Figure 1. Frame-based stereotaxis involves several coordinate spaces, which include the anatomical space , the frame space , and the head-stage space . These spaces are defined in surgery once a frame is attached to a patient and has undergone stereotactic localization (Left). Then, one can perform coordinate transformations between all three systems and the anatomical space to an atlas for which the mathematical computations are referenced in later sections of the article (Right, A-D). Conversion D can also be utilized with computed tomography (CT) or magnetic resonance imaging (MRI) rather than an atlas. Important reference points include the anterior commissure (AC), posterior commissure (PC), and the midline. In frames that operate around an isocenter, which are known as target-centered frames, the head-stage rotates around the frame-space. Three angles can be applied to the head-stage, which are in frame space the ring angle , the arc angle , and the axial angle representing rotations about the x-axis, the y-axis, and the z-axis, respectively. In anatomical space, represents the sagittal angle, whereas represents the coronal angle. The angle can also be used for electrode rotation. In addition, there are movements in the head-stage along the AP, LAT, and Vertical (depth) axes.

AP = Antero-Posterior; LAT = Lateral; VERT = Vertical; X = Anatomical space; X' = Frame-based space; X'' = Head-stage space; AC = Anterior Commissure; PC = Posterior Commissure; Midline = Midline structure

Left Image Pane. Depiction of the Cosman-Roberts-Wells (CRW) Stereotactic Apparatus (Radionics CRW Stereotactic System, Integra LifeSciences Corporation, Plainsboro, New Jersey)