Experience with a Multitransducer Ultrasound System for Localized Hyperthermia of Deep Tissues

A system employing six planar ultrasound transducers has been utilized for preclinical and pilot clinical studies with the aim of producing therapeutic heating preferentially at depth. The array consists of six 7 cm diameter PZT-4 disks mounted on a spherical shell section with a 26 cm radius of curvature. The crystals operate at different frequencies a few kilohertz above their fundamental frequencies of approximately 350 kHz for near-field peak suppression, and each has a few percent modulation to minimize standing wave effects. In water, the system can be focused to produce a high intensity region near the isocenter with a full width half maximum of approximately 1.5 cm in all directions. In attenuating tissue, the high intensity region is closer to the array of transducers by a few centimeters. For heating of realistic tumors at depth, small wedges are used to rotate the transducer axes a few degrees away from the radial direction, yielding a waist rather than a single point where the individual beam central axes come closest to each other. The waist is 3-6 cm in diameter, producing, ideally, ellipsoidal shaped temperature distributions centered deep in perfused tissue. Quantitative power deposition profile mapping, as well as qualitative studies using liquid crystal sheets, have been performed in water phantoms to characterize the system for different transducer orientations.