Investigating the impact of skull vibrations on motor responses to focused ultrasound neuromodulation in small rodents and methods to mitigate them.

Objective: Focused ultrasound (FUS) neuromodulation non-invasively alters brain activity, likely via acoustic radiation force (ARF) with dynamics of the pulse repetition frequency (PRF). PRF impacts neuromodulation as well as indirect auditory activation, a confound linked to skull vibrations. This study aimed to minimize these vibrations, by adjusting and randomizing PRF, and determine their impact on FUS-induced motor activity. We hypothesized that: the skull would vibrate most at a specific PRF; randomizing PRF would reduce skull vibrations without affecting motor responses; and FUS would yield motor activity while actuator-induced skull vibrations would not. Approach: Three objectives were studied in parallel using C57Bl/6 mice ( n = number of mice used per objective). First, skull vibration amplitude, measured as a percentage of maximum amplitude per treatment, was recorded via contact microphone over a range of PRFs to assess the PRF-dependency of skull vibrations ( n = 19). Vibrations were then compared between random and fixed PRFs ( n = 15). Lastly, motor responses were compared between fixed 1.5 kHz PRF FUS, random PRF FUS, air-puff stimulation, sham stimulation, and vibration induction via piezoelectric actuator ( n = 30). Main Results: The study found amplitude peaked at 1.51 kHz (88.1 ± 11.5%), significantly higher than at 0.54 kHz (75.5 ± 15.1%; p = 0.0149). Random PRF reduced amplitude by 4.2% ( p = 0.0181). Motor response rates to actuator-induced skull vibrations at the PRF (5.73 ± 6.96%) and its third harmonic (22.9 ± 22.7%) were not significantly different than sham (14.1 ± 11.6%), but lower than FUS (70.2 ± 16.3%; p < 0.0001). Significance: Based on these results, PRF near 0.5 kHz may best avoid skull vibrations, while random PRF could be utilized to slightly reduce vibration amplitude. The results also suggested that skull vibrations likely do not significantly impact motor responses to FUS neuromodulation.