Biopsy: The removal of a small amount of tissue to make a specific diagnosis.

Catheter: a narrow tube, used to drain fluid, inject fluid or move body fluid from one area to another.

Cerebrospinal fluid (CSF): A fluid that surrounds the brain and spinal cord. Its main purpose is to maintain constant pressure. CSF circulates in cavities within the brain and between the brain and its covering (the meninges) and is eventually reabsorbed into the blood.

Endoscopic: Surgical procedures or examinations which use only small incisions, transmitted images, and miniaturized surgical tools to access sites inside the body. Because incisions are smaller and less surrounding tissue is disrupted, such surgeries often have shorter recovery periods with less pain.

Hemorrhage: Blood vessel rupture. In the brain, hemorrhage can be even more dangerous than elsewhere in the body, as brain tissue becomes irritated and inflamed and the pressure inside the skull increases.

Intrauterine surgery: Surgery on a fetus while it remains in the mother's womb.

Meninges: The membranes that enclose the brain and spinal cord, composed of three layers: arachnoid, dura mater, and pia mater.

MRI perfusion studies: These tests use magnetic resonance imaging to compare the relative amount of blood flow on each side of the brain. They also determine how rapidly blood gets to the brain and whether there are any differences in blood flow from one area of the brain to another.

Pial synangiosis: One of several possible surgical approaches to treatment of Moyamoya. The temporal scalp artery is moved to run under a section of the skull and stitched to the surface of the brain. New blood vessels grow spontaneously to feed the oxygen-deprived areas of the brain. Pial synangiosis does not require severing and reconnecting the tiny scalp and brain arteries -- a significant advantage, especially in young children.

Shunt: A narrow tube that diverts fluid from one space to another. The word can refer either to an anatomical shunt or to an implanted medical device designed to correct fluid imbalance. A ventriculoperitoneal shunt is most common in the treatment of hydrocephalus. It sends cerebrospinal fluid from the over-filled ventricles of the brain to the space in the belly between organs, where it is easily absorbed. A one-way valve prevents fluid from the peritoneal space from finding its way back to the brain and regulates the pressure at which the shunt is activated. Under certain circumstances, neurosurgeons may also use a ventriculopleural shunt (deposits CSF in the space around the lungs) or a ventriculoatrial shunt (ends in the right atrium of the heart).

Stereotactic Radiosurgery: Stereotactic radiosurgery is not actually surgery at all, but can destroy small tumors by focusing several arcs of radiation so they intersect at the exact location of the tumor or malformation. The target area receives a destructive dose of radiation, but the surrounding tissues get only a mild dose. Sterotactic radiosurgery kills, but does not remove, the tumor or malformation.

Stroke: Blockage or rupture of a blood vessel in the brain. Typical symptoms include loss of consciousness, slurred speech, and inability to move or feel, especially on one side of the body.

Transient ischemic attack (TIA): Temporary blockage of blood vessels in the brain, usually leading to symptoms such as slurred speech, difficulty thinking, numbness or uncoordinated movement in part of the body.

Undifferentiated cells: In the development process, a single cell multiplies into many different kinds of specialized tissues. Undifferentiated cells are those that have not yet taken on the traits of the tissue they will become.