Unfocused Field Analysis of a Density-Tapered Spiral Array for High-Volume-Rate 3-D Ultrasound Imaging

Spiral array transducers with a sparse 2-D aperture have demonstrated their potential in realizing 3-D ultrasound imaging with reduced data rates. Nevertheless, their feasibility in high-volume-rate imaging based on unfocused transmissions has yet to be established. From a metrology standpoint, it is essential to characterize the acoustic field of unfocusedtransmissions fromspiral arrays not only to assess their safety but also to identify the root cause of imaging irregularities due to the array's sparse aperture. Here, we present a field profile analysis of unfocused transmissions from a density-tapered spiral array transducer (256 hexagonal elements, 220-mu melement diameter, and 1-cm aperture diameter) through both simulations and hydrophonemeasurements. We investigatedplane- and diverging-wave transmissions (five-cycle, 7.5-MHz pulses) from 0 degrees to 10 degrees steering for their beam intensity characteristics and wavefront arrival time profiles. Unfocused firings were also tested for B-mode imaging performance (ten compounded angles, -5 degrees to 5 degrees span). The array was found to produce unfocused transmissions with a peak negative pressure of 93.9 kPa at 2 cm depth. All transmissions steered up to 5 degrees were free of secondary lobes within 12 dB of the main beam peak intensity. All wavefront arrival time profiles were found to closely match the expected profiles with maximum root-mean-squared errors of 0.054 mu s for plane wave (PW) and 0.124 mu s for diverging wave. The B-mode images showed good spatial resolution with a 27 penetration depth of 22 mm in PW imaging. Overall, these 28 results demonstrate that the density-tapered spiral array 29 can facilitate unfocused transmissions below regulatory 30 limits (mechanical index: 0.034; spatial-peak, pulse-average 31 intensity: 0.298 W/cm(2)) and with suppressed secondary 32 lobes while maintaining smooth wavefronts.