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

In hyperthermia induced by electromagnetic applicators, one way to obtain i nformation about the energy absorption is by measurement of electric field strength (E). This paper describes a system which can measure E-distributio ns, using a two-dimensional array of diodes. It was designed to be used on patient skin, during hyperthermia treatments of superficial tumours, provid ing additional data for applicator power control. The first prototype consi sts of a sheet with Schottky diode sensors with a spacing of 2.5cm, connect ed to high-resistance leads, printed with carbon ink. The rectified diode v oltages 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 d istributions obtained with 64 sensors are updated within is. Phantom measur ements, performed with the sensor matrix under a Lucite Cone Applicator (LC A) and under a 2 x 2 array of Current Sheet Applicators (CSAs) were compare d with infrared measurements of the temperature rise after a short power pu lse. A fair agreement was found between the square of the diode voltages an d the infrared distributions. Movement of a single CSA over the sensor matr ix can be visualized clearly by the system. The diode matrix E-field measur ement system is sufficiently fast and accurate to give valuable feedback fo r power steering for an array of LCAs and CSAs. The system has the potentia l of being a helpful tool in other fields of quality assurance as well.