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 mu m 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.