Fig. 2.

Tissue-mimicking phantom to be imaged. (a) The dimensions of the phantom are $4\times 8\times 8{\mathrm{cm}}^3$ ($X\times Y\times Z$). A 5-mm-thick scattering layer with reduced scattering coefficient ${\mu }_s^{\prime }=6{\mathrm{cm}}^{-1}$ was sandwiched between two transparent gelatin gel layers. (b) In the mid-$X$ plane of the scattering layer, two optical absorbers and a needle were embedded. The two absorbers were made by infusing optically absorbing India ink. The needle was used to align the two ultrasonic transducers with respect to the two absorbers.