Integrated photoacoustic and oblique incidence diffuse reflectance system for quantitative optical sensing in turbid media

Abstract

The photoacoustic signal of an optical absorber in a turbid medium is proportional to the local laser fluence, the optical absorption coefficient and the Gruneisen parameter. The local fluence at a subsurface absorber is determined by the initial incident fluence and optical properties of the media. Knowledge of laser fluence at subcutaneous tissue locations will improve our ability to estimate local chromophore concentrations and will lead to more quantitative estimates of blood oxygen saturation with photoacoustics. By integrating an oblique incidence reflectance (OIR) system in a photoacoustic imaging system, we are able to estimate optical properties of the turbid medium. To do this, we use a unique photoacoustic probe consisting of a 45-degree optical prism in an optical index-matching fluid. An oblique CWlaser beam interrogates the tissue surface at the same location as a pulsed laser, used for photoacoustic interrogation. Photoacoustic signals collected from the tissue are deflected by the prism to a focused 10 MHz ultrasound transducer. Diffuse light from the CW-laser is collected by a CCD camera and analyzed to estimate the bulk absorption and scattering coefficients. We fixed a tube filled with known concentrations of an absorbing dye below the probe in an Intralipid bath. We obtained the OIR and photoacoustic measurements for different Intralipid concentrations (providing a μs' between 1 and 10 cm^-1). The OIR measurements were used to estimate the bulk optical parameters. Using these values, models of light transport were then used to calculate the local laser fluence to normalize the photoacoustic measurements. The corrected photoacoustic signals show direct proportionality to the tube dye concentrations irrespective of bulk turbid medium properties.