In-situ determination of concentration and degree of oxygenation of hemoglobin in neural tissue by pulsed photoacoustic spectroscopy

Pulsed-photoacoustic spectroscopy (PPAS) is an in-situ technique used for quantitative monitoring of brain-tissue hemoglobin concentration and its oxygenation state. In contrast to most spectroscopic techniques that measure infrared absorption PPAS does not require knowledge of the pathlength of light traveling through tissues in order to determine the absorption coefficient and hence the concentration of absorbing species. The photoacoustic response (PAR) is produced by light absorption. Light scattering modifies the spatial distribution of the absorption. PPAS uses a pulsed, tunable optical source coupled to a 1mm diameter fiber optic cable to transmit optical energy to the tissue. The fiber can be placed on the exterior surface or inserted into the tissue. An ultrasonic signal is produced by light absorbed in the tissue. Since the rate of conversion of laser light to heat is rapid and the laser pulse much shorter than the tissue thermal-diffusion length, the ultrasonic signal amplitude is proportional to the energy absorbed. Spectra of absorbing compounds can be obtained by measuring the variation in the acoustic signal with source wavelength. Our studies demonstrate that acoustic spectra obtained both in-vitro and in-vivo allows relative changes in the concentration of oxy- and de-oxyhemoglobin to be determined.