A 256-element phased array has been designed, constructed, and tested for ablative treatment of large focal volumes of deep seated tissue. The array was constructed from a 1.1-MHz, 1-3 composite piezoelectric spherical shell with a 10-cm radius of curvature and a 12-cm diameter. The array was tested to determine its electroacoustic efficiency and inter-element coupling under high acoustic power conditions. A series of in vivo porcine experiments demonstrated the ability to produce deep seated tissue lesions in thigh muscle using the large scale phased array. The array was used to heat and coagulate tissue volumes >5 cm(3) in a single ultrasound exposure using multiple foci and temporally scanned power deposition patterns. The spatial and temporal experimental results for large, heated focal volumes correlated very well with the simulated temperature response model for homogeneous tissue. A 25-cm(3) tissue volume was coagulated in a 90-min period using overlapping large ultrasound exposures.

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Temporal switching has been simulated and implemented in vivo experiments as a method to optimize thermal dose in ultrasound surgery. By optimizing the thermal dose over a tissue volume, the peak temperature is decreased, less average power is expended, and overall treatment time is shortened. To test this hypothesis, a 16 element, spherically sectioned array has been constructed for application in ultrasound surgery guided by magnetic resonance imaging. A simulation study for the array was performed to determine an optimal treatment from a set of multiple focus fields. These fields were generated using the mode scanning technique with power levels determined numerically using a direct weighted gradient search in the attempt to create an optimally uniform thermal dose over a 0.6x0.6x1.0 cm(3) tissue volume. Comparisons of the switched fields and a static multiple focus field indicate that the switching technique can lower power requirements and decrease treatment time by 20%. More importantly, the peak temperature of the sonication was lowered 13 degrees C, thus decreasing the possibility of cavitation. The simulated results of the 16 element array were then experimentally tested using MRI to noninvasively monitor temperature elevations and predict lesion size in rabbit thigh muscle in vivo. In addition, the results show that the switching technique can be less sensitive to tissue inhomogeneities than static field sonication while creating contiguous necrosis regions at equal average powers.

A driving system has been designed for phased array ultrasound applicators. The system is designed to-operate in the bandwidth 1.2 to 1.8 MHz, with independent channel power control up to 60 W (8 bit resolution) for each array element. To reduce power variation between elements, the system utilizes switching regulators in a feedback loop to automatically adjust the DC supply of a class D/E power converter. This feedback reduces the RF electrical power variation from 20% to 1% into a 16 element array. DC-to-RF efficiencies close to 70% for all power levels eliminates the need for large heat sinks. In addition to power control, each channel may be phase shifted 360 degrees with a minimum of 8 bit resolution. To ensure proper operation while driving ultrasound arrays with varying element sizes, each RF driving channel implements phase feedback such that proper phase of the driving signal is produced either at the amplifier output before the matching circuitry or after the matching circuitry at the transducer face. This feedback has been experimentally shown to increase the focal intensities by 20 to 25% of two tested phased arrays without array calibration using a hydrophone.