DICOM is one of the most interesting image formats if not the most. DICOM files are typically generated from medical imaging devices directly such as CT, MRI scanner. The value of each pixel in DICOM files are the raw values coming out of the scanner. When we use or visualize these pixel values, transformations are often necessary. The mechanism used in these transformations are call LUT, look up table.
DICOM standard is full of the LUTs of various kinds. Among them, the most basic one is the modailty LUT. Modality LUT is used to transform the output from the scanner to a industry standard. For example, in CT scan, modality LUT is used to transform the output from the device to Hounsfield. The radiointensity of water at standard temperature and pressure is 0, The raidiointensity of air at standard temperature and pressure is 0. The device maufacturer need to calibrate the CT machine before shipping to the buyer to ensure the correctness of the modality LUT. A typical modality LUT is stored in the DICOM file as a slope and an intercept of a linear transformation.