Hyperthermia by magnetic induction: I. Physical characteristics of the technique

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      Several electromagnetic techniques are currently used in hyperthermia therapy for cancer. This report discusses the magnetic induction technique, in which application of an alternating magnetic field to a conductor results in induction of eddy current flow and power deposition via ohmic losses. The power density in tissues depends upon the heterogeneous tissue conductivities and dielectric constants, magnetic field intensity and distribution, and eddy current radius. Using a newly developed magnetic field probe, the magnetic fields produced at 13.56 MHz by electrodes of a commercial magnetic induction device have been accurately mapped and used to calculate power densities in static phantoms. Calculated and observed temperature elevations in a cylindrically symmetric phantom agree well, confirming the simple formula in this case relating power density to magnetic field strength. The efficiencies of three commercially available electrodes have been accurately measured using calorimetric techniques and phantom loads modeling human anatomy and electrical conductivity. The effect of eccentric positioning of the load has been studied using magnetic field mapping techniques. Power densities in a very simplified model of human anatomy that retains cylindrical symmetry were calculated. Effects of inhomogeneities in conductivity have been investigated qualitatively using composite static phantoms modeling human cross-sectional anatomy and thermographic camera recordings of surface temperature distributions. The advantages and disadvantages of this heating technique are discussed from the point of view of the power density distributions in heterogeneous materials. Evaluation of power density distributions is essential for optimizing this heating technique using various electrode arrangements and/or load modifications, for providing part of the information needed for solution of the bioheat transfer equation in living subjects, and for predicting the ability of the technique to heat specific tumors.


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