(a) Depth map of the acinar glands detected at 2850 cm-1 calculated using the time arrival of OA signals, depth is color-coded according the contrast bar to the left. Two points (P1 and P2) from two different structures at different depths are indicated. (b) OA intensity map of a showing also P1 and P2 for comparison. (c) 3D surface plot of the depth map in a. (d) Comparison of the time arrival of the OA signal detected at P1 and P2. Data in a-d are representative of three independent experiments.