SIMON FRASER / SCIENCE PHOTO LIBRARY SIMON FRASER / SCIENCE PHOTO LIBRARY
Arachnoid cyst. Coloured magnetic resonance imaging (MRI) scan of a sagittal (side) section through the head of a 13 year old boy. The front of the head is at left. The scan shows a large left middle cranial fossa arachnoid cyst (blue). This is an abnormal collection of CSF (cerebral spinal fluid), which is walled off and acts like a mass. It is benign but can cause a variety of symptoms including seizures. MRI uses radio waves and a magnet to obtain "slice" body images.
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