Ultrasound allows precise focusing of power for hyperthermia. However, simple focused ultrasound beams are too small (1-2 mm full width at half maximum) to heat a tumor volume unless scanned. The authors have successfully reconstructed complex foci with a single transducer coupled to a lens. The lenses are fabricated on a computer-controlled milling machine from cross-linked polystyrene. Using a lens allows many more elements (500-2000) than currently available in two-dimensional arrays. Using a polystyrene lens coupled to a 2.3-MHz transducer, the authors have created a complex focus consisting of 18 focus points in a 1-cm cube (9 points each at 5.0 and 6.0cm from the transducer). Controlling the phases of the individual focus points increases the intensity in the focal plane and reduces hot spots away from the focus. Temperature distributions measured in tissue-equivalent phantoms for the lens system agree well with predictions of thermal modeling.
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