Self-focusing HIFU source for large therapy volumes

In medical thermo-therapy with ultrasound, extended focusing sources generate small focal spots with ultrasonic intensities high enough to generate fine tissue necroses. These focal spots are scanned over the target tissue volume. Scanning time in the therapeutic procedure can be reduced by the lateral enlargement of these spots. Multielement ultrasonic sources do this job on the basis of expensive multichannel electronics. The authors' HIFU system, however, bases on a single element 60 mm diameter piezoelectric source mounted onto a spherical lens with a focal distance of 70 mm and a focal width (-6 dB) of 1.5 mm. Center frequency is 1.7 MHz. This system, source and lens together, is diced into 4 sectors and remounted at radial non-symmetric locations. Computer simulations under the condition that the prefocal spots were not enlarged defined the proper location of the sectors. The asymmetry with respect to the location is /spl plusmn/0.4 mm. The resulting theoretical acoustic field pattern for one-channel driving electronics shows a focus split into 4 peaks representing the broadening into a width of about 3 mm. This system will generate a clear time reduction for the therapeutic procedure without increasing technological and electronic expenditure.