Front elevation of (a) the sample, which consists of four different patterns: two lengthways black adhesive tape dots stuck to the front, two transverse black adhesive tape dots stuck to the second layer, a lower half ring of black adhesive tape stuck to the third layer, and an upper half ring of black adhesive tape stuck to the back of three pieces of polymethylmethacrylate. (b–e) are the corresponding reconstructed PA images of the four parts of the sample.