Transcranial Focused Ultrasound Generates Skull-Conducted Shear Waves: Computational Model and Implications for Neuromodulation

Abstract

Focused ultrasound (FUS) is an established technique for non-invasive surgery and has recently attracted considerable attention as a potential method for non-invasive neuromodulation. While the pressure waves generated by FUS in this context have been extensively studied, the accompanying shear waves are often neglected due to the relatively high shear compliance of soft tissues. However, in bony structures such as the skull, acoustic pressure can also induce significant shear waves that could propagate outside the ultrasound focus. Here, we investigate wave propagation in the human cranium by means of a finite-element model that accounts for the anatomy, elasticity and viscoelasticity of the skull and brain. We show that, when a region on the frontal lobe is subjected to FUS, the skull acts as a wave guide for shear waves, resulting in their propagation to off-target structures such as the cochlea. This effect helps explain the off-target auditory responses observed during neuromodulation experiments and informs the development of mitigation and sham control strategies.

Auditory confounds can drive online effects of transcranial ultrasonic stimulation in humans

“…Where less acoustic energy was delivered to the brain, more acoustic energy was absorbed by the skull. This energy is thought to propagate via shear waves to the cochlea (Braun et al, 2020;Salahshoor et al, 2020), particularly when square wave amplitude modulation is administered as in the present study. With greater energy deposition in the skull, increased amplitude shear waves would likely result in greater perceived sound volumes (Stenfelt & Håkansson, 2002).…”

Image-Based Polygonal Lattices for Mechanical Modeling of Biological Materials: 2D Demonstrations

“…The image-based analysis significantly benefits from many new technologies − that reveal the detailed features of biological materials, such as scanning electron microscopy (SEM), X-ray computed tomography (XCT), and dual-energy X-ray absorptiometry (DXA) . For example, high-resolution magnetic resonance (MR) head images have enabled personalized numerical simulations of the mechanical response of brains under impact − or decompressive craniectomy . DXA-based patient-specific finite element simulations can analyze the stress state of hips and assess the fracture risk. − The major advantage of the image-based mechanical analysis is the convenience of utilizing the native uniform meshes from image pixels .…”