  Photoacoustic fluctuation imaging, which exploits randomness in photoacoustic
generation, provides enhanced images in terms of resolution and visibility, as
compared to conventional photoacoustic images. While a few experimental
demonstrations of photoacoustic fluctuation imaging have been reported, it has
to date not been described theoretically. In the first part of this work, we
propose a theory relevant to fluctuations induced either by random illumination
patterns or by random distributions of absorbing particles. The theoretical
predictions are validated by Monte Carlo finite-difference time-domain
simulations of photoacoustic generation in random particle media. We provide a
physical insight into why visibility artefacts are absent from second-order
fluctuation images. In the second part, we demonstrate experimentally that
harnessing randomness induced by the flow of red blood cells produce
photoacoustic fluctuation images free of visibility artefacts. As a first proof
of concept, we obtain two-dimensional images of blood vessel phantoms.
Photoacoustic fluctuation imaging is finally applied in vivo to obtain 3D
images of the vascularization in a chicken embryo.
