Radiofrequency capacitive heaters: the effect of coupling medium resistivity on power absorption along a mouse leg.

A capacitive radiofrequency source in conjunction with a temperature-controlled electromagnetic coupling medium has the potential of delivering uniform heating distributions in a mouse leg for experimental studies to investigate the use of hyperthermia as a treatment for cancer. The system has been adopted by a number of groups who have confirmed that uniform temperatures can be achieved in the presence of blood flow along a one-dimensional line which extends between the plates across the leg. In this paper, a simple mathematical model is presented and verified experimentally to demonstrate that parallel-plate capacitive radiofrequency heaters produce an inherent absorbed power distribution along the leg which is determined by the impedances across the loads. Hence, the thicker or thinner regions of the leg can be preferentially heated by using a coupling medium with lower or higher salinity, respectively. Uniform power absorption along the mouse leg required that the coupling medium had equivalent electrical properties to those of tissue.

Development of an inductive, non-invasive RF applicator for studying hyperthermia in a rat brain tumour model.

A non-invasive inductive RF applicator has been designed specially for local heating of 6 mm diameter tumours growing in the brains of F344 rats (144-148 MHz). It is shown that an inductive applicator is the choice for this particular case. The power deposition pattern is analysed by modelling the applicator with a three-dimensional array of magnetic dipoles. The total hyperthermia system is briefly described and results of phantom and in vivo experimental measurements are presented. These confirm the theoretical analysis.

Two-point control of temperature profile in tissue.

A strategy for controlling the temperature profile in the tissue with a single applicator hyperthermia system is described. By manipulating the cooling water temperature as well as the heating power, the tissue temperatures in two selected locations can be controlled. By proper choice of these two locations and the corresponding temperature set-points, a temperature maximum can be obtained in a fairly superficial tumour. If the tissue composition and consequently the temperature distribution is fairly regular, a temperature profile above 43 degrees C in the tumour and below that in normal tissue can be obtained along an axis perpendicular to the surface. The controller is self-tuning and provides dynamic decoupling, bumpless transfer and anti-reset windup. Test of the controller by simulation and on a phantom indicates it is superior to the single point controller currently used.