Thermal-tagging Photoacoustic Remote Sensing Flowmetry

Ultrasound coupling is one of the critical challenges for the traditional photoacoustic (or optoacoustic) microscopy (PAM) technique transferred to the clinical examinations of chronic wounds and open tissues. A promising alternative potential solution for breaking the limitation of ultrasound coupling in PAM is photoacoustic remote sensing (PARS), which implements all-optical non-interferometric photoacoustic measurements. Functional imaging of PARS microscopy was demonstrated from the aspects of histopathology and oxygen metabolism, while its performance in hemodynamic quantification remains unexplored. In this work, we present an all-optical thermal-tagging flowmetry approach for PARS microscopy and demonstrate it with comprehensive mathematical modelling and ex vivo and in vivo experimental validations. Proof-of-concept experimental results demonstrated that the measurable range of blood flow speed was from 0 to 12 mm/s with a high accuracy (±1.2%) at 10-kHz laser pulse repetition rate. The proposed all-optical thermal-tagging flowmetry offers an effective alternative approach for PARS microscopy realizing non-contact dye-free hemodynamic imaging.