On the Physical Mechanisms in Ultrasound Neuromodulation

Ultrasound Neuromodulation is a technique that can potentially be used for non-invasive neuromodulation. A transducer is placed on the scalp and insonicates the brain. By focusing the ultrasound beam, a subcortical region in the brain can be targeted with high resolution in the order of centimeters in axial direction and millimeters in the lateral direction. However, the mechanism of interaction between the ultrasound wave and the neurons is still unclear. Multiple mechanisms have been proposed (e.g.: intramembrane cavitation, mechanosensitive ion channels, changes in membrane capacitance). It is likely that they all contribute but it is unclear to what extent. These mechanisms are playing a role on the neuron level and are therefore called biophysical mechanisms. This study focuses on the physical mechanisms of the wave inside the brain and not on the beforementioned biophysical mechanisms. To this end, a realistic head model is used. The medium properties are retrieved from a CT scan. This is used to run a pressure simulation. From the pressure field, the acoustic radiation force (ARF) is calculated. The ARF is then used as an input to simulate the displacement of the brain tissue. Prior to the simulation of the head model, the grid sensitivity needed for both simulations is investigated in order to have reliable results. It is found that the location maximal pressure is the same as the maximal ARF. This is also the case for the displacement in the beginning. However, as time progresses, the displacement field changes and so do the point of maximal displacement and the resolution.