1. A method for determining perfusion in tissue using fluorescence imaging, comprising: defining a static region of interest (ROI) in an image of the tissue; measuring fluorescence intensity values of image elements (pixels) located within the ROI; and determining a blush value from an average of the intensity values of image elements located within a smallest contiguous range of image intensity values containing a first predefined fraction of a total measured image intensity of all image elements within the ROI.