Abstract
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.
Highlights
Optical-resolution photoacoustic microscopy (OR-PAM) converts absorbed optical energy into ultrasonic wave, offering optical absorption contrast at sub-cellular spatial resolutions [1,2,3,4,5]
3.1 Numerical simulation We numerically evaluate the impact of absorption saturation on the accuracy of sO2 measurement under different sO2 values and different axial resolutions shown
We develop a nonlinear method to compensate for the absorption saturation in OR-PAM sO2 imaging
Summary
Optical-resolution photoacoustic microscopy (OR-PAM) converts absorbed optical energy into ultrasonic wave, offering optical absorption contrast at sub-cellular spatial resolutions [1,2,3,4,5]. When the absorption coefficient is high, the PA amplitude may become a nonlinear function of the absorption coefficient, referred to as the absorption saturation effect [29]. In such a case, the linear PA assumption will not be accurate and may lead to biased sO2 measurements. The absorption saturation is related to the absorber’s thickness, the axial resolution, and the optical wavelength [30]. The axial resolution and the optical wavelengths usually compromise with other parameters, such as imaging depth, sensitivity, laser energy, and laser pulse repetition rate. Compensation for the absorption saturation may improve the accuracy in OR-PAM sO2 imaging
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