Quantitative Measurement of Tissue Optical Absorption Spectrum in a Scattering Medium by Photoacoustic Technique

The photoacoustic technique can be used to quantify tissue absorption spectrum in a wide spectral range from visible to near infrared. As the photoacoustic signal intensity is proportional to tissue optical absorption coefficient and light fluence, it is important to know the local light fluence at the regional target in order to obtain the accurate absorption spectrum because the tissue optical properties including scattering and absorption are wavelength dependent and affect the distribution and intensity of light in the sample. In this work, an optical contrast agent has been employed to enhance the performance of spectroscopic photoacoustic technique. From the photoacoustic measurements with and without the contrast agent in a target tissue, the spectroscopic local light fluence in the tissue can be determined. Then a quantified measurement of the tissue optical absorption spectrum can be realized in a strong scattering medium without need to know the wavelength-dependent optical properties in the scattering medium. A commercially available dye which has strong absorption in the wavelength range of interest was used. The results of the spectroscopic photoacoustic measurements on fresh canine blood specimens in scattering media such as milk and chicken breast tissue have been presented. It was found that photoacoustic measurements after employing this new technique have an improved match with the standard absorption spectra of both oxygenated and deoxygenated blood.