ ible material, such as conductive rubber. Referring now to Figure 4, the manner in which belt dosimeter 60 may be affixed around the object 78 is shown. It should be noted that object 78 need not be centered in aperture 80 formed by the coil 62 of dosimeter 60. Fur¬ ther, it should be noted that the aperture 80 need not be circular in shape. By utilizing the high resistivity leads and shieldinq, as previously discussed, the exact position of conductor 78 within aperture 80 is not critical. How¬ ever, it should be understood that the length of the coil 62, or its circumference when connected together as in Fiq- ures 2 and 3, should be less than three time the wavelength of the freguency being measured. Ideally, the circumfer¬ ence of coil 62 should be less than the wavelength of the frequency beinq measured, but experiments have shown that it is usable with acceptable error up to three times the wavelenqth frequency. Referrinq now to Figure 5, clamp-on ammeter 90 op¬ erating according to the principals of the subject inven¬ tion is shown. Ammeter 90 includes a coil 92 and a pair of leads 94 and 96 extendinq back from the ends 98 and 100 of coil 92. The ends 98 and 100 should be spaced apart so that the exitinq point for leads 94 and 96 are spaced from one another by an amount substantially the same as the dis¬ tance between adjacent turns on coil 92. Leads 92 and 94 may be buried beneath the flexible shield 99 of coil 92 and extend throuqh a main handle 102 from which they exit and are applied to a detector, such as the high impedance volt¬ age detector 24 shown in Figure 2. A second handle 104 is also provided and angularly spaced apart from main handle 102. When handle 104 is rotated toward handle 102, the ends 98 and 100 of coil 92 are separated, as shown in the dashed lines, and the conductor to be tested may be in¬ serted into the aperture 106 defined by coil 92. When han¬ dle 104 is released, the spring tension of coil 92 reposi- tions dosimeter 92 to the configuration shown by the solid lines of in Figure 5. Additional springs may be positioned between handles 102 and 104 " to assist in the closure of clamp—on ammeter 90. Referring again to Figure 1, one may use any of the dosimeters described with respect to Figures 2 through 5 to measure the current I flowing through the leg of patient 10. It can be expected that the current I through the leg of patient 10 will manifest the aberrant heating caused by the energy from applicator 12. In order to determine the occurrence of this aberrant heating, a dosimeter 14 is placed at various places along the extremities of person 10. As seen in Figure 1, dosimeter 14 is placed around the ankle of person 10. Other similar dosimeters may be placed at other places along the legs, arms or neck of person 10 to monitor the aberrant heating. When aberrant heating is found by noting a high current reading on detector 24, the physician or technician may adjust the position of the limbs, or change the f e— guency of enerqy from applicator 12 or chanqe the focal point of the applicator 12, energy. Rather than them using multiple dosimeters, the clamp-on dosimeter 90 shown in Figure 5 may be moved along each limb to monitor for aber¬ rant heating. Dosimeter 15 is placed around the abdomen of person 10, either as a belt type device shown in Figure 3, or as part of applicator 12 itself. Dosimeter 15 is intended to monitor the SAR in the region being treated rather than monitor aberrant heating. Where the reading on detector 24 is to low, it may indicate insufficient energy is being applied or that applicator 12 is not properly focused. If the reading from dosimeter is too high, the energy applied by the applicator 12 should be reduced.

Claims: 1. A current detector <14, 15, 22, 60 or 90 ) for de ¬ tecting high frequency currents (I) flowing through an ob ¬ j ect (10, 30 or 78), said detector (1.4, 15, 22, 60 or 90 ) including a conductor (32, 62 or 92) wound in multiple turns around a substantially closed loop core (34 or 64 ) , said loop (34 or 64) having an aperture (28, 80 or 106 ) qreater than the corresponding dimensions of said object (10, 30 or 78) to be measured, characterized by said con¬ ductor (32, 62 or 92) being a hiqh resistivity material, by said core (34 or 64) beinq non-ferromagnetic and by the ratio of said conductor (32, 62 or 92) turns spacing to said core (34 or 64) cross-sectional area beinq constant at any given point along said loop (34 or 64). 2. The invention according to claim 1 characterized in that the ends (42, 44) of said wound conductor (32, 62 or 92) are coupled through second high resistance conductors (46, 48 or 72, 74) to a voltage detector (24). 3. The invention according to claim 2 characterized in that said loop (34 or 64) includes a shield (36 or 70) of a high resistivity material. 4. The invention according to claim 1 characte ized in that said loop (34 or 64) includes a shield (36 or 70) of a high resistivity material. 5. The invention according to claim 3 or 4 character¬ ized in that said shield (36 or 70) includes a gap (38) around said loop (34 or 64). 6. The invention according to claim 3 or 4 character¬ ized in that said shield (36 or 70) includes a gap (38) around said loop (34 or 64) and said shield (36 or 70) is selected to eliminate capacitive coupling between said high resistivity conductor (32, 62 or 92) and said object (10, 30 or 78) . 7. The Invention according to claim 6 characterized in that said gap (38) extends entirely along said closed loop (34 or 64) . 