Real-time imaging of brain displacement during FUS neuromodulation in rodents in vivo

Focused ultrasound (FUS) is capable of modulating the central and peripheral nervous system. Temperature increase, cavitation, and acoustic radiation force have been proposed as potential mechanisms. Thus, tissue displacement imaging during FUS modulation is critical to understanding its physical mechanism. A cranial window was formed to perform imaging during FUS neuromodulation in C57BL/6 mice. FUS sonications were performed with a 4 MHz single-element transducer with a focal volume 0.2 × 0.9 mm (Sonic Concepts, WA), an applied tone burst of 0.5 ms and peak positive pressures varying from 1.9 to 6.7 MPa. Real-time channel data using plane wave sequences were acquired with an 18 MHz linear array (Vermon, France) and axial displacements throughout the entire brain were estimated using 1D cross-correlation (9λ window, 99% overlap) at 1 kHz frame rate. Downward displacements were only detected during FUS within the focal region. During modulation, the cumulative displacement ranged from 0.021 ± 0.017 μm (n = 3) at 3.1 MPa to 0.24 ± 0.004 μm at 6.7 MPa (n = 3). Displacement decreased post-FUS and returned to null within 1–2 ms. Unlike in a preliminary phantom study, no shear waves were detected. Real-time displacement imaging could thus provide effective targeting and monitoring as well as unveil the mechanism during FUS modulation.