An electric field measurement system, using two-dimensional array of diodes.

In hyperthermia induced by electromagnetic applicators, one way to obtain information about the energy absorption is by measurement of electric field strength (E). This paper describes a system which can measure E-distributions, using a two-dimensional array of diodes. It was designed to be used on patient skin, during hyperthermia treatments of superficial tumours, providing additional data for applicator power control. The first prototype consists of a sheet with Schottky diode sensors with a spacing of 2.5cm, connected to high-resistance leads, printed with carbon ink. The rectified diode voltages are passed through a multiplexer unit to an AD-DA card in a PC. The sensitivity of the sensors is linearly proportional to the electric field and to the length of the extended diode connection pins. Relative E-field distributions obtained with 64 sensors are updated within 1s. Phantom measurements, performed with the sensor matrix under a Lucite Cone Applicator (LCA) and under a 2 x 2 array of Current Sheet Applicators (CSAs) were compared with infrared measurements of the temperature rise after a short power pulse. A fair agreement was found between the square of the diode voltages and the infrared distributions. Movement of a single CSA over the sensor matrix can be visualized clearly by the system. The diode matrix E-field measurement system is sufficiently fast and accurate to give valuable feedback for power steering for an array of LCAs and CSAs. The system has the potential of being a helpful tool in other fields of quality assurance as well.