Identification of red blood cell rouleaux formation using photoacoustic ultrasound spectroscopy

Red blood cell (RBC) rouleaux formation is a reversible phenomenon that occurs during low blood flow and small shearing forces in circulation. Certain pathological conditions can alter the molecular constituents of blood and properties of the RBCs leading to enhanced rouleaux formation, which results in impaired perfusion and tissue oxygenation. In this study rouleaux were artificially generated using Dextran-70 and examined using a photoacoustic (PA) microscope. Individual rouleau were irradiated with a 532 nm pulsed laser focused to a 10 mu m spot size, and the resulting PA signals recorded with a 200 MHz transducer. The laser and transducer were co-aligned, with the sample positioned between them. The frequency-domain PA ultrasound spectra were calculated for rouleaux with lengths ranging from 10 to 20 mu m. For the rouleaux, a single spectral minimum at 269 +/- 4 MHz was observed. The spectral minima were in good agreement with a theoretical thermoelastic expansion model using an infinite length cylindrical absorber, bearing a diameter equivalent to an average human RBC (7.8 mu m). These results suggest that PA ultrasound spectroscopy can be potentially used as a tool for monitoring blood samples for the presence of rouleaux.