SIMON FRASER / SCIENCE PHOTO LIBRARY SIMON FRASER / SCIENCE PHOTO LIBRARY
Arachnoid cyst. Magnetic resonance imaging (MRI) scan of an axial (horizontal) section through the brain of a 13 year old boy. The front of the head is at top. The scan shows a large left middle cranial fossa arachnoid cyst (at right, pale). 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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