Brain cyst. Coloured magnetic resonance imaging (MRI) scan of a section through the brain of an 11 month old child. At lower right a cyst (red) has formed in the region of the cerebellum. The brain's ventricle (pink) can also be seen in the image. Cysts are abnormal lumps or swellings filled with fluid or semi-solid material. They can occur in any organ or tissue. Cysts are usually harmless, but if they grow disproportionately large they may disrupt the function of the tissue. MRI scanners produce slice images though the body. The images are built up from radio signals and a magnetic field.