Optimization of thermal dose using switching mode patterns of a spherically shaped square element phased array

A 16 element, spherically sectioned array has been constructed for application in ultrasound surgery guided by magnetic resonance imaging. To reduce the peak temperature and cooling time interval, a treatment based on rapid switching between multiple focus fields during a ten second sonication has been investigated. First, 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 to create an optimally uniform thermal dose over a 0.6×0.6×1.0 cm³ volume. Second, the simulated results were experimentally tested using MRI to non-invasively monitor temperature elevations and predict lesion size in rabbit thigh muscle in vivo