Frameless localization for functional neurosurgical procedures: a preliminary accuracy study

Targeting for functional stereotactic procedures is traditionally carried out in a noninteractive fashion, without real-time positional feedback. In addition, stereotactic frames are uncomfortable for patients and may impede intraoperative neurological evaluation. As an initial step toward a fully frameless approach to functional surgery, we have investigated the use of an image-guided microdrive coupled to a stereotactic frame system.

For initial laboratory tests, a stereotactic phantom was imaged using high-resolution CT scanning. Three representative targets were chosen within the phantom. Targeting was carried out in the usual fashion using the StealthStation planning suite, utilizing the Radionics CRW system. An LED-equipped reference arc was attached to the CRW base ring. Registration of the base ring was accomplished using a spherical probe. A custom-built microdrive was fitted with an LED array, mounted on the CRW arc and tracked by the StealthStation. The distance between the Stealth real-time localization and the CRW localization was measured on-screen. To evaluate the accuracy of the system in the operating room, a similar procedure was carried out in 13 functional neurosurgical operations (pallidotomy or deep brain stimulator placement).

Errors of localization in the laboratory setting ranged from 0.53 to 0.70 mm. In 11 operative cases, the average difference between the CRW localization and the Stealth localization was 1.77 mm. In the remaining 2 cases, equipment malfunction prevented measurement of localization error.

Frameless image-guided localization compares favorably to targeting performed noninteractively. In addition, real-time positional feedback confers advantages in target region visualization and confidence in placement of lesions and stimulators for functional procedures. With small improvements in accuracy and system reliability, fully frameless functional procedures could be safely carried out.