UCLA

Transcranial focused ultrasound stimulation (tUS) is an emerging technique for non-invasive brain stimulation (NIBS). tUS offers the option of higher spatial resolution and modulation of deeper neural structures, compared to other NIBS techniques. However, the neuromodulatory effects of tUS are still poorly understood, and the few experiments published thus far have shown variable outcomes between regions and studies.

To better understand the strengths, weakness, and feasibility prospects of tUS for use in humans, I compared tUS to an established NIBS technique, transcranial magnetic stimulation (TMS), in a cortical region with previously unclear tUS results but established TMS results: primary motor cortex (M1). I found no change in ongoing muscle activity concurrent with tUS stimulation. I also found no significant change in M1 cortical excitability due to tUS exposure.

To explore causes of variability in tUS, I performed acoustic simulations that replicated conditions of the M1 experiments. Specifically, I used participant-specific magnetic resonances images (MRI) to find where in the brain tUS pressure fields landed and how strong those pressure fields were. With this simulation data, I explored the spatial precision and volume of tUS, the accuracy of tUS device placement in my own data, and whether there was a correlation between tUS exposure and M1 excitability. I found that tUS produced a narrow ellipsoid intracranial pressure field with a full width at half maximum roughly 4 mm wide. While the simulations showed that tUS exposure levels varied, I found no clear correlation between tUS exposure and M1 excitability from these data.

Lastly, acoustic simulations provide significant insight for any tUS experiment, since it provides investigators estimates of the location and intensity of the intracranial ultrasonic field. However, implementing such simulations is challenging and time-consuming, which may explain why many published tUS studies do not perform acoustic simulations. To make acoustic simulation more accessible for tUS investigators, I built an open-source MATLAB toolbox that integrates medical imaging and neuronavigation with acoustic simulation. An increased adoption of acoustic simulation in tUS experiments should accelerate collective progress within the field.