Significance: We demonstrate the potential of probing the sO₂ change under blood flow in vivo using photoacoustic (PA) imaging and sheds light on the complex relationship between RBC aggregation and oxygen delivery.

Aim: To conduct in vivo assessments of the sO₂ in the radial artery of healthy volunteers and simultaneously probe the relation between the sO₂ and hemodynamic behavior such as red blood cell (RBC) aggregation.

Approach: The effects of PA-based measurements of blood hemodynamics were studied as a function of the subjects’ age (20s, 30s, and 40s). The pulsatile blood flow in the human radial artery of 12 healthy subjects was imaged in the 700 to 900 nm optical wavelength range using a linear array-based PA system.

Results: The PA power when blood velocity is minimum (Pa_max) was larger than the one attained at maximum blood velocity (Pa_min), consistent with predictions based on the cyclical variation of RBC aggregation during pulsatile flow. The difference between Pa_min and Pa_max at 800 nm (ΔPa₈₀₀) increased with age (1.7, 2.2, and 2.6 dB for age group of 20s, 30s, and 40s, respectively). The sO₂ computed from Pa_max was larger than the one from Pa_min.

Conclusions: The ΔPa₈₀₀ increased with participant age. The ΔPa₈₀₀ metric could be a surrogate of noninvasively monitoring the age-induced changes in RBC aggregation. The sO₂ change during a cycle of pulsatile blood flow also increased with age, demonstrating that RBC aggregation can affect the sO₂ change.