Contrast enhancement is a ubiquitous term in radiology and can be used in two ways.
Firstly, it may refer to methods of exaggerating the visible difference between adjacent structures on scans by administering contrast 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 bodily tissues is physiologically limited. In pathologies such as cancer, blood vessels grow which are abnormal (i.e. neoangiogenesis) with increased leaking of contrast agent from the blood vessels, results in the lesion being much more visible on contrast-enhanced scans.
In the case of the CNS, the blood-brain barrier (BBB) limits diffusion of contrast agents. The BBB is often disrupted in pathologies of the CNS, leading to their enhancement after the administration of contrast agents on CT and MRI scans.