Simultaneous measurement of erythrocyte aggregarion and oxygen saturation under in vitro pulsatile blood flow by high-frequency photoacoustics

In this paper, high-frequency photoacoustic (PA) imaging is proposed to simultaneously measure erythrocyte aggregation (EA) and oxygen saturation (SO₂). EA is a reversible phenomenon where red blood cells aggregate under flowing conditions and it becomes pathological when enhanced in a number of circulatory disorders. Here we investigate the feasibility of PA imaging in detecting EA-induced changes in SO₂ in a simulated circulatory flow system. For all optical wavelengths of illumination (750 and 850 nm), the mean PA amplitude inside the region of interest cyclically varied at intervals corresponding to the beat rate (30, 60, and 90 bpm). The vessel diameter also cyclically varied at the same time interval, but the phase of its variation was reversed compared to the PA amplitude variations. This was expected: as the blood velocity decreased, the shear rate in the radial direction also decreased, resulting in increased EA thus enhancing the PA amplitude due to the increased effective absorber size. When the velocity is increased, the aforementioned process is reversed, resulting in decreased EA and PA amplitude. The cyclic variation in SO₂ was evident for the highest beat rate, and differences in the mean PA amplitude at 750 and 850 nm were detected for all beat rates. This indicates that the SO₂ was varying while blood was flowing with the different beat rates. The temporal variation in SO₂ can be correlated to EA, since it has been reported that oxygen release is inhibited by EA.