Simultaneous estimation of chromophore concentration and scattering distributions from multiwavelength photoacoustic images

In biomedical photoacoustic tomography of soft tissue, the initial acoustic pressure distribution following the absorption of a short excitation laser pulse, is recovered as a function of position. This initial pressure distribution is proportional to the absorbed optical energy density, and is thus related (albeit indirectly) to the tissue optical coefficients. When imaging soft tissue which contains several absorbing chromophores (such as oxy- and deoxy-haemoglobin, water, etc.), the primary quantity of interest is the concentrations of the chromophores at each point in the tissue, and not the absorbed optical energy density, which is nonlinearly related to the chromophore concentrations, and also depends on the distribution of scattering. Estimating the distribution of the concentration of a chromophore therefore requires the recovery of two unknown functions (chromophore concentration and scattering distributions) from measurements of one (absorbed energy density). For measurements made at a single optical wavelength, this problem suffers from nonuniqueness, and cannot be solved without additional information being incorporated. A simulated example is used here to demonstrate that, in principle, by using multi-wavelength data and incorporating the known wavelength dependence of the chromophore absorption and the scattering as prior information, a chromophore concentration and spatial dependence of the scattering can be recovered simultaneously. This step opens the way to physiological and molecular imaging using multispectral photoacoustic tomography.