CT-target determination in postero-ventral pallidotomy: a universal method. Technical note.

Stereotactic targeting of the postero-ventral pallidum (PVP) presents special challenges to the surgeon 1) The target is in intimate relation to the optic tract and the internal capsule. 2) Proper angulation of the trajectory is crucial to achieve optimal effects given the long craniocaudal extension of the PVP. 3) The PVP is difficult to identify on computerized tomography (CT): the border between the internal capsule and the pallidum is usually not apparent. Potential sources of error in target determination include: - angulation of the scanning plane in relation to the intercommissural plane; - projection of the reference points (anterior commissure/posterior commissure [AC/PC]) results in the foreshortened intercommissural line, affecting the Y co-ordinate; - small tilts between the CT gantry and the stereotactic frame affect determination of the X and Z co-ordinates. Correction of these sources of error is done with basic trigonometric algorithms. The authors have developed a rapid method of stereotactic co-ordinate calculation which avoids the need to perform error-prone calculations under the pressure of operating room conditions. 1) The frame is applied with its basal plane corresponding roughly to the orbito-meatal line. 2) Thin CT slices (2 mm increments) are obtained through the area of interest and the slices are printed. 3) The major reference points (Foramen of Monro, AC, PC) are identified and marked. The PC point is projected onto the film containing AC (or viceversa if the PC slice is caudal to AC). 4) The intercommissural distance is measured, and the true length is obtained from a correction graph. The Y co-ordinate is then calculated as 1/2 ICL + 2 mm [towards AC]. 5) The slice corresponding to the target [Z co-ordinate] is obtained from a correction graph that takes into consideration the gap [number of slices] between AC and PC. 6) The X co-ordinate is placed 20 to 22 mm from midline. A graph that takes into consideration the coronal tilt of the stereotactic frame in relation to the CT gantry allows for final corrections of the X and Z co-ordinates. This step-by-step simple method of co-ordinate calculation can be used with any CT-compatible stereotactic frame.