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An extensive comparison between measured and computed heat absorption distributions in several phantoms inside the 'coaxial TEM' applicator is presented. The influence of the width of the aperture and of the presence of a PVC tube is especially considered.

Based upon a capacitive system a quasi-microwave cavity operating at a frequency of 13.56 or 27.12 MHz has been developed. The prototype consisted of two capacitive plates with a circular aperture at the centre of each plate in which a cylindrically shaped tissue volume can be placed. Phantom measurements showed that a second-generation applicator, consisting of two narrow rings with equal inner and outer diameter, gave identical results. Due to the positioning of the rings along the enclosed tissue cylinder, the electrical field is mainly parallel to the body axis. SAR distributions were measured by infrared thermography in cylindrical, muscle equivalent phantoms enclosed in PVC-tubes using the 'split phantom' technique. For phantom diameters up to 13.5 cm a homogeneous heating, SAR 70-100 per cent of the maximum SAR, has been obtained over the tissue volume between the inner edges of the two rings. For these measurements the non-isolated ring electrodes are placed directly onto the PVC cover. When the phantom diameter, excluding PVC cover, is increased to 22.5 cm the SAR values at the centre vary from 30 to 40 per cent of the maximum SAR value which is located near the inner edge of the rings. In this case a 1 cm gap between the rings and the PVC cover was used in order to reduce the intensity of the hot spots. In all experiments no cooling of the phantom surface or ring electrodes has been used. The results from this initiating study indicate the feasibility of this type of applicator for regional deep heating, although more experimental work is needed when the applicator is used to heat tissue bodies with a diameter larger than 13.5 cm. Already, the applicator in its most simple design may be of clinical value for hyperthermic treatment of tumors in arms or legs.

A rigorous three-dimensional electromagnetic model predicting the complete field distribution in the space interior to the 'coaxial TEM' applicator is presented. The applicator consists of a radiating ring aperture, with a given electric-field distribution in the wall of a hollow circular cylinder. The method of calculation employs the spatial Fourier transform of all field quantities with respect to the axial co-ordinate, after which the field equations are solved in the spectral domain. Subsequently, an inverse Fourier transform is carried out to compute the quantities that are of interest to the optimum clinical deep-body hyperthermia system. Numerical results for a number of representative applicator configurations are given. The present results are compared with the ones that are obtained by a model based on the far-field approximation of a distribution of dipole sources located in the ring aperture.

The computational model developed by van den Berg has been used to perform two-dimensional simulations of tissue-equivalent phantoms heated by a 27 MHz ridged waveguide. The program is able to calculate temperature distributions for realistic inhomogenous tissue configurations (as derived from CT scans) in about 20 min on a PDP 11-44. Results for tissue-equivalent phantoms are evaluated with respect to their accuracy in predicting actual temperature distributions by comparing them with measured temperature distributions. In general the computer simulation tends to overestimate the depth of penetration; this can be ascribed to the two dimensional approach followed which does not take into account the divergence of the electric field in the third dimension. This effect can to a certain degree be compensated by assigning a higher value for the electrical conductivity of the medium. For homogenous phantoms this results in a satisfactory agreement between measured and calculated temperature distributions. Furthermore, a correct approximation for the electric field distribution over the aperture appears to be of importance, since it significantly influences the calculation results, especially for waveguide loads whose size exceeds the width of the ridge area. For inhomogeneous media the simulation results, using realistic (literature) values for the electrical conductivity, can be used with these limitations in mind, as a best-case estimate of the absorbed power distribution at depth. For more accurate simulations a three-dimensional computer model is ultimately needed.

The electromagnetic heat dissipation in a radially layered biological tissue inside a circular cylinder has been investigated theoretically. The theory is based on a three-dimensional model and the electromagnetic field is assumed to be generated by a prescribed electric field along a ring-shaped aperture. The method of computation employs the spatial Fourier transform of all field quantities with respect to the axial coordinate, after which the field equations are solved in the spectral domain. Subsequently, an inverse Fourier transform is carried out to compute the quantities that are of interest to the clinical deep-body hyperthermia system at hand. For a number of representative configurations numerical results at 70 MHz are given.