The invention relates to an apparatus (1) and method for automatically determining the blood flow direction (42) in a blood vessel (14) using the fluorescence light from a fluorophore (16). Blood flow direction (42) is determined by first identifying a blood vessel structure (38) in an input frame (6) from a camera assembly (2) using a pattern recognition module (26). Blood flow direction (42) is determined from the spatial gradient (dl/dx) of the fluorescence intensity (I) along the identified blood vessel structure (38) and the temporal gradient (dl/dt). An output frame (48) is displayed on a display (36) with time-varying marker data (52) overlaid on the identified blood vessels structure (38) and representative of the blood flow direction (42).
