Effects of wavelength-dependent fluence attenuation on the noninvasive photoacoustic imaging of hemoglobin oxygen saturation in subcutaneous vasculature in vivo

Quantitative measurements of the oxygen saturation of hemoglobin (sO_2) in a blood vessel in vivo presents a challenge in photoacoustic imaging. As a result of wavelength-dependent optical attenuation in the skin, the local fluence at a subcutaneous vessel varies with the optical wavelength in spectral measurement and hence needs to be compensated for so that the intrinsic absorption coefficient can be recovered. Here, by employing a simplified double-layer skin model, we demonstrate that although the absolute value of sO_2 in a vessel is seriously affected by the volume fraction of blood and the spatially averaged sO_2 in the dermis, the difference of sO_2 between neighboring vessels is minimally affected. Experimentally, we acquire compensational factors for the wavelength-dependent optical attenuation by measuring the PA spectrum of a subcutaneously inserted 25 µm thick black film using our PA microscope. We demonstrate in vivo that the difference in sO_2 between a typical artery and a typical vein is conserved before and after spectral compensation. This conservation holds regardless of the animal's systemic physiological state.