a system for the analysis of image data 48 that has been subject to one or more processing operations is described . the image data is displayed using a display palette 30 which maps pixel values pv to displayable appearances rgb , i . under user control , a sub - section of the pixel values are mapped to enhanced displayable appearances so as to make small variations in the pixel values more visible . a palette scale 54 illustrating the mapping being performed by the palette 30 is displayed adjacent to the image 48 being analyzed . in addition , a graph 60 of the variation of pixel value along a given analysis line 62 is displayed to assist quantative analysis .