The anterior and posterior commissures (AC;PC) are the most frequently used reference points for target determination in functional stereotactic surgery. Computerized tomography (CT) has been slow in replacing ventriculography for identification of these points, primarily because in most methods described to date, the AC/PC plane had to be identified by reformatted CT images or cumbersome mapping procedures. The authors describe their methodology for determination of functional CT-stereotactic coordinates using the Brown-Robert-Wells (BRW) stereotactic system, axial CT imaging, and a proportional method to rapidly scale standard stereotactic map coordinates to the diencephalic size of any individual patient. 1. CT gantry angulation coinciding with Twining's line allows rapid identification of the AC/PC plane on axial CT. 2. Determination of the AC/PC distance with the "measure distance" function of the scanner enables the surgeon to rapidly find the proportional coordinates for either V.O.P., V.im., or V.C. 3. This localization method takes little more time than a routine stereotactic biopsy. Its application is easy, demanding only a few minutes of CT-console time, employing the standard computer software available in every current CT scanner. The method was been successfully employed in 25 functional thalamic procedures.
