A variety of devices exist to assist the neurosurgeon to know where he/she is, within the brain, at any time during a neurosurgical operation, within 1-2 mm of accuracy.  The patient's brain MRI or CT is registered to a computer, and then, using a reference arc seen in the picture, either infrared, ultrasound, or magnetic signals are emitted to allow the computer to guide the neurosurgeon during the surgery.