A sector-vortex phased array capable of generating directly annular-shape foci is analyzed. By driving the sectors of the array with signals whose phase rotates M times around the circular track, annular foci with the same acoustic-signal-phase rotation are produced in the geometrical focal plane of the array. Because of this phase modulation around the focal annuli and the resulting high-spatial-frequency content, the produced acoustic fields are free from secondary foci both behind and in front of the focal plane. The diameter of the focal annuli can be increased by increasing the mode number M. By providing the array with multiple tracks, it is possible to get larger focal annuli than with a single track with the same mode number M. It is also possible to achieve some control of the power deposition patterns in the depth direction in this way. Using a dual track sector-vortex array with practical ranges of the aperture size and number of elements, acoustic power deposition patterns capable of heating the peripheral and central regions of a nonsuperficial tumor a few centimeters in diameter are obtained by computer simulation.
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