NONINVASIVE PREDICTION OF SAR DISTRIBUTIONS WITH AN ELECTROOPTICAL E-FIELD SENSOR

An integrated electro-optical (eo) E field sensor is developed on the basis of a Ti:LiNbO3 Mach-Zehnder interferometer. A measuring device b ased on the lock-in principle is introduced to register the E field in phase and amplitude using this E field probe. Segmented electrodes ar e used to minimize influences from the dielectric surroundings on the base point capacitance of the receiving dipole. The operating point is stabilized against drift phenomena resulting from optical damage and pyroelectric effect. Sensitivity, dynamic range, harmonic distortions and mechanical properties of a prototype of this electro-optical E fie ld sensor are evaluated. A phantom setup in the SIGMA-60 applicator wa s developed to test this electro-optical sensor for hyperthermia appli cations. Power deposition patterns of various standard adjustments of the SIGMA ring are visualized in an elliptical lamp phantom. Simultane ously, E field in phase and amplitude is determined on a closed curve in 10 degrees steps around the phantom in a substitute bolus. The numb ers are stored and utilized as boundary conditions in a two-dimensiona l finite elements code which calculates the SAR distribution on an app ropriate triangular grid inside the closed curve. An excellent qualita tive agreement is obtained between visualized and calculated SAR patte rns. This novel measurement method is therefore suitable for noninvasi ve monitoring of SAR patterns during clinical application of regional radiofrequency hyperthermia.