Displacement and Cavitation Monitoring During Focused Ultrasound Neuromodulation of the Sciatic Nerve In Vivo

Focused ultrasound (FUS) can be used to excite peripheral neurons, inducing local motor activity. Sciatic nerve displacement, the amount by which FUS physically moves the nerve, has strongly correlated to the degree of motor activation; however, the potential relationship between in vivo cavitation and nerve activation remains unclear. In this study, sciatic nerve displacement and cavitation were imaged during FUS neuromodulation. Nerve activation was recorded using electromyography (EMG) electrodes and the amplitude of compound muscle action potentials (CMAPs) was correlated to both displacement and cavitation. FUS pulses that induced CMAPs were associated on average with a peak interframe nerve displacement of 46.1 ± 24.1 μm compared to only 20.6 ± 16.5 μm for pulses with no activation. In contrast, no significant differences were found between passive cavitation images, stable cavitation doses (SCD) or inertial cavitation doses (ICD) in activation and no activation conditions. Peak interframe displacement also demonstrated a strong correlation (r = 0.53) with CMAP amplitude, in contrast to both ICD and SCD. Thus, nerve displacement appears to be the driving mechanism behind FUS peripheral nerve excitation.