Label-free functional imaging of single red blood cells (RBCs) in vivo holds the key to uncovering the fundamental mechanism of oxygen metabolism in cells. To this end, we developed single-RBC photoacoustic flowoxigraphy (FOG), which can image oxygen delivery from single flowing RBCs in vivo with millisecond-scale temporal resolution and micrometer-scale spatial resolution. Using intrinsic optical absorption contrast from oxyhemoglobin (HbO_2) and deoxyhemoglobin (HbR), FOG allows label-free imaging. Multiple single-RBC functional parameters, including total hemoglobin concentration (C_(Hb)), oxygen saturation (sO_2), sO_2 gradient (∇SO_2), flow speed (v_f), and oxygen release rate (rO_2), have been quantified simultaneously in real time. Working in reflection instead of transmission mode, the system allows minimally invasive imaging at more anatomical sites. We showed the capability to measure relationships among sO_2, ∇SO_2, v_f, and rO_2 in a living mouse brain. We also demonstrated that single-RBC oxygen delivery was modulated by changing either the inhalation gas or blood glucose. Furthermore, we showed that the coupling between neural activity and oxygen delivery could be imaged at the single-RBC level in the brain. The single-RBC functional imaging capability of FOG enables numerous biomedical studies and clinical applications.
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