Optical-resolution photoacoustic microscopy of oxygen saturation with nonlinear compensation

Optical-resolution photoacoustic microscopy (OR-PAM) of oxygen saturation (sO₂) offers high-resolution functional information on living tissue. Limited by the availability of high-speed multi-wavelength lasers, most OR-PAM systems use wavelengths around 532nm. Blood has high absorption coefficients in this spectrum, which may cause absorption saturation and induce systematic errors in sO₂ imaging. Here, we present nonlinear OR-PAM that compensates for the absorption saturation in sO₂ imaging. We model the absorption saturation at different absorption coefficients and ultrasonic bandwidths. To compensate for the absorption saturation, we develop an OR-PAM system with three optical wavelengths and implement a nonlinear algorithm to compute sO₂. Phantom experiments on bovine blood validate that the nonlinear OR-PAM can improve the sO₂ accuracy by up to 0.13 for fully oxygenated blood. In vivo sO₂ imaging has been conducted in the mouse ear. The nonlinear sO₂ results agree with the normal physiological values. These results show that the absorption saturation effect can be compensated for in nonlinear OR-PAM, which improves the accuracy of functional photoacoustic imaging.