Combined imaging and therapy with piezocomposite phased arrays

Recently, we have developed temperature estimation algorithms based on signal processing of pulse-echo ultrasound radio frequency (rf) data at diagnostic levels. A modified commercially available scanner was used to obtain 2D temperature estimates that were color coded and overlaid on real-time B-scan images to show the location and extent of the therapeutic beam with respect to the target as well as any critical structures in the treatment region. Recent advances in piezocomposite transducer technology have allowed the development of high power phased arrays for high-intensity focused ultrasound (HIFU) generation with excellent cross coupling characteristics. Furthermore, fractional bandwidths of 30-60% are now possible with such arrays. Therefore, we have investigated the feasibility of utilizing a 64-element piezocomposite therapeutic array in an imaging mode. The array is concave on a spherical shell with a radius of curvature of 100 mm with elements arranged in a linear configuration (2 mm×50 mm). The array operates at a center frequency of 1.25 MHz and has a fractional bandwidth of 37%. Wire target images as well as images from a tissue mimicking cyst phantom were reconstructed using a synthetic aperture technique. Excellent image quality was achieved in a region extending 70 mm axially and 60 mm laterally and centered around the geometric center of the array. Furthermore, we have experimentally established that the speckle generated by this array can be used to extract temperature information based on our previously described algorithm. This was demonstrated by using a 4×64 matrix switch and a high speed digitizer to acquire image frames during a slow heating experiment.