Hyperthermia by magnetic induction: experimental and theoretical results for coaxial coil pairs.

High-frequency magnetic induction for producing elevated temperatures in human tumors can be accomplished with several electrode arrangements. We describe the physical characteristics of a coaxial pair of electrodes or current loops placed on either side of the body. 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"). We report results of measured and calculated magnetic field distributions and power density distributions obtained with a coaxial pair. The effect of varying load geometry upon the power density distribution was shown to be 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 simple model of body tissue conductivities. The power density was found to diminish at given depths as the electrode diameter decreased. Finally, values of magnetic fields were measured during treatment of human subjects with a coaxial electrode pair and were found to vary 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.