Laser optoacoustic imaging of turbid media: determination of optical properties by comparison with diffusion theory and Monte Carlo simulation

Laser optoacoustic imaging based on time-resolved detection of laser-induced thermoelastic pressure waves can be potentially used in medicine as a diagnostic tool (tomography) and as means to measure tissue optical properties in vivo. Information on tissue optical properties in vivo is important for laser dosimetry and tissue characterization. Analysis of the profile and amplitude of laser-induced transient stress can provide direct information about absorbed energy distribution in irradiated volume. Pressure wave profiles exactly correspond to temperature distributions generated in tissues under irradiation conditions of temporal stress confinement. In this study we experimentally measured pressure wave profile and amplitude upon irradiation of highly scattering gel phantoms and aqueous solutions colored with potassium chromate and made turbid with polystyrene spheres. A wide-band lithium niobate acoustic transducer was used to detect laser-induced pressure waves. The experimentally measured laser-induced stress profiles and the results of a Monte Carlo simulation were in excellent agreement. Computer code was developed to calculate separately absorption and scattering coefficients from transient stress profiles. Our study has demonstrated feasibility of time-resolved detection of laser-induced acoustic transients as a method for tissue optical properties measurements in vivo.