Design and experimental verification of thin acoustic lenses for the coagulation of large tissue volumes

Large focal volumes are desired in ultrasound surgery to reduce the total treatment time when large tumours are thermally coagulated. Phased arrays are capable of producing enlarged focal volumes in addition to providing the ability for on-line modification of focal shape and location. Although phased arrays have several advantages over their non-phased counterparts, the complexity of these arrays also presents some disadvantages regarding cost and complexity. One less costly alternative is the use of thin acoustic lenses to alter the field shape of a single-focus transducer. Four polystyrene lenses have been designed using the sector-vortex principle developed for phased arrays by Cain and Umemura. Measurements of the acoustic fields produced with the lenses are in good agreement with the simulated fields. The transmission measurements through each of the four lenses ranged from 76% to 84%, and over 52 W of total acoustic power has been delivered through each of the lenses during in vivo experiments without any damage to the lenses or the transducer. The in vivo results showed an increase in rate of necrosis to 10.1 +/- 1.4 cm(3) h(-1) using the mode 4 lens, or 5.2 +/- 0.7 times higher than the focused transducer alone.