Multi-Directional Excitation System and Clinical Coil Model for Soft-heating Hyperthermia

Abstract

The soft-heating method induces hyperthermia with elongated heating elements implanted in the body. The element produces heat under high-frequency magnetic field generated by an excitation coil placed outside the body. The element has thermo-sensitive ferrite in a metal loop. Notably, the temperature of the element does not exceed the Curie point, providing an automatic mechanism that limits the heating temperature and enables safe treatment. The heat output of the element depends on magnetic field strength, excitation frequency, and the angle between the element and the incident magnetic field. For example, heating becomes largest when the incident magnetic field is parallel to the long axis of the element. However, the heating area of an element is limited, and multiple elements must be implanted to target a tumor from various angles. Excitation using a solenoid coil or a flat coil is only in one direction, but magnetic field needs to be generated in multiple directions. We use two coils because magnetic field sources in at least two directions allow for a multi-directional magnetic field. It is well known that a current phase difference of 90° between two coils results in a rotating magnetic field. However, induced current might damage the power supply if inductive coupling occurs between the two coils. In addition, a coil without electromagnetic coupling is difficult to apply to the human body. Accordingly, we propose a new method for realizing a multi-directional magnetic field, as well as a clinical coil model. The method uses two excitation frequencies, and a multi-directional magnetic field is obtained at a certain position. Additionally, an important characteristic is that the power supply is safe even if electromagnetic coupling occurs in the coil model. Using this technique, we successfully heated elements placed in various directions.