An arteriovenous malformation (AVM) is an abnormal cluster of blood vessels in the brain or spine in which one or more arteries are directly connected to one or more veins. Arteriovenous fistulas (AVF) are similar to AVMs. AVFs are found in the coverings of the brain and spinal cord whereas AVMs are found within the tissue of the brain or spinal cord. Endovascular embolization of an AVM or AVF is a minimally invasive procedure in which embolic material is injected, shutting off blood flow to the affected arteries. Embolization may be sufficient to cure an AVM or AVF, or it may be performed as a preliminary step in preparation for other surgical procedures (microsurgical resection or stereotactic radiotherapy).