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.