Detection of the PA signal from the human calcaneus bone in vivo. (a) The schematic diagram of the PA measurement of a human heel. (b) The schematic diagram of the QUS measurement of a human heel. (c, d) The calcaneus bone PA signal before and after the compensation of the ultrasound attenuation, respectively. The signal marked between the two red dashed lines is from the calcaneus bone, where the right and the left boundaries are marked by L1 and L2, respectively. The large and saturated signal at the right side of the green dashed line marked with “S” is from the soft tissue covering the bone.