Instrument guidance for stereotactic surgery

A method and apparatus for positioning a surgical instrument during stereotactic surgery using a guidance fixture. The guidance fixture includes an upper portion, which includes an instrument guide for moving the surgical instrument along a constrained trajectory relative to the upper portion, and includes an adjustable base supporting the upper portion, which includes a mounting base and an adjustment mechanism. The adjustable base can include a rotating collar attached to the mounting base and a pivoting collar coupling the rotating collar to the upper portion. A remote sensing device, such as a camera array sensing LEDs on the fixture, is used to determine the orientation of the upper portion of the guidance assembly in relation to the body.