Acoustic holographic lenses for transcranial focusing in an ex vivo human skull

The skull has long been an obstacle for transcranial ultrasound therapy and neuromodulation, leading to waveform distortion and focal points misalignment. In this work, we propose a dual-focus transcranial ultrasound focusing method for ex vivo human skulls to bridge this barrier, which is demonstrated by using a 2 MHz single-element transducer placed outside the occipital bone, coupled to a 3D-printed acoustic holographic lens. The experimental results show that the proposed method allows for achieving high-precision single- and dual-focus ultrasound focusing through human skulls with the maximum target registration error less than one wavelength in water. The in-target ratio spans a range from 72% to 92%, with an average of 80.7%, while the out-of-target ratio varies from 0% to 43.9%, with an average of 18.3%. Additionally, the focal spots are substantially free of the skull-induced distortion, closely matching the ideal ones, and maintain a clearly identifiable circular focus shape. The method opens up a route toward the subsequent applications in clinical research and may enable to implement transcranial ultrasound therapy and neuromodulation.