Brain, x-ray
The blood vessels in the brain are a little different from those in the rest of the body. In ordinary blood vessels, dissolved substances including contrast media (CM) can "leak out" into the small intercellular spaces that are always found between the individual cells. The blood vessels in the brain, however, are provided with a "leak-proof barrier", the so-called blood-brain barrier. This protects the brain cells, which are so sensitive to harmful substances that even minute concentrations could damage the vital brain cells.
However, there are a number of conditions that can damage the blood-brain barrier, such as cancer, inflammation and bleeding. If the blood-brain barrier has been damaged, then CM can leak out into the interstitial spaces between brain cells. In that case, the damaged area will be lighter in colour than the healthy brain tissue on the X-ray image, allowing us to pinpoint the site of the disease.
Thus, the kidneys and the brain are special cases, and we can turn the special characteristics of these tissues to our advantage, as an aid in diagnosis. But there are also other ways to distinguish between various tissues and organs, and between healthy and diseased tissue. That is because the blood supply to different tissues varies. Tumours and infections often have a richer supply of blood than normal, healthy tissue, and these sites therefore acquire a higher concentration of CM.
One of the reasons we call a tumour malignant is that it often grows quickly and presses against or displaces the healthy tissue. In order to grow quickly, the tumour needs a rich supply of blood and nutrients. Very large tumours often have dead tissue at the centre, which will not absorb any CM. On a contrast enhanced X-ray, such tumours will be dark in the centre and very light around the edges. Fluid-filled cavities, such an infected cyst filled with pus or other matter, will not absorb CM, either, and will be dark on the image.
GE Healthcare Glossary