Photoacoustic imaging has been popular in the past two decades; its nature is literally an inverse source problem, but the media are often coupled and cannot use a simple model to describe the ultrasound wave propagation. For many media, the properties are easier to quantify and tend to be homogeneous/isotropic. For some other media, like bones, the properties are rather more difficult to quantify with an elastic model only, probably due to their anisotropy and porous structures. Reconstruction under such assumptions is challenging.