measures the ability 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. For instance, a 12-bit representation and hence 12-bit contrast resolution is customary in MR imaging and computed tomography CT . Owing, however, to image display characteristics and the limitations of the human visual system the perceived contrast resolution of the 12-bit image is typically reduced to about 6- to 8-bit.
From GE Medcyclopaedia