Fig. 1.

Principle of flow measurement by PAM based on cross correlation. (a) Two laser beams (blue and red arrows) illuminate the measurement area of a blood vessel alternately. The axes of the two beams are separated by distance $L$. Sequential A-lines are acquired at (b) $A_v$ and (c) $B_v$ at an interval time of $2t_p$. (d) The slow-time photoacoustic (PA) profiles from $A_v$ and $B_v$ are shifted in time by $\mathrm{\Delta }t$.