Local determination of hemoglobin concentration and degree of oxygenation in tissue by pulsed photoacoustic spectroscopy

Pulsed-photoacoustic spectroscopy (PPAS) in the near-infrared portion of the optical spectrum was used as a local technique for quantitative monitoring of tissue hemoglobin concentration and its oxygenation state. A pulsed, tunable optical source coupled to a 1mm-diameter optical fiber cable was used to deliver optical energy to the tissue under study. The fiber was placed either on the exterior surface or inserted into the tissue. An ultrasonic signal was produced in the tissue as a result of the absorbed light pulse energy. Since the rate of conversion of laser light energy to heat was rapid and the laser pulse was much shorter than the tissue thermal-diffusion length, the ultrasonic signal amplitude was proportional to the energy absorbed. Spectra of absorbing compounds were obtained by measuring the Variation in the acoustic signal with source wavelength. In contrast with near-infrared spectroscopic techniques that measure diffuse light transmission and assume knowledge of the pathlength of light traveling through tissues in order to determine the absorption coefficient, the photoacoustic response is produced directly by light absorption. Light scattering merely modifies the spatial distribution of the absorbed energy. Our studies demonstrate that photoacoustic spectra obtained both in-vitro and in-vivo allow determination of relative changes in the concentration of oxy- and de-oxyhemoglobin.