Preliminary photoacoustic imaging of the human radial artery for simultaneous assessment of red blood cell aggregation and oxygen saturation in vivo

Abstract

Our group has previously demonstrated that lower shear rate in the pulsatile blood flow yields greater red blood cell (RBC) aggregation which in turn results in a higher oxygen saturation (sO 2 ) level and a higher photoacoustic (PA) signal. Higher shear rates led to disaggregation thereby decreasing the PA signal amplitude and the sO 2 . These results suggest that the interrelationship between the sO 2 and RBC aggregation may provide a new biomarker in the diagnosis of diseases that alter blood rheological properties. In this paper, we present a pilot study where high-frequency photoacoustic imaging (PAI) is used for the simultaneous assessment of RBC aggregation and sO 2 in vivo in the human radial artery (RA). The ultrasound (US) and PA images in the RA were acquired using a US/PA imaging system equipped with a 21 MHz linear-array probe (Vevo LAZR; LZ250, FUJIFILM VisualSonics, Canada), varying the wavelength of optical illumination (700, 750, 800, 850 and 900 nm). The blood flow velocity at the RA was assessed by pulsed wave Doppler in the same device. The PA signals inside the RA were observed at all wavelengths. The PA power increased with the illumination wavelength. At each wavelength, the PA power varied as function of time. The phase of variation in PA signals was inversely proportional to that in systolic blood flow velocity. The sO 2 is proportional to the cell surface area exposed to the surrounding media, and RBC aggregation decreases the exposed area of the aggregate. As such, the sO 2 was higher for aggregated cells (diastolic state) than single cells (systolic state). This PAI study of RA RBC aggregation and sO 2 in vivo is the first attempt to study the hemodynamics and physiological function of RBC, which can be used as a potential tool for the diagnosis of blood flow conditions in the RA.