3D-printed gradient-index phononic crystal lens for transcranial focused ultrasound

Abstract

Transcranial focused ultrasound (tFUS) shows great promise as a noninvasive tool to treat neurological conditions such as essential tremor. The existing clinical phased array systems are mostly intended for ultrasound delivery to the center of the brain (as in thalamotomy for essential tremor), in addition to being complex and expensive. To seek an alternative focusing approach especially for the brain periphery, we explore a 3D-printed gradient-index (GRIN) lens as a simple and an orders of magnitude more cost-effective approach. The lens is constructed using a phononic crystal (PC) architecture with varying lattice geometry and hence refractive index distribution. Specifically, the lens uses an axisymmetric hyperbolic secant refractive index profile to focus ultrasonic waves generated by a 1 MHz single-element ultrasonic transducer. Finite element simulations are performed to design and analyze the GRIN-PC lens, and to explore the effects of various parameters such as the distance from the skull and the incidence angle. The numerical results are validated experimentally for a 3D-printed lens by scanning the 3D pressure field generated through a temporal bone. This cost-effective approach to tFUS can open new possibilities to 3D print lenses based on patient computed tomography scans for various applications from tissue ablation to neurostimulation.