Assessment of Time Reversal Methods Used For RF Hyperthermia in Cancer Treatment
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Radio frequency (RF) hyperthermia, for which cell temperatures are increased to 41-45 °C, is a well-recognized method for treatment of malignant tumors. However, it suffers from the lack of spatial selectivity towards the target when non-invasive external sources are used. An optimized hyperthermia method, time reversal (TR) technique, is introduced to enhance specific heating on the target while reducing the non-specific heating elsewhere. TR is applied to electromagnetics (EM) recently due to its intrinsic temporal and spatial focusing properties coming from the linear reciprocity of the Maxwell’s equations. It utilizes the time reversal mirror (TRM) to record the EM waves emitted from a source and reemit the time-reversed version back to the source. By collecting the scrambled information dispersed in the reflection and scattering, the original source signal is rebuilt and a high-intensity focusing is observed at the source location. Owing to its focusing properties, EM TR has been applied in areas such as wireless communication and geophysics. In this dissertation the possible application of TR in hyperthermia is assessed and investigated. A simple analytical model is proposed to analyze the TR problem in a pure EM way. The model converts the spatial focusing problem into a constrained optimization based on an asymptotic study and provides information for the choices of signal types, frequencies, bandwidths, and number of antennas. In addition, two non-invasive focusing methods are discussed. The first approach is to mark the target with an active virtual source. The real signals used for selective heating are obtained from the simulation on the personalized permittivity map generated from MRI and CT. The other way is to place one or more passive nonlinear element, such as a ferromagnetic material, instead of an active source to spatially encode the tumor location. The feasibility of these two methods is evaluated by simulations in CST MICROWAVE STUDIO® for a reverberating system. The follow-up experiments conducted in the same environment clearly demonstrates the intensity and selectivity of the heating effects from the EM TR techniques.