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Criteria for the design and characterization of large-area arrays for transskull ultrasound surgery and therapy

The concept of a noninvasive ultrasound transducer array for therapeutic and surgical procedures in the brain introduces a number of unique design and measurement considerations. The primary problem is to produce an array that can correct for the distortion of a focused beam caused by the skull bone and at the same time avoid significant skull heating during high power sonications. The number of array elements must be sufficient to correct for ultrasound diffraction and scattering, permit steering of the beam whether mechanically or electronically, and possess the ability to deliver therapeutic power levels. The surface area of the array must be large enough to minimize levels of skull heating to the point where it is safe for the patient as well as insignificant to cause severe changes in the acoustic properties of the bone. Further, the frequency must be optimized to allow for the maximum gain between the skull surface and the transducer. Each of these problems is addressed, resulting in a hemisphere-shaped transducer array design (r=150 mm) optimized in frequency (f=0.67 MHz) and element number (N=64). A second array (f=0.81 MHz, N=500 MHz) is designed to both restore the focus through the skull as well as electronically shift the beam.