These are externally administered pharmaceutical agents given during an imaging examination to highlight normal or diseased brain structures. The additional information provided by a contrast agent may or may not be necessary to make an accurate diagnosis. Contrast agents are most often administered by intravenous injection, but may in certain cases require intrathecal (spinal) injection using a lumbar puncture (spinal tap) procedure. Because they are water soluble they are normally removed from the body by the kidneys, though when the kidneys are diseased the liver may also contribute to their elimination. CT contrast agents are typically iodine-containing compounds that transiently increase the density of structures that they pass through. MRI contrast agents contain the heavy metal gadolinium, which changes the inherent T1 and T2 relaxation parameters of tissues.
The normal path of intravenously-injected contrast agents is through the heart, lungs and arteries of the chest and neck before entering the head. Once in the head, contrast passes first into the arterial system of the brain and its coverings, then through the cerebral microcirculation to supply the brain itself, ultimately passing into the intracranial venous system. Once it has passed into the large veins in the head, contrast is transmitted into the veins of the neck. From there, the contrast enters the heart for the second time, beginning the process of recirculation. The relative timing of the scan with respect to the location of the contrast agent within the blood pool allows detailed imaging of the arteries, brain, or veins.
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 MRI 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.