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SAR distributions were measured in the CDRH phantom, a 1 cm fat-equivalent shell filled with an abdomen-equivalent liquid (sigma = 0.4-1.0 S m-1; dimensions 22 x 32 x 57 cm) to demonstrate the feasibility of the ring applicator to obtain deep heating. The ring electrodes were fixed in a PVC tube; diameter 48 cm, ring width 20 cm and gap width between both rings 31.6 cm. Radio-frequency energy was fed to the electrodes at eight points. The medium between the electrodes and the phantom was deionised water. The SAR distribution in the liquid tissue volume was obtained by a scanning E-field probe measuring the E-field in all three directions. With equal amplitude and phase applied to all feeding points, a uniform SAR distribution was measured in the central cross-section at 30 MHz. With RF energy supplied to only four adjacent feeding points (others were connected to a 50 omega load), the feasibility to perform amplitude steering was demonstrated; SAR values above 50% of the maximum SAR were measured in one quadrant only. SAR distributions obtained at 70 MHz showed an improved focusing ability; a maximum at the centre exists for an electric conductivity of the abdomen-equivalent tissue of 0.6 and 0.4 S m-1.

An efficient iterative method for solving quasi-static electromagnetic field problems is presented. A relaxation function is introduced in the quasi-static field equations. Then, the resulting equations can be solved by iteration. The method is similar to the one of solving a Laplace equation by computing the stationary state of a diffusion equation. Next, for a radially layered configuration the numerical results are compared with the results from an existing integral equation method. Subsequently, for a realistic three-dimensional model of a human knee numerical results are arrived at.

The energy deposition pattern within a radially layered fat-muscle phantom, diameter 135 mm, heated by a novel ring capacitor applicator has been determined experimentally as well as theoretically. Good to excellent agreement is found between measured and predicted energy distributions. For the specific absorption rate in the muscle tissue the differences are in general smaller than 6%. When the ring electrodes are placed directly on the phantom surface both measured and predicted energy distributions show the presence of superficial hot spots located within the fat layer at the site of the ring electrodes. The theoretical distributions showed that the radial component of the E-field contributes for more than 90% to the energy absorption at the hot spot in the fatty tissue in front of the ring electrodes. Introducing a small air gap (10 mm) between the phantom surface and the ring electrode results in a decrease of the energy absorption within the fatty tissue at the hot spot location by 30%. Further theoretical analysis of the energy distribution within the inhomogeneous model showed that the intensity of the hot spots at the ring electrodes can be controlled by adjustment of the applicator configuration. Independent of the size of the electrode to phantom gap the specific absorption rate values predicted in the fat-muscle model show a more favorable distribution at a frequency of 27.12 MHz than at 13.56 MHz. For a similar electrode to phantom gap the specific absorption rate within the fatty tissue is approximately two times lower at 27.12 than at 13.56 MHz. For the model calculations performed the best ratio of fat to muscle SAR (0.2) is obtained with distilled water as bolus medium in the gap.

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 ring capacitor (RC) applicator has been investigated. A quasi-static model is introduced to compute the relevant electromagnetic field quantities. The method of computation employs the spatial Fourier transform of all field quantities with respect to the axial coordinate. After an iterative solution of a dual boundary value problem for the electric potential and the current density at the electrodes, an inverse Fourier transform is carried out to compute the quantities that are of interest to the deep-body system at hand. Comparison of numerical results with phantom measurements shows excellent agreement.

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.