VARIABLES AFFECTING THE ACCURACY OF STEREOTAXIC LOCALIZATION USING COMPUTERIZED-TOMOGRAPHY

Stereotactic localization using computerized tomography (CT) is increa singly employed to guide neurosurgical procedures in crucial areas of the brain such as the brain stem. This technique allows the surgeon to resect a lesion in its entirety while sparing critical areas of the b rain. Thus, the parameters used for scanning should be selected for ma ximum accuracy. While the small pixel size of CT scanners suggests a h igh degree of precision in localization, there have been few systemati c studies of this accuracy. The authors have studied the amount of err or in localization created by variables such as CT scan thickness, int erscan spacing, size of lesion, and method of computation when using t he Brown-Roberts-Wells (BRW) stereotactic system. Over 1000 CT scans w ere made of a phantom composed of spheres of differing diameter and lo cation. The CT slice thickness was varied from 1.5 to 5.0 mm, and inte rscan spacing was varied from 0.5 to 3.0 mm. The coordinates of the ce nter of the spheres were calculated independently using the laptop com puter supplied with the unit and also by a stereotactic computer which automatically calculates the center of the fiducials. The actual BRW coordinates of the sphere center were obtained using the phantom base and were then compared to the computer-calculated coordinates to deter mine error in localization. Variables with a significant effect on err or included the scan thickness, interscan spacing, and sphere size. Th e mean error decreased 23% as the scan thickness decreased from 5.0 to 1.5 mm and 45% as the interscan spacing decreased from 3.0 to 0.5 mm. Mean error was greatest for the smallest sphere sizes. The two comput ational methods did not differ in error. This study suggests that, for critical areas of the brain or for small lesions, a scan thickness of 1.5 mm and interscan spacing of 0.5 mm should be employed.