MR images can be manipulated for evaluation in various ways. Postprocessing includes: Subtraction, addition, rotation, inversion, multiplanar reconstruction (MPR), maximum intensity projection (MIP), etc.
Subtraction is particularly useful in contrast enhanced MRI examinations (for example breast MRI, brain MRI). The pre contrast images are subtracted from the images after an injection of contrast agents (sometimes also called dye) for a better tumor detection.

See also Computer Aided Detection