The purpose of a LUT is to take in one value and to output another using a scale. It is a technique to perform a conversion on pixel data. When the pixel values are decoded they are in a normalised range. To transfer to a grayscale the normalised data is windowed into a series of sub-ranges which contain all the important information and a non-linear conversion is then applied. For each modality the conversion is based on different tables, i.e. the CT pixel values are converted into the Hounsfield scale, whereas the pixel values of an x-ray are proportional to the x-ray intensity.