# Holograms to focus arbitrary ultrasonic fields through the skull

## TL;DR

This paper introduces 3D-printed acoustic holographic lenses that enable precise focusing of ultrasonic fields through the skull, allowing complex spatial targeting for biomedical applications like neuromodulation.

## Contribution

The study demonstrates a novel method for designing holographic lenses that can focus ultrasound on complex targets inside the skull, overcoming phase aberrations caused by skull refraction and attenuation.

## Key findings

- Successfully focused ultrasound on multiple targets within a skull phantom.
- Achieved accurate beam bending along arbitrary paths inside the skull.
- Demonstrated overlap of focus with target volume in experiments and simulations.

## Abstract

We report 3D-printed acoustic holographic lenses for the formation of ultrasonic fields of complex spatial distribution inside the skull. Using holographic lenses, we experimentally, numerically and theoretically produce acoustic beams whose spatial distribution matches target structures of the central nervous system. In particular, we produce three types of targets of increasing complexity. First, a set of points are selected at the center of both right and left human hippocampi. Experiments using a skull phantom and 3D printed acoustic holographic lenses show that the corresponding bifocal lens simultaneously focuses acoustic energy at the target foci, with good agreement between theory and simulations. Second, an arbitrary curve is set as the target inside the skull phantom. Using time-reversal methods the holographic beam bends following the target path, in a similar way as self-bending beams do in free space. Finally, the right human hippocampus is selected as a target volume. The focus of the corresponding holographic lens overlaps with the target volume in excellent agreement between theory in free-media, and experiments and simulations including the skull phantom. The precise control of focused ultrasound into the central nervous system is mainly limited due to the strong phase aberrations produced by refraction and attenuation of the skull. Using the present method, the ultrasonic beam can be focused not only at a single point but overlapping one or various target structures simultaneously using low-cost 3D-printed acoustic holographic lens. The results open new paths to spread incoming biomedical ultrasound applications including blood-brain barrier opening or neuromodulation.