Hyperthermia by Magnetic Induction: Experimental and Theoretical Results for Coaxial Coil Pairs

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Abstract
High-frequency magnetic induction for producing elevated temperatures in human tumors can be accomplished with several electrode arrangements. The physical characteristics of a coaxial pair of electrodes or current loops placed on either side of the body are described. This electrode arrangement can improve the depth of power deposition within the body relative to that obtained with an electrode circumferentially encircling the body or a single surface applicator (pancake coil). Results of measured and calculated magnetic field distributions and power density distributions obtained with a coaxial pair are reported. The effect of varying load geometry on the power density distribution was significant in several static phantom experiments. Electrode geometry was also important, as shown by calculations of power deposition as a function of electrode diameter in a sample model of body tissue conductivities. The power density diminished at given depths as the electrode diameter decreased. Values of magnetic fields were measured during treatment of human subjects with a coaxial electrode pair and varied significantly from subject to subject. This resulted from variation in the geometry and electrical characteristics of tissues among subjects and indicates the difficulty of formulating a standard accurate model for use with this magnetic induction technique.