Contrast agents in neuroimaging are usually given to evaluate blood vessels or to assess the integrity of the blood-brain barrier. In the former case, CT or MR angiography can diagnose cerebral aneurysms, vascular malformations, and narrowed or occluded arteries. In the latter, enhancement of the brain itself is used for the diagnosis of disease. The cells that line the capillaries of the normal brain are tightly bound together to form the “blood-brain barrier,” which allows the passage of oxygen and nutrients into the brain but prevents the transit of disease-causing organisms and large molecules, including contrast agents. Brain tumors, infections and inflammatory processes often disrupt the blood-brain barrier, giving rise to abnormal enhancement within the brain. Unenhanced (labeled “-”) and contrast-enhanced (labeled “+”) images from CT (top row) and MRI (bottom row) in two different patients with brain tumors. Contrast enhanced images show both normal blood vessels (red arrows) and tumor enhancement from breakdown of the blood-brain barrier (yellow arrows).
Source: Google Knol (Distributed under the terms of the Creative Commons Attribution License.)