Acoustic properties across the human skull

Abstract

Transcranial ultrasound is emerging as a noninvasive tool for targeted treatments of brain disorders. Transcranial ultrasound has been used for remotely mediated surgeries, transient opening of the blood-brain barrier, local drug delivery, and neuromodulation. However, all applications have been limited by the severe attenuation and phase distortion of ultrasound by the skull. Here, we characterized the dependence of the aberrations on specific anatomical segments of the skull. In particular, we measured ultrasound propagation properties throughout the perimeter of intact human skulls at 500 kHz. We found that the parietal bone provides substantially higher transmission (average pressure transmission 31±7%) and smaller phase distortion (242±44 degrees) than frontal (13±2%, 425±47 degrees) and occipital bone regions (16±4%, 416±35 degrees). In addition, we found that across skull regions, transmission strongly anti-correlated (R = −0.79) and phase distortion correlated (R = 0.85) with skull thickness. This information guides the design, positioning, and skull correction functionality of next-generation devices for effective, safe, and reproducible transcranial focused ultrasound therapies.

Controlled delivery of ultrasound through the head for effective and safe therapies of the brain

“…Across 8 ex-vivo skulls, we found that the ultrasound intensity delivered into a deep brain location (see Methods) is attenuated by a factor of 11.4 ± 6.8 (mean±S.D. ), which replicates previous findings 43 . In principle, the attenuation could be estimated using tabulated values (e.g., 20,21 ), but the high variability of the attenuation across individuals makes such estimates inaccurate and uncertain (Fig.…”

“…RTT is limited to transducer geometries and sections of the skull for which a through-transmit path can be established. We chose to place transducers parallel over the left and right sides of the skull as 1) there is a through-transmit path with minimal incidence angle to the skull 2) the parietal bone has favorable acoustic properties 19 , and 3) this geometry provides convenient coupling. Deviations from this geometry may incur loss of performance or practicality.…”

“…However, the effectiveness and safety of these emerging approaches have been hampered by a formidable barrier: the acoustically complex human head. The human skull alone attenuates the ultrasound by a factor of 4.5–64 depending on individual and skull segment 19–21 . Hair 22,23 , acoustic coupling to the head 24,25 , and entrapped bubbles or air pockets 26 present additional significant barriers.…”

BabelBrain: An Open-Source Application for Prospective Modeling of Transcranial Focused Ultrasound for Neuromodulation Applications

“…A pressure transmission coefficient T S was calculated with T S = √ E L . The values of T S calculated with each of the different mapping approaches were compared against experimental values of sub-MHz pressure transmission (T E ) from literature reports on pressure [18,29,[32][33][34][35][36] and energy [28,37,38] transmission. For the latter, T E was approximated by the square root of the reported energy transmission.…”

Low Intensity Focused Ultrasound for Epilepsy— A New Approach to Neuromodulation

“…Computational skull modeling, derived from each subject’s head CT or brain MRI, is a complex process which is currently being developed and validated with ongoing studies. 40-42 Further study in this area will also allow for use of LIFUS in a (paradoxically) less focused manner, to perhaps treat a broader area of cortex known to be epileptogenic, such as in some frontal lobe and other epilepsies.…”