Cartesian system, Ox y (green lines) and polar system O′x′y′ (blue lines). An ROI center (A) has the A(x, y) Cartesian coordinates and A(ρ, θ) polar coordinates. The green area represents the position of piezoelectric crystals in the probe and the red lines show the imaging aperture of the convex probe. The depth of the imaging system was set to 16 cm.