In this paper the possibility of using phased array generated multiple-focus patterns to reduce the overall treatment time in ultrasound surgery while restraining prefocal heating was investigated. This was done by comparing through computer simulation the performance of different possible schemes, i.e., single-focus scans, multiple-focus scans, and simultaneous multiple focusing without scanning, when used to "ablate" a 10/spl times/10/spl times/10 mm/sup 3/ tissue volume 100 mm deep. In all cases, 41 foci were used to cover the treatment volume. Multiple-focus scans were arranged into nine groups which were scanned in a raster fashion, as with single-focus scans. Keeping the treatment time constant, the maximum intensities, maximum thermal doses, dose distributions, and prefocal necrosis zones for the different schemes were compared. It was found that the nonscanned simultaneous multiple-focus case required the smallest maximum intensity and dose, and resulted in the most even dose distribution. Single-focus raster scanning of individual lesions, as currently used with fixed-focus transducers, gave the worst results. These results show that multiple-focus patterns help considerably in reducing the maximum intensity and dose, and in generating a more even dose distribution assuming the same treatment time and prefocal heating. Alternatively, multiple-focus patterns can be used to significantly reduce treatment time while keeping the maximum intensity and prefocal heating below predetermined limits.
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