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Calibration-free quantification of absolute oxygen saturation based on the dynamics of photoacoustic signals.

Photoacoustic tomography (PAT) is a hybrid imaging technique that has broad preclinical and clinical applications. Based on the photoacoustic effect, PAT directly measures specific optical absorption, which is the product of the tissue-intrinsic optical absorption coefficient and the local optical fluence. Therefore, quantitative PAT, such as absolute oxygen saturation (sO₂) quantification, requires knowledge of the local optical fluence, which can only be estimated through invasive measurements or sophisticated modeling of light transportation. In this Letter, we circumvent this requirement by taking advantage of the dynamics in sO₂. The new method works when the sO₂ transition can be simultaneously monitored with multiple wavelengths. For each wavelength, the ratio of photoacoustic amplitudes measured at different sO₂ states is utilized. Using the ratio cancels the contribution from optical fluence and allows calibration-free quantification of absolute sO₂. The new method was validated through both phantom and in vivo experiments.