Fig. 4 The changes reflecting blood flow over time（Class 1-4）.

a. C1: There is no inflow of arterial blood, the area remains black.
b. C2: The whole area is white, blood vessel shadows are unclear. The whole image has a cloudy appearance.
c. C3: The background of the whole image is white, which reflects mild venous stasis.
d. C4: The background of the image is black and the white branches of the capillaries can be observed.