Investigating the impact of skull vibrations during focused ultrasound neuromodulation and methods to mitigate them

Focused ultrasound (FUS) neuromodulation offers the potential to non-invasively alter brain activity at targets throughout the brain, circumventing limitations of other neurostimulation techniques. FUS likely modulates neural tissue excitability through acoustic radiation force (ARF). The ultrasound signal exerts a mechanical force with dynamics of the carrier envelope, determined by the pulse repetition frequency (PRF). Thus, PRF has been tied both to modulatory effects of FUS and indirect auditory activation caused by FUS delivery through the skull. This auditory confound challenged motor responses to FUS, a common proxy measure of neuromodulation efficacy in rodents, and presents a hurdle for translation. Auditory activation has been linked to skull vibrations related to the PRF. Randomized PRF could prevent constructive interference of skull vibrations caused by ARF and minimize possible sensitivity of the skull to vibrate at PRFs. We hypothesized that randomizing the PRF would reduce skull vibrations without affecting motor response rates. We also hypothesized that while FUS delivery may cause skull vibrations, this auditory artifact was not the cause of motor responses and FUS neuromodulation would produce motor activity more reliably than piezoelectrically-induced skull vibrations. This study first aimed to record the amplitude of skull vibrations during FUS over a range of PRF values to assess the likelihood of skull vibrations depending on PRF. Second, vibrations were compared between randomized and fixed PRF conditions. Lastly, motor responses to FUS neuromodulation were compared between fixed PRF, random PRF, a positive control of air-puff stimulation, a sham condition, and vibration induction via a piezoelectric actuator. To obtain motor responses, isoflurane anesthesia was reduced during stimulus delivery. Motor activity was recorded via video and electromyography. A contact microphone recorded skull vibrations. The results of this work revealed the influence of PRF on skull vibrations and suggested that they do not significantly impact motor responses to FUS neuromodulation. The random PRF method slightly reduced skull vibrations and may be adapted in future studies of the neuromodulatory effects of FUS.