  Non-invasive imaging of deep blood vessels for mapping hemodynamics remains
an open quest in biomedical optical imaging. Although pure optical imaging
techniques offer rich optical contrast of blood and have been reported to
measure blood flow, they are generally limited to surface imaging within the
optical diffusion limit of about one millimeter. Herein, we present
photoacoustic vector tomography (PAVT), breaking through the optical diffusion
limit to image deep blood flow with speed and direction quantification. PAVT
synergizes the spatial heterogeneity of blood and the photoacoustic contrast;
it compiles successive single-shot, wide-field photoacoustic images to directly
visualize the frame-to-frame propagation of the blood with pixel-wise flow
velocity estimation. We demonstrated in vivo that PAVT allows hemodynamic
quantification of deep blood vessels at five times the optical diffusion limit
(more than five millimeters), leading to vector mapping of blood flow in
humans. By offering the capability for deep hemodynamic imaging with optical
contrast, PAVT may become a powerful tool for monitoring and diagnosing
vascular diseases and mapping circulatory system function.
