Methods to design and evaluate transcranial ultrasonic lenses using acoustic holography

Ultrasonic three-dimensional printed holograms are getting increasing interest for transcranial therapies since they can correct skull aberrations and, simultaneously, adapt the acoustic field to particular brain targets. However, evaluating the targeting performance of these systems requires the measurement of complex volumetric acoustic fields, which in many practical situations cannot be estimated by direct hydrophone measurements. In this work, we apply single-plane holographic measurement techniques to experimentally calibrate and measure the full volumetric field produced by holographic lenses. Two ex vivo test cases are presented, a four-foci lens and a preclinical case, both targeting through a macaque skull for potential applications in blood--brain barrier opening (BBBO) studies. Time-reversal and angular spectrum projection methods are compared to direct experimental measurements. Results show that holographic projection methods can reconstruct the complex acoustic images produced by holographic lenses, matching direct measurements in all test cases. However, while direct measurements are restricted to transverse-field cross sections, holographic projection allows estimating the field on the whole targeting volume. In this way, the location and the full three-dimensional shape of all acoustic foci can be obtained. Furthermore, these techniques can provide the field at the surface of the lens to compare it to the design phase distribution. Using this procedure, complex volumetric acoustic fields can be reconstructed, saving significant measurement time and computational resources, and enabling an accurate characterization of phase plates and other holographic lens topologies.