Enhanced Numerical Method for the Design of 3-D-Printed Holographic Acoustic Lenses for Aberration Correction of Single-Element Transcranial Focused Ultrasound

Abstract

The correction of the aberration of transcranial focused ultrasounds is a relevant issue for enhancing various non-invasive medical treatments. Emission through multi-element phased arrays has been the most widely accepted method to reduce aberrations in recent years; however, a new disruptive technology, based on 3D printed acoustic lenses, has recently been proposed with lower cost and comparable accuracy. The number and size of transducers in phased array configurations was a bottleneck limiting the focusing accuracy, but once the submillimeter precision of the latest generation 3D printers has overcome this limitation, the challenge is now to improve the accuracy of the numerical simulations needed to design the lens. This study introduces and evaluates two improvements to the numerical model applied in previous works that proposed 3D lenses, which consist in the direct calculation of the phase pattern from the propagation of oscillating magnitudes in complex form, and in the introduction of the absorption phenomenon into the set of equations that describe the dynamics of the wave in both fluid and solid media. Numerical experiments are performed analysing the quality of the aberrated-corrected focus in different configurations. The results obtained show that the inclusion of absorption significantly improves focusing, especially where the thickness of the skull is more irregular.

A Clinical System for Non-invasive Blood–Brain Barrier Opening Using a Neuronavigation-Guided Single-Element Focused Ultrasound Transducer

“…Third, the proposed system is currently capable of compensating for skull-induced aberrations by predicing, and accounting for, the shift using a simulation framework similar to the one presented herein but not by phasing the elements in a way similar to multi-element hemispherical arrays. Another possible solution is to use 3-D printed holographic phase plates tailored to each skull contour and targeted structure (Melde et al 2016;Ferri et al 2018;Maimbourg et al 2018). Accurate knowledge of the intracranial pressure is impossible; therefore, one needs to simulate the pressure field within the targeted location on a patient-bypatient basis using head CT scans.…”

On the Evaluation of the Suitability of the Materials Used to 3D Print Holographic Acoustic Lenses to Correct Transcranial Focused Ultrasound Aberrations

“…Most acoustic holographic techniques are based on phased arrays with a large number of transducers [1,2], requiring sophisticated electronic circuits highly sensitive to calibration inaccuracies. However, new advances in metamaterials and the design capabilities of the state of art 3D printers allow the development of passive structures capable of shaping complicated acoustic fields [3,4,5,6,7,8,9].…”

“…An ambit in which precise holographic reconstruction of acoustic fields has been gaining importance in recent years is in focused ultrasound for medical applications (FUS), mainly in non-invasive treatments where the ultrasound propagates through tissues with very different acoustic impedances, as is the case of transcranial propagation [8,9,10,11,12,13,14,15,16]. The first successful transcranial ablation of animal brain tissue was achieved by Fry and Goss in 1980 [16].…”

“…In fact, the aberration correction by multi-element phased arrays is subjected to technological limitations associated with the size and number of singular elements that can be implemented in the phased arrays. These limitations can be overcome by shaping the wavefront using high transparency 3D printed refractive ultrasound lenses [6,8]. In fact, the work published by Maimbourg et al [6] demonstrates that the spatial resolution, associated with the voxel size achieved by the latest generation of 3D printers, dramatically improves the space resolution of phased arrays, leading to better focusing.…”

“…However, other aspects, such as those related to the shape of the lens and the sound speed in the polymer, which are intimately linked to each other, require a more detailed quantitative study. In fact, in previous works, polymers with lower [6] or higher [8,9] than water wave speeds have been indistinctly used to build the lenses without explicitly expressing a suitability criterion. In both slow and fast materials, the p-wave speeds condition the thickness and shape of the lenses.…”