Scanning E-field sensor device for online measurements in annular phased-array systems

Purpose: A measurement device for noninvasive and simultaneous control of a ntennas during regional radiofrequency (rf) hyperthermia and, subsequently, the estimation of the power distribution in the interior of patients are e ssential preconditions for further technological progress. Aiming at this, the feasibility of an electro-optical electric field sensor was investigate d during clinical rf hyperthermia. Material and Methods: The electro-optical electric field (E-field) sensor i s based on lithiumniobate crystals and the Mach-Zehnder interferometer stru cture, and was tested in an earlier phantom study. For this study, a mechan ical scanning device was developed allowing the registration of the E-field during clinical application. Data were recorded along a curve in the water bolus of the SIGMA 60 applicator of the annular phased-array system BSD-20 00 (BSD Medical Corp., Salt Lake City, UT) close to the base points of the hat biconical dipole antennas. The results were compared with modeling calc ulations using the finite-difference time-domain (FDTD) method. For the lat ter, different antenna models were assumed. For systematic registration of the E-field curves in amplitude and phase, we employed an elliptical lamp p hantom with fat-equivalent ring (filled with saline solution) and an ellipt ical polyacrylamide phantom with acrylic glass wall. Further measurements w ere carried out during the treatment of 5 patients with 20 hyperthermia tre atments. Results: Data of both phantom and patient measurements can be satisfactoril y described by the FDTD method, if the antenna model is refined by taking i nto account the conical form of the dipoles and the special dielectric envi ronment of the feeding point. Phase deviations can be entered ex posteriori for correction in the calculation algorithm. A comparison of amplifier pow er measurement (forward and backward power) and bolus E-field scans near th e antenna base points demonstrates that E-field measurements between antenn as and patient are a necessity for the appropriate characterization of ante nna radiation properties. These measurements are sensitive to variations of the lossy medium in position and shape, and can be correctly predicted wit h current models. However, the differences between different patients are m oderate and unspecific in both calculations and measurements, with fluctuat ions at maximum of 30 degrees in phases and 40% in amplitudes. Conclusions: The measurement method presented here turned out to be a pract ical tool for online registration of E-fields in phases and amplitudes alon g arbitrary curves in a water bolus or phantom. It can be utilized to evalu ate antenna design and modeling calculations and leads, thus, to a better u nderstanding of complicated multiantenna systems. In clinical routine, it c an be employed as input for patient-specific hyperthermia planning and, fin ally, for the realization of online control with subsequent optimization of the power distribution in the patient. (C) 1999 Elsevier Science Inc.