A craniotomy is a surgical operation in which a bone flap is temporarily removed from the skull to access the brain. Craniotomies are often critical operations, performed on patients who are suffering from brain lesions, such as tumors, blood clots, removal of foreign bodies such as bullets, or traumatic brain injury (TBI), and can also allow doctors to surgically implant devices, such as deep brain stimulators for the treatment of Parkinson's disease, epilepsy, and cerebellar tremor. The procedure is also used in epilepsy surgery to remove the parts of the brain that are causing epilepsy.