Quantitative photoacoustic imaging: fitting a model of light transport to the initial pressure distribution

Photoacoustic imaging, which generates a map of the initial acoustic pressure distribution generated by a short laser pulse, has been demonstrated by several authors. Quantitative photoacoustic imaging takes this one stage further to produce a map of the distribution of an optical property of the tissue, in this case absorption, which can then be related to a physiological parameter. In this technique, the initial pressure distribution is assumed to be proportional to the absorbed laser energy density. A model of light transport in scattering media is then used to estimate the distribution of optical properties that would result in such a pattern of absorbed energy. The light model used a finite element implementation of the diffusion equation (with the δ-E(3) approximation included to improve the accuracy at short distances inside the scattering medium). An algorithm which applies this model iteratively and converges on a quantitative estimate of the optical absorption distribution is described. 2D examples using simulated data (initial pressure maps) with and without noise are shown to converge quickly and accurately.