Multi-resolution simulation of focused ultrasound propagation through ovine skull from a single-element transducer

Abstract

Transcranial focused ultrasound (tFUS) is emerging as a non-invasive brain stimulation modality. Complicated interactions between acoustic pressure waves and osseous tissue introduce many challenges in the accurate targeting of an acoustic focus through the cranium. Image-guidance accompanied by a numerical simulation is desired to predict the intracranial acoustic propagation through the skull; however, such simulations typically demand heavy computation, which warrants an expedited processing method to provide on-site feedback for the user in guiding the acoustic focus to a particular brain region. In this paper, we present a multi-resolution simulation method based on the finite-difference time-domain formulation to model the transcranial propagation of acoustic waves from a single-element transducer (250 kHz). The multi-resolution approach improved computational efficiency by providing the flexibility in adjusting the spatial resolution. The simulation was also accelerated by utilizing parallelized computation through the graphic processing unit. To evaluate the accuracy of the method, we measured the actual acoustic fields through ex vivo sheep skulls with different sonication incident angles. The measured acoustic fields were compared to the simulation results in terms of focal location, dimensions, and pressure levels. The computational efficiency of the presented method was also assessed by comparing simulation speeds at various combinations of resolution grid settings. The multi-resolution grids consisting of 0.5 and 1.0 mm resolutions gave acceptable accuracy (under 3 mm in terms of focal position and dimension, less than 5% difference in peak pressure ratio) with a speed compatible with semi real-time user feedback (within 30 s). The proposed multi-resolution approach may serve as a novel tool for simulation-based guidance for tFUS applications.

Transcranial focused ultrasound stimulation of motor cortical areas in freely-moving awake rats

“…For example, imperfections in applying the sonication (e.g., mechanical slippage during application or due to the growth of cranium) can result in slight misalignments of the sonication target. Acoustic reverberation inside a small cavity of the rat skull [ 40 , 56 ] with the potential to create multiple sonication foci may be another possible cause. It is also plausible that the twitches from the head/neck/ears and chewing behaviors were not seen in the previous studies due to the weight of transducer/coupling devices (water bags or plastic standoffs were used along with much bigger/heavier transducers), which became detectable in the present study using a light-weight wearable tFUS apparatus.…”

Effects of sonication parameters on transcranial focused ultrasound brain stimulation in an ovine model

“…We previously reported our investigation on the effects of FUS on stimulation of the sensorimotor and visual cortices in an ovine model [31]. We chose to study sheep due to the structural similarities between the sheep and human craniums in regards to thickness, radius of curvature, and porosity [46, 47] and the neuroanatomical structures that are non-homogeneous and gyrencephalic [48]. Moreover, available clinical models of epilepsy [49], stroke [50], and brain injury [51] make sheep an attractive species to study en route to human application.…”

Virtual Brain Projection for Evaluating Trans-skull Beam Behavior of Transcranial Ultrasound Devices

“…It was later estimated using computational modeling that unforeseen side lobes of high intensities accumulated at the skull interface within the near-field of the FUS beam, causing the adverse events. Computer modeling is valuable when using subject-specific images in semi-real-time analysis for steering FUS beams to new locations for attenuation estimation, defocusing correction, and possible warnings of undesired standing wave formations (Yoon et al 2018). The porous microstructure and curvature of skull tissue has a major impact on the amount of attenuation a FUS beam experiences while being transmitted through the skull, which computational modeling cannot totally account for given the limited spatial resolution of Xray computed tomography used in the modeling.…”