Synthesized acoustic holography: A method to evaluate steering and focusing performance of ultrasound arrays

Acoustic holography is a commonly used tool to characterize the three-dimensional acoustic fields and the vibration patterns of ultrasound (US) transducers and arrays. It involves recording the pressure distribution over a transverse plane in front of the transducer via a two-dimensional hydrophone scan, and subsequent forward or backward field projection. For multi-element arrays capable of electronic focus steering, a separate hologram is needed to describe each beam configuration of interest. Since medical US arrays commonly use tens to hundreds of beam configurations, their characterization is very time consuming. Here, we show that holograms for the field of each array element can be recorded with a single hydrophone scan by pulsing the elements sequentially at each location. This approach was validated using a 1 MHz 64-element diagnostic-therapeutic linear array. Holograms of each element combined with backpropagation to the array surface revealed the variability of vibration patterns and crosstalk between channels and elements. Electronically steered beam configurations resulting from boundary conditions synthesized from elemental holograms and directly measured holograms were found to be in excellent agreement. The results demonstrate the method's potential in detecting defects and other nonideal array behavior and in rapid and accurate characterization of all relevant beam configurations.