Integrated neuronavigation and acoustic-thermal modeling for simultaneous transcranial ultrasound stimulation (TUS) and functional MRI.

Abstract

Concurrent transcranial ultrasound stimulation (TUS) and functional MRI (fMRI) requires precise and stable MRI-compatible transducer positioning and fixation with reliable target verification within the confined MRI head-coil environment. Here, we present and validate an integrated workflow for precise, stable, and reproducible targeting of a TUS transducer inside the MRI scanner using subject-specific imaging, neuronavigation, acoustic-thermal modeling, transducer positioning and fixation, and in-scanner verification. For demonstration and validation, a stimulation target was defined in the primary motor cortex (M1) hand knob region using structural imaging and task-based fMRI collected during a planning session, then transferred to neuronavigation for trajectory planning and scalp marking. Acoustic and thermal simulations were performed to characterize the acoustic beam geometry, ensure target engagement, and adjust ultrasound power to achieve the desired acoustic field at the target while satisfying safety constraints. During the experiment scanning session, low-resolution T1-weighted localizer images were used to visualize transducer fiducials and the planned target directly on the scanner console, enabling iterative in-bore refinements. Coupled with a custom MRI-compatible transducer fixation cap enabling controlled multi-axis adjustments, this workflow achieved a mean post-refinement off-target distance of 1.4±1.0 mm across five participants over 19 sessions, falling within one fMRI voxel (2.5 mm) of the planned target in 17 of 19 sessions (89%). Repeat transducer position verification approximately 40 minutes after initial setup showed no detectable within-session position drift. This workflow establishes a practical experimental foundation for reproducible, millimeter-precision in-scanner targeting and verification for integrated human TUS-fMRI studies.