Contrast enhancement is a ubiquitous term in radiology and can be used in three ways.
Firstly, it may refer to any method of exaggerating the visible difference between adjacent structures on imaging by administering contrast media/agents. This includes differentiating between normal structures. There are many different types, which vary by modality (see the article on contrast agents for further information).
Secondly, contrast enhancement can also refer to characteristics of abnormal lesions in the body. The diffusion of the contrast agents from the bloodstream to body tissues is physiologically limited. In some pathologies e.g. cancer, new abnormal blood vessels may develop (i.e. neoangiogenesis), which tend to be leakier than normal capillaries, resulting in the lesion being much more visible on contrast-enhanced scans.
In the case of the CNS, the blood-brain barrier (BBB) limits the diffusion of contrast agents. The blood-brain barrier is often disrupted in pathologies of the CNS, leading to lesional enhancement post-administration of contrast agents on CT/MRI scans.
A third use of the term contrast enhancement is in image post-processing. The improvement of lesion conspicuity is obtained by redistributing the grey scale of the images in a non-linear fashion to improve the separation of subtle or obscured variations in pixel intensity into a more visually discernible distribution, thus taking advantage of human vision physiological attributes.