Multifrequency and multifocal acoustic holograms in complex media: Applications to transcranial ultrasound focusing

Advancements in ultrasonic beam shaping using multi-element arrays have significantly improved the ability to focus ultrasound through aberrating media, such as the human skull, enabling rapid growth in transcranial-focused ultrasound therapies. However, the high cost of these technologies presents a barrier to widespread adoption. Acoustic lenses offer a promising, low-cost alternative for correcting skull-induced aberrations. We will show that computed tomography-based simulations allow to design silicone acoustic lenses for single-element transducers. This approach demonstrates a 10-fold increase in energy deposition at the target when lens-based aberration correction is applied. Additionally, we will show that the same acoustic lens can produce multifrequency holograms across a 500 kHz to 1 MHz range, validated both in free-field conditions and through human skulls, provided the phase is unwrapped before the estimation of the thickness of the lens. Finally, we will demonstrate that the unwrapped acoustic lenses allow multifocal refocusing and we will discuss potential applications in transcranial ultrasonic neurostimulation for the treatment of drug-resistant depression. Together, these results emphasize the necessity of aberration correction to mitigate the field distortions induced by the human skull and highlight the potential of tailored acoustic lenses as a cost-effective solution for advancing transcranial ultrasound therapies.