1. A method for processing digital image data, comprising: extracting a first set of intensity values corresponding to a first pixel from a plurality of sets of image data, each set of image data having a corresponding time value, each intensity value in the first set being extracted from a corresponding set of image data; identifying a first subset of the first set of intensity values, the intensity values of the first subset having a substantially linear relationship with each other; and performing regression on the first subset, the regression deriving a first regression slope corresponding to the first pixel.