Transcranial Focusing of Ultrasonic Vortices by Acoustic Holograms

Acoustic vortex beams have great potential for contactless particle manipulation and torque-based biomedical applications. However, focusing acoustic waves through highly aberrating layers such as the human skull at ultrasonic frequencies results in strong phase aberrations, which prevent the generation of sharp acoustic images. In the case of a wavefront containing phase dislocations, skull aberrations can inhibit the focusing of acoustic vortex beams inside the cranial cavity. In this paper, we demonstrate that phase-conjugated acoustic holograms can encode time-reversed fields, allowing compensation of the aberrations of the skull and, simultaneously, the generation of a focused vortex inside an ex vivo human skull. The method is applied to single-element geometrically focused sources and results in a very simple and compact ultrasonic system. This work will pave the way to designing low-cost particle-trapping systems, clot manipulation, and the exertion of acoustic-radiation forces and torques in the brain for biomedical applications.