8. The invention according to claim 7 characterized in that the ratio of the volume resistivity to the thickness of the shield (38) material is in the range of 1000 to 1 ,000,000 ohms. 9. The invention according to claim 6 characterized in that said shield (38) material is spaced from said wound high resistivity conductor (32, 62 or 92) by an amount to permit usable sensitivity and frequency response. 10. The invention according to claim 1, 2 or 4 charac¬ terized in that said loop (34 or 64) has a circumference of less than three times the wavelength of the frequency of the current (I) beinq detected. 11. The invention according to claim 1 or 4 charactei— ized in that the ends (42, 44) of said wound conductor (32, 62 or 92) are coupled through respective high resistivity leads (46, 48 or 72, 74) to a high impedance voltage de- tector (24) and in that a resistance (52) is coupled to one of said hiqh resistivity leads (46 or 48) . 12. The invention according to claim 11 characterized in that a rectifier (53) is coupled between said wound conduc¬ tor (32) and said high resistivity leads (46). 13. A detector (14, 15, 22, 60 or 90) for non-invasively determininq the specific absorption rate of hiqh frequency electromaqnetic energy in an animal body (10) subjected to such radiation by measurinq the electric current (I) flow- 5 inq through said body (10) as a manifestation of said spe¬ cific absorption rate, said detector (14, 15, 22, 60 or 90 ) beinq characterized by current detector means (14, 15, 22, 60 or 90), includinq a helical coil (32, 62 or 92) of elec¬ trical conductor material having two ends (42 and 44) and a - ■ - lead (46, 48 or 72, 74) extending from each end (42 and 44); by flexible means (34 or 64) for containing said cur¬ rent detector means (14, 15, 22, 60 or 90) and for being affixed around a portion of the body (10) so that said two ends (42 and 44) are substantially juxtaposed to one an- 5 other to form a closed loop coil (42 and 44) around said body (10) portion; and by means (24) coupled to said leads (46, 48 or 72, 74) for detecting the voltaqe induced in said coil (42 and 44) by the current (I) flowing throug said body ( 10) . 14. The invention according to claim 13 characterized in that said coil (34, 64) conductor material is of a hiqh resistivity material. 15. The invention according to claim 14 characterized in said coil (34, 64) includes a shield (36) therearound and in that the ratio of the volume resistivity to the thick¬ ness of the shield (36) material is in the range of 1000 to 5 1,000,000 ohms. 16. The invention accordinq to claim 13, 14 or 15 char¬ acterized in that said coil (34, 64) has an non-ferromag¬ netic core. 17. The invention according to claim 16 characterized in that said coil (34, 64) turns are spaced apart to maintain the ratio of the turns spacing to the coil (34, 64) cross- sectional area as a constant value. 18. The invention according to claim 13, 14 or 15 char¬ acterized in that said coil (34, 64) has a high resistivity shield (36) therearound. 19. The invention according to claim 18 characterized in that said shield (36) has a gap (38) therein extendinq en¬ tirely alonq said closed loop. 20. The invention accordinq to claim 19 characterized in that said shield (36) material is spaced from said coil (34, 64). 21. The invention accordinq to claim 18 characterized in that said loop has a circumference of less than three times the wavelength of said frequency of said radiation. 22. The invention accordinq to claim 13, 14 or 15 char¬ acterized in that said ends (42 and 44) of said coil are coupled throuqh respective hiqh resistivity leads (46, 48 or 72, 74) to a current detector (24); and in that a shunt - resistance (52) is coupled between said respective hiqh resistance leads (46, 48 or 72, 74). 23. The invention accordinq to claim 22 characterized in that said current detector (24) has a high impedance. 24. A method for detecting aberrant heating in an animal body (10) undergoing an application of electromagnetic en¬ ergy characterized by the steps of placing a current de¬ tecting probe (14, 15), which does not significantly pei— 5 turb said electromaqnetic energy, around at least one por¬ tion of said body (10); by monitoring said probe (14, 15) for an indication of voltaqe induced in said probe (14, 15) in excess of a certain threshold as manifestation of aber¬ rant heatinq; and by affecting the application of said en- 0 ergy to reduce said aberrant heating, in response to said indication. 25. The method according to claim 24 characterized in that said step of affecting includes altering the position of said body ( 10) . 26. The method according to claim 24 characte ized in that said of affecting includes changing the freguency of said energy. 27. The method according to claim S4 characterized in that said step of affecting includes moving the point of application of said energy. 28. The method according to claim 25, 26 or 27 charac¬ terized in that said step of placing includes encompassing said portion (10) with high resistivity coil (32, 62 or 92), and said step of monitoring includes reading the volt- ~ aqe induced in said coil (32, 62 or 92). 29. The invention accordinq to claim 25, 26 or 27 char¬ acterized in that said step of placinq includes placing probes (14, 15) at multiple positions on said body (10).

Inventors: BABIJ TADEUSZ M, HAGMANN MARK J, BABIJ, TADEUSZ, M., HAGMANN, MARK, J.
Applicants: UNIV FLORIDA INT, FLORIDA INTERNATIONAL UNIVERSITY
Application year: 1988
Main classification: A61B5
IPC codes: A61B5