Fig. 4. Shear shock wave formation in numerically simulated tissue during scanning along closed circular trajectories.

(A) Wake patterns trailed the scanning focus location. Because of shock formation, wakes were longest at higher scanning speeds v ≥ 5 m/s, corresponding to Mach numbers M ≥ 1 [oscillation velocity u(x, t) is shown]. Right column: Temporal compositing in the moving reference frame of the source, $\overline{u}(\mathbf{x})$, revealed spiral wake patterns. (B) Time-averaged wave fields; RMS oscillation velocity is shown. For Mach numbers M ≥ 1, waves radiated outward from the circular scanning trajectory. Constructive interference in the region circumscribed by the scanning trajectory yielded the greatest wave amplitude along a smaller circle whose radius decreased as M increased. Wave oscillations were greatly attenuated throughout a central quiescent disc, consistent with theoretical predictions (see also fig. S1) (26).