In vitro photoacoustic spectroscopy of pulsatile blood flow: Probing the interrelationship between red blood cell aggregation and oxygen saturation.

We investigate the optical wavelength dependence in quantitative photoacoustic (QPA) assessment of red blood cell (RBC) aggregation and oxygen saturation (sO2 ) during pulsatile blood flow. Experimentally, the pulsatile flow was imaged with a 700 to 900 nm laser using the VevoLAZR. Theoretically, the photoacoustic (PA) signals were computed based on a Green's function integrated with a Monte Carlo simulation of radiant fluence. The pulsatile flow created periodic conditions of RBC aggregation/nonaggregation, altering the aggregate size, and, in turn, the sO2 . The dynamic range, DR (a metric of change in PA power) from 700 to 900 nm for nonaggregated RBCs, was 5 dB for both experiment and theory. A significant difference in the DR for aggregated RBCs was 1.5 dB between experiment and theory. Comparing the DR at different wavelengths, the DR from nonaggregated to aggregated RBCs at 700 nm was significantly smaller than that at 900 nm for both experiment (4.0 dB < 7.1 dB) and theory (5.3 dB < 9.0 dB). These results demonstrate that RBC aggregation simultaneously affects the absorber size and the absorption coefficient in photoacoustic imaging (PAI) of pulsatile blood flow. This investigation elucidates how QPA spectroscopy can be used for probing hemodynamics and oxygen transport by PAI of blood flow.