Three-layer model for maximum pressure transmission in transcranial ultrasound stimulation

Brain Stimulation Lab
Graph and computer imagery showing three-layer model for maximum pressure transmission in transcranial ultrasound stimulation
 
Led by: Dr David Atalli, University of Paris
 
University of Plymouth lead: Dr Elsa Fouragnan
 
Partners: Drexel University, Philadelphia, Universite Lyon; Vanderbilt University Institute of Imaging Sciences, USA; University of Toronto, Canada; Krembil Research Institute, Canada; Harvard Medical School, USA; Imperial College London; University of Oxford; Radboud University, The Netherlands; Stanford University, USA.
 
2023
Transcranial ultrasound stimulation (TUS) has been shown to be safe and effective for non-invasive superficial and deep brain stimulation. The safety of this relies on estimating the ‘acoustic attenuation’ of the skull – the energy loss occurring at the depth of ultrasound stimulation. This research addresses the mechanical bioeffects of TUS and aims to fill the gap in international guidelines for estimating the impact of the skull bone.
 
The role of bone and tissue absorption in TUS
The US Food and Drug Administration (FDA) developed a tissue independent, arbitrary derating for diagnostic ultrasound that takes into account tissue absorption (0.3 dB/cm-MHz). But, applying this guideline to transcranial ultrasound imaging fails to take into account absorption by the skull bone and brain tissue. 
This results in an overly conservative absorption estimate, that has the potential to limit TUS applications.
Three-layer model
Researchers propose a three-layer model – the skin, skull and brain – to inform international guidelines on acoustic attenuation. This includes bone absorption to calculate the maximum pressure transmission through the skull for frequencies ranging between 100 kHz and 1.5 MHz. The calculated pressure transmission decreases with the frequency and thickness of the skull bone, with peaks for each thickness corresponding to a multiple of half the wavelength. 
The team recommended the maximum pressure transmission for each ultrasound beam diameter and frequency to facilitate the safe adjustment of acoustic pressure for short ultrasound pulses used in TUS.
Publication
Attali D, Tiennot T, Schafer M, Fouragnan E, Sallet J, Caskey C, Chen R, Darmani G, Bubrick EJ & Butler C (2022) 'Three-layer model with absorption for conservative estimation of the maximum acoustic transmission coefficient through the human skull for transcranial ultrasound stimulation' Brain Stimulation 16, (1) 48-55, DOI Open access.
Leading the way in transcranial ultrasound stimulation
The University of Plymouth's pioneering research in transcranial ultrasound stimulation has the potential to improve the lives of millions of people with mental health conditions as well as neurological disorders like Parkinson’s disease.
Dr Elsa Fouragnan is a leading authority on transcranial ultrasound stimulation research in the UK, and is recognised internationally. Her lab is one of the few in the UK to apply TUS in humans.
This research is hosted at the University’s Brain Research & Imaging Centre (BRIC) – the most advanced multi-modal brain research facility in the South West.
Dr. Elsa Fouragnan