We investigate the feasibility of photoacoustic (PA) imaging for assessing the correlation between red blood cell (RBC) aggregation and the oxygen saturation (sO2) in a simulated pulsatile blood flow system. For the 750 and 850 nm illuminations, the PA amplitude (PAA) increased and decreased as the mean blood flow velocity decreased and increased, respectively, at all beat rates (60, 120 and 180 bpm). The sO2 also cyclically varied, in phase with the PAA for all beat rates. However, the linear correlation between the sO2 and the PAA at 850 nm was stronger than that at 750 nm. These results suggest that the sO2 can be correlated with RBC aggregation induced by decreased mean shear rate in pulsatile flow, and that the correlation is dependent on the optical wavelength. The hemodynamic properties of blood flow assessed by PA imaging may be used to provide a new biomarker for simultaneous monitoring blood viscosity related to RBC aggregation, oxygen delivery related to the sO2 and their clinical correlation.

We investigate the optical wavelength dependence in quantitative photoacoustic (QPA) assessment of red blood cell (RBC) aggregation and oxygen saturation (sO ) 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 sO . 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.

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The present study investigates cyclic variation in blood echogenicity (CVBE) in vivo using high-frequency ultrasound (HFUS). Blood echogenicity (BE) and vessel diameter (VD) were obtained from cross-sectional B-mode images of the radial artery of six volunteers (three young and three old volunteers) acquired at a frequency of 20 MHz. The magnitudes of the cyclic variations in BE and VD were 0.83 ± 0.18 dB and 0.29 ± 0.05 mm, respectively. CVBE was observed to be out of phase with the cyclic variation in VD, which is known to be in phase with blood flow velocity. This result is different from those in previous studies, which were performed in the carotid artery at lower frequencies. In addition, the magnitude of CVBE in the older group (0.96 ± 0.05 dB) was higher than that in the younger group (0.63 ± 0.06 dB, p \ 0.005), whereas the magnitude of variation in VD was not significantly different between the two groups (p = 0.119). This feasibility study suggests that HFUS B-mode blood imaging of human small vessels is useful for the noninvasive measurement or monitoring of the dynamic variation of hemorheological properties in human blood.