Simulation study of transcranial ultrasound delivery to the hippocampus to test the feasibility of non-invasive neuromodulation

Transcranial ultrasound stimulation (TUS) is an emerging technology for non-invasive neuromodulation of localised sites in the brain. Here simulations using the k-Wave toolbox, were used to determine an appropriate ultrasound source to target the hippocampus (a relatively deep target in the brain) and the perirhinal cortex which is a neighbouring site that can be used as control for neurostimulation studies. Anonymised T1-weighted MR images were segmented and used as the 3-D simulation space. A virtual source was placed at the target location and captured on a surface outside the head. Phase-conjugation was employed to design a lens for an unfocused transducer and forward propagation employed to predict the acoustic field in the head. The size of the transducer frequency, transducer diameter, and the lens material were each varied. The simulation results suggested that the optimal configuration was a 65 mm diameter, 500 kHz transducer fitted a PDMS lens. Targeting was accurate to within 1 mm with a focal volume less than 30 mm3. In comparison, using a single element transducer with a fixed radius of curvature produce a focal volume greater than 1300 mm3. This motivates developing and validating a lens based TUS with neuromodulation of the hippocampus.