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 gado