Fig. 2.

Principle of the VD-based SAFT strategy. (a) Geometric illustration of SAFT. Red dots: targets (synthesized imaging points). Blue dots: artifacts of the target, generated due to the signal blurring in AR-PAM. $\mathrm{z}$: depth of the synthesized imaging point. $z_f$: depth of the focal point (VD). $r_i$: distance between the synthesized point (target location) and the focal point. $r_{i+1}$: distance between the detected signal of the target (artifact) and the focal point when the UST moves to position i + 1. $r_{i+1}^{\prime }$: distance between the synthesized point and focal point when the UST moves to position i + 1. $∆x$: scanning step of the UST between A-lines. (b) Synthesis of new A-lines: Left: original detected A-lines; Middle: delayed versions of A-lines; Right: newly generated A-lines after summation.