Electronically steerable large-scale ultrasound phased-array for noninvasive transcranial therapy

A hemispherical focused ultrasound phased-array was designed and fabricated for noninvasive transcranial therapy. The array had a diameter of 31 cm. It was constructed with 1372 custom-made 6 mm long, 10 mm and 7.2 mm outer and inner diameter, cylindrical shape PZT-4 tube elements. The elements were driven at length-mode resonance (306 kHz) or third harmonic (840 kHz) frequency, which reduced the electrical impedance at least an order of magnitude compared with standard PZT plates driven at thickness mode. A scanning laser vibrometer was used to demonstrate clear length mode oscillation of the element. The ultrasound field measurements in water showed that the effective beam steering ranges, measured at a 50% of its maximum pressure-squared amplitude, were 100 times 100 times 80 mm³ at 306 kHz and 30 times 30 times 30 mm³ at 840 kHz, respectively. The focal spot sizes at the geometric focus were approximately 2.3 mm in diameter at 306 kHz and 0.7 mm in diameter at 840 kHz, respectively. The functionality of the array, including its beam steering and focusing capabilities, was evaluated with an ex vivo human skull.