A device for performing a biopsy on a brain is disclosed. The device includes a mount adapted for attachment to a burr hole on the skull of a patient. The mount has a body with a threaded surface and a cup-shaped surface forming a socket. The surface at the bottom of the socket defines a surgical aperture. A first arm extends away from the body, and has one fiducial marker. A second arm extends away from the body and has two spaced-apart fiducial markers. An instrument guide has a tubular body with a ball joint at one end with and an instrument insertion opening at an opposite end, configured to receive an instrument therethrough. The ball joint inserts into the socket, allowing the instrument guide to be positioned to a desired angle. A locking cup is attached to the body to secure the instrument in a fixed position.