An alternating focused ultrasound system for thermal therapy studies in small animals

Purpose: To develop an alternating focused ultrasound system (AFUS) for preclinical studies of thermal and acoustic responses of tumors in small animal models. This work was motivated by the need of noninvasively creating relatively small spheroidal thermal lesions in small targets (e.g., a murine tumor) without damaging the surrounding tissues. Methods: The AFUS consists of two lead zirconate titanate (PZT-4) spherically curved ultrasound transducers with focal zones crossing each other at a 90 angle. The transducers were independently powered following a programed alternating firing scheme. Before the device design and construction, an acoustic and biothermal model was developed to simulate the ultrasound pressure field and the resulting temperature and thermal dose distributions. A shape factor, sphericity, to quantify the roundness of the lesions was calculated based on the 240 equivalent minutes at 43 degrees C thermal dose contours. A prototype of the AFUS was constructed with two identical transducers of an operating frequency of 2.25 MHz, 38 mm in diameter, and F-number equal to 1.33. To evaluate the performance of the AFUS experimentally, a series of heating in polyacrylamide phantoms, ex vivo porcine liver tissues, and in implanted mouse tumors fibrosarcoma (FSaII) in vivo was conducted. In these experimental cases, the sphericity was calculated and compared based on the visible lesion (a marked change in coloration). Results: As shown in the simulations, the lesions induced in polyacrylamide phantoms, ex vivo porcine liver tissues, and in vivo mouse tumors, the sphericities of the lesions yielded by AFUS heating were approximately 50% higher than those of single focused ultrasound heating as long as moderate intensities were used and the duty cycle pulses were distributed equally among the transducers. Conclusions: The AFUS is a device capable of noninvasively creating spheroidal thermal lesions in small targets such as murine tumors. (C) 2011 American Association of Physicists in Medicine. [DOI:10.1118/1.3553405]