Computational sensitivity evaluation of ultrasound neuromodulation resolution to brain tissue sound speed with robust beamforming.

Low-intensity focused ultrasound (LIFU) neuromodulation requires precise targeting and high resolution enabled by phased array transducers and beamforming. However, focusing optimization usually relies on phantom measurements or simulations with inaccurate acoustic properties to degrade neuromodulation resolution. Therefore, this work analyzes the sensitivity of neuromodulation resolution, measured by off-target activation area (OTAA), to brain tissue sound speed. A Robust Optimal Resolution (ROR) beamforming method is proposed to minimize the worst-case OTAA with restricted sound speed inaccuracy and propagation information estimated with deviated sound speed. The propagation estimation model utilizes equivalent source method (ESM) to map sound field between different acoustic parameter sets. Simulation in a human head model validates the effectiveness of the proposed propagation estimation model, and shows that ROR beamforming method can significantly reduce the worst-case OTAA compared to benchmark methods by [Formula: see text] on average and up to [Formula: see text], improving the robustness of stimulation and addressing the sensitivity issue. This allows reliable high-resolution neuromodulation in potential clinical applications with reduced invasive acquisition of propagation measurements for focusing optimization.

Introduction

Purpose Transcranial ultrasound stimulation

Study Objective To analyze the sensitivity of LIFU neuromodulation resolution (measured by OTAA) to brain tissue sound speed and to develop a Robust Optimal Resolution (ROR) beamforming method that minimizes the worst-case OTAA under sound speed uncertainty.

MRI or image guidance method Human head model constructed from imaging data (modality not specified) with the target region marked in the model; targeting performed via phased‑array beamforming simulated in k-Wave (model‑based image guidance)

Outcomes and Safety

Summary of Outcomes Robust Optimal Resolution (ROR) beamforming combined with an Equivalent Source Method (ESM) propagation estimator markedly improved intracranial low‑intensity focused ultrasound neuromodulation by reducing worst‑case off‑target activation area (OTAA) on average by 78% and up to 90%, producing higher focality and robustness to brain sound‑speed deviations. Successful configurations reported were ROR beamforming (and its simplified variant ROR‑1) with ESM propagation estimation (ESM‑1), applied to phased‑array transducers using continuous‑wave or long‑pulse stimulation; benchmark methods without ROR showed much greater OTAA sensitivity.

Safety-related matter The paper highlights heating and cavitation as the main safety concerns for LIFU and recommends quantifying them using thermal index (TI), spatial-peak temporal average intensity (I_spta) and mechanical index (MI), citing guideline thresholds I_spta ≤ 720 mW/cm^2, TI ≤ 6 and MI ≤ 1.9 and presenting a preliminary framework to integrate these constraints into beamforming. No adverse effects were observed or reported in the simulations.

Brain Region

Visualization unavailable

Ultrasound Parameters

Ultrasound instrument Intracranial phased array transducer (simulation)

FUS Frequency 250 kHz

FUS Mode continuous

Duration of a single FUS session 100 ms

Focal Characteristics focal depth: None, focal length: None, aperture size: None

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