Visualization by a matrix of light-emitting diodes of interference effects from a radiative four-applicator hyperthermia system.

A simple, unique tool has been constructed at a very low cost for visualization of the electric field distribution in a phantom exposed to radiofrequency fields from a four-applicator hyperthermia system. The tool consists of a thin Perspex plate to which light-emitting diodes (LEDs) have been attached in a regular pattern. The leads of these LEDs have been bent in the form of dipoles. Electric field distributions can be made visible, because the light intensity from an individual LED dipole in a RF field is a function of the local electric field strength. The LED dipoles were arranged in a 11 x 15 matrix with a mutual distance of 2 cm. Experiments have been performed in a cylinder with a length of 100 cm and with an elliptical cross-section of 24 cm x 36 cm. The cylinder was filled with saline to simulate the electric properties of biological tissue. The matrix plate was shaped like the cross-section inside the phantom, and could be moved to any position under the externally applied antennae. In our department the applicators installed form a system of four coherent waveguide radiators operating at a frequency of 70 MHz. Photographs have been taken from the distribution of the light intensity through transparent windows at the outer ends of the phantom. The advantage of the LED matrix is the fact that the effect of changing phase relations, amplitudes or position of any applicator on the electric field distribution can be seen instantaneously. The interference effects of multiple fields, i.e. extinction and reinforcement, become easily visible. Typical examples demonstrate the necessity to obtain the field distribution not only along some previously chosen lines, but in the whole transversal midplane. The attenuation of field strength due to the presence of the matrix varies only slightly from the edge to the centre of the matrix, so the pattern of light intensity is in good qualitative agreement with the pattern of field strength. The LED matrix has been very useful in the verification of pretreatment planning calculations, and is already used for intercomparison and evaluation studies of similar hyperthermia equipment.