Contrast resolution in radiology refers to the ability of any imaging modality to distinguish between differences in image intensity. The inherent contrast resolution of a digital image is given by the number of possible pixel values, and is defined as the number of bits per pixel value.
Imaging modalities have an inherent contrast resolution intrinsic to the modality itself, e.g. MRI, but this baseline resolution is commonly further modified by the tweaking of scan parameters and/or the use of contrast media.