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Fig. 2 Image processing in PIV analysis. (a) Raw brightfield images showing a 5 dpf zebrafish tail. A complete dataset is made up by 100 successive frames. (b) Contrast images obtained by subtracting the average intensity image to every brightfield image. They show blood cells as bright spots. (c) Segmented image of vessels superimposed in red color on a raw image. (d–e) Blood cell shift between one frame (d) and the successive one (e). ROIs are depicted as green rectangles. They are placed on two particular vessels, namely the caudal vein (blue vector, small shift) and the caudal artery (red vector, large shift). (f) Planar vector maps obtained with the PIV algorithm. Blue and red asterisks indicate the caudal vein and the caudal artery, respectively. Red arrows show the instantaneous velocity of blood cells. (g) Speed versus time profiles for two vectors chosen in correspondence of the asterisks in (f). The red curve corresponds to arterial speed profile and shows peaks that relate to the cardiac cycle phases. The blue one corresponds to venous speed profile and shows a more continuous trend, with only large oscillations slightly shifted in phase with respect to the aortic one. Scale bars are: (a–c, f) 50 µm; (d–e) 30 µm.