Calculation of stereotactic coordinates from the computed tomographic scan.

A protocol has been developed to use the GE 8800 scanner and its resident programs to calculate stereotactic coordinates, which has made it possible to use any stereotactic apparatus without modifying the apparatus in order to introduce a cannula into any lesion visualized on a computed tomographic (CT) scan to biopsy tumors, drain abscesses, implant radioisotopes, etc. The CT scanning is done in a routine fashion except that a lateral ScoutView, with the planes of each CT slice indicated, is included. Once the CT scan has been completed, resident programs for measuring distances are used to establish a zero point on a reference plane, from which all other coordinates can be defined. The stereotactic procedure is done at a separate time in the operating room, using the coordinates derived from the CT scan. The ScoutView image is compared to the lateral x-ray film taken during the stereotactic procedure to establish the location of the targets. It has been estimated that the accuracy of this system is 3 mm. Abscesses less than 1 cm in diameter deep within the cerebral hemisphere have been accurately aspirated and tumor biopsies have been successfully taken.