Cross-correlation-based flowmetry using optical-resolution photoacoustic microscopy with a digital micromirror device

Noninvasive and accurate blood flow measurement is critical for medical diagnoses. We proposed a cross-correlationbased method to quantitatively measure transverse flow velocity, using an optical-resolution photoacoustic microscope with a digital micromirror device (DMD). The DMD alternately delivers two spatially separated laser beams to the target. The slow-time photoacoustic signal profiles measured from the two beams are cross-correlated. The magnitude and sign of the time shift in the cross-correlation profile are used to simultaneously calculate the speed and direction of transverse flow. The proposed method was first demonstrated in an aqueous suspension of microspheres flowing in capillary tubing. Using 10-μm-microspheres, transverse flows in the range of 0.50–6.84 mm/s were measured with a root-mean-squared accuracy of 0.22 mm/s. Using three different sizes of microspheres (3, 6, and 10 μm in diameter), we proved experimentally that the flow measurements were independent of the particle size for flows in the velocity range of 0.55–6.49 mm/s. We also observed an expected parabolic distribution of flow velocity along the depth direction. Finally, we used this method to measure blood flow in a mouse ear in vivo.