Concentric-ring phased arrays subdivided into sectors (radial slices) can, with appropriate phasing, produce variable diameter annular (or ring) focal regions. Such focal rings can be effective in heating tumors if they are directed around the tumor periphery. Phasing methods which produce focal ring diameters over a wide range are developed. Scanning the focal ring diameter and depth provides considerable heating flexibility. Appropriate phasing of the rings and sectors can produce patterns which are circularly asymmetric. By controlling these asymmetries, nonspherical tumors can be heated more optimally.
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