Gain Optimization of a Near-Field Focusing Array for Hyperthermia: Theory and Experiment

170 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1986.A new variation of the array gain optimization problem has arisen in the study of microwave arrays used for hyperthermia, the heating of biological tissue. For a given array configuration and arbitrary medium, we show how to maximize the (VBAR)(')E(VBAR)('2) power deposition at the observation point in the near field of the array, analogous to the loss- less case of radiation maximization in a particular direction. In this formulation the medium need be neither homogeneous nor lossless, and the array elements need not be identical. Some practical cases are simulated with both lossy and lossless media showing how a closed-form solution gives results that closely follow the optimum when the dominant polarization from linearly polarized radiators is maximized. A comparison is also made with the conjugate-field or time-reversal excitation, a scheme which is slightly less efficient in most cases and which causes significantly higher focal shifts for some off-axis scan situations. In a medium as lossy as water, the field strength decays away from any practical array.A seven-element array of dielectric-loaded open-ended wave- guides totally immersed in a water tank is tested as a possible hyperthermia applicator. Experimental results show the substantial focusing ability of the array if a conjugate matching scheme is used to adjust the phase of each element excitation. This scheme offers a practical procedure for operating a focused hyperthermia appli- cator in a living patient. Computed power patterns are fitted to experimental data.A new geometrical optics formulation is derived for refraction at a planar interface between arbitrarily lossy media. This and other ray methods are compared against the evaluation of the exact Sommerfeld integral for observers in various lossy dielectrics adjacent to free space containing a point source. As the loss increases, only the new method gives useful results.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD