From the first reference: 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.