Numerical evaluation of the skull for human neuromodulation with transcranial focused ultrasound

Abstract

Objective. Transcranial focused ultrasound is an emerging field for human non-invasive neuromodulation, but its dosing in humans is difficult to know due to the skull. The objective of the present study was to establish modeling methods based on medical images to assess skull differences between individuals on the wave propagation of ultrasound. Approach. Computational models of transcranial focused ultrasound were constructed using CT and MR scans to solve for intracranial pressure. We explored the effect of including the skull base in models, different transducer placements on the head, and differences between 250 kHz or 500 kHz acoustic frequency for both female and male models. We further tested these features using linear, nonlinear, and elastic simulations. To better understand inter-subject skull thickness and composition effects we evaluated the intracranial pressure maps between twelve individuals at two different skull sites. Main results. Nonlinear acoustic simulations resulted in virtually identical intracranial pressure maps with linear acoustic simulations. Elastic simulations showed a difference in max pressures and full width half maximum volumes of 15% at most. Ultrasound at an acoustic frequency of 250 kHz resulted in the creation of more prominent intracranial standing waves compared to 500 kHz. Finally, across twelve model human skulls, a significant linear relationship to characterize intracranial pressure maps was not found. Significance. Despite its appeal, an inherent problem with the use of a noninvasive transcranial ultrasound method is the difficulty of knowing intracranial effects because of the skull. Here we develop detailed computational models derived from medical images of individuals to simulate the propagation of neuromodulatory ultrasound across the skull and solve for intracranial pressure maps. These methods allow for a much better understanding of the intracranial effects of ultrasound for an individual in order to ensure proper targeting and more tightly control dosing.

Neuromodulation with single‐element transcranial focused ultrasound in human thalamus

“…Detailed computational models of the skull are used in applications including aberration correction in high intensity focused ultrasound for thermal ablation (Marquet et al, ) and nonthermal ablation with microbubble enhanced focused ultrasound (Top, White, & McDannold, ). We were not able to acquire CT scans of the participants in this study and given the initial model shown here (despite a good conservation of beam geometry) it is recommended for future subcortical single‐element transcranial ultrasound studies that CT scans of individuals are acquired and detailed computational models run to ensure accurate target localization and transducer placement (Mueller et al ). This may not be feasible in some cases or not recommended to expose participants to unnecessary ionizing radiation.…”

“…CT images were used to construct the acoustic model of the skull, while MR images were used to target tFUS at the unilateral sensory thalamus based upon individual brain morphology. Details of the modelling parameters can be found in Mueller, Ai, Bansal, and Legon (2017).…”

“…We were not able to acquire CT scans of the participants in this study and given the initial model shown here (despite a good conservation of beam geometry) it is recommended for future subcortical singleelement transcranial ultrasound studies that CT scans of individuals are acquired and detailed computational models run to ensure accurate target localization and transducer placement (Mueller et al 2017). This may not be feasible in some cases or not recommended to expose participants to unnecessary ionizing radiation.…”

Intrinsic Cell-type Selectivity and Inter-neuronal Connectivity Alteration by Transcranial Focused Ultrasound

“…The CT images of the Wistar rat were aquired from the scan using a Siemens Inveon microCT machine at the University Imaging Center of the University of Minnesota. The simulation study was following a similar protocol described in (Mueller et al, 2017) and using the acoustic parameters listed in Table 2. The skull was deemed to be immersed in water and its acoustic properties were mapped from the Hounsfield unit acquired from the CT images and thus the porosity readings.…”

Transcranial Pulse Stimulation with Ultrasound in Alzheimer's Disease—A New Navigated Focal Brain Therapy

“…[11,12] TPS extends current literature by providing a new navigated stimulation technique which is clinically certified (CE mark). [14,15] We also present the first patient study (Alzheimer's disease, AD) and first investigation of long-term effects (up to three months) for ultrasound brain stimulation. [14,15] We also present the first patient study (Alzheimer's disease, AD) and first investigation of long-term effects (up to three months) for ultrasound brain stimulation.…”