1. A medical image mapping method; the method comprising the steps of: receiving a primary image having a region of interest the primary image comprising a plurality of pixels wherein each pixel has a primary image data value within a primary image data value range wherein the primary image is characteristic of a first imaging modality; receiving a secondary image corresponding to at least the region of interest the secondary image comprising a plurality of pixels wherein each pixel has a secondary image data value within a secondary image data value range; wherein the secondary image is characteristic of a different imaging modality to the first imaging modality; fusing the primary image with the secondary image to create a fused image comprising a plurality of pixels; wherein for at least the region of interest pixel in the fused image has an image value determined by: identifying a first range within the primary image data value range wherein the first range is narrower than the primary image data value range; selecting from the primary image a first pixel group of one or more pixels having a primary image data value within the first range; mapping the primary image data value of each pixel from the first pixel group to a grayscale image value representing a grayscale intensity wherein the lowest data value within the first range is mapped to black and the highest data value within the first range is mapped to white and wherein the relationship between the grayscale image value and the primary image data value is described by a monotonic function; identifying at least a second range within the primary image data value range which is distinct from the first range; selecting from the primary image a second pixel group of one or more pixels having a primary image data value within the at least a second range; for each pixel in the second pixel group, mapping the secondary image data value of each corresponding pixel from the secondary image to a color image value representing the intensity of at least a first color; wherein the relationship between the color image value and the secondary image data value is described by a monotonic function.