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.

Mechanics Of Ultrasonic Neuromodulation In A Mouse Subject

“…These low values evince the insulating effect of the cranium, which effectively carries the shear waves away from the head into the vertebral column, as well as the low energy/momentum absorption of the US by the soft tissue. A similar shear-wave insulating effect of the brain by the cranium, or 'cloaking', is observed in simulations of FUS applied to the human head [22]. The transmission coefficients for waves crossing the various soft tissue and bone interfaces shed quantitative light on the extreme pressure-shear asymmetry observed in the calculations.…”

Brain aging mechanisms with mechanical manifestations

“…Work has been done on TBI ( Hajiaghamemar et al, 2020 ; Li et al, 2017 ; Giordano et al, 2017 ; Ghazi et al, 2021 ), chronic traumatic encephalopathy ( Noël and Kuhl, 2019 ; van den Bedem and Kuhl, 2017 ; Bakhtiarydavijani et al, 2020 ), and the mechanical-electrophysiological coupling in dislocation injury ( Kwong et al, 2019 ). There are many other examples, including models of neuronal transport ( Li et al, 2021 ), and medical applications such as a decompressive craniectomy ( Weickenmeier et al, 2017 ; Bing et al, 2020 ), deep brain stimulation ( Bikson et al, 2012 ), and focused ultrasound techniques ( Salahshoor et al, 2020 ). With respect to aging, recent studies modeled cerebral atrophy in the brain during healthy ( Harris et al, 2019 ) and accelerated aging ( Weickenmeier et al, 2018 ; Schäfer et al, 2019 ; Blinkouskaya and Weickenmeier, 2021 ).…”

Application of Data-Driven computing to patient-specific simulation of brain neuromodulation

“…We demonstrate the range and scope of the model-free Data-Driven brain mechanics framework through an application to transcranial ultrasound stimulation. Transcranial ultrasound has elicited growing interest in both low-intensity mode, suited to neuromodulation applications [2,10,[14][15][16][18][19][20], as well as in high-intensity mode, 3/8 germane to tumor ablation applications [11,23].…”