In vivo photoacoustic oxygen saturation imaging without the need for fluence estimation

Due to the difference between the absorption spectra of reduced and oxygenated haemoglobin, spectroscopic photoacoustic imaging can be used to determine, in vivo and non-invasively, the local blood oxygen saturation. Conventional methods achieve this by computing the absolute concentrations of reduced and oxygenated haemoglobin, which require models for, or computations of, the local fluence at depth. However, such estimates are potentially inaccurate or expensive to compute. In this work, fluence estimation is circumvented by modelling the absorption spectra. Over local wavelength ranges the absorption spectra of reduced and oxygenated haemoglobin can accurately be represented by linear functions, and extrapolating the best-fit straight lines to zero absorption yields a fluenceindependent quantity that is readily converted to a blood oxygen saturation level. Using a clinical photoacoustic scanner, spectroscopic images were obtained from the second author´s inner elbow in the wavelength range between 761 and 793 nm. Regions of interest containing signal from either an artery or a vein were analysed with the new method, and yielded oxygen saturation values of 97 % and 70 %, respectively, which are in agreement with literature. Thus, these preliminary results suggest that the presented method can effectively determine local oxygen saturation in vivo and at depth.