Accuracy of true frameless stereotaxy: in vivo measurement and laboratory phantom studies - Technical note

The authors present the results of accuracy measurements, obtained in both laboratory phantom studies and an in vivo assessment, for a technique of fr ameless stereotaxy. An instrument holder was developed to facilitate stereo tactic guidance and enable introduction of frameless methods to traditional frame-based procedures. The accuracy of frameless stereotaxy was assessed for images acquired using 05-tesla or 1.5-tesla magnetic resonance (MR) ima ging or 2-mm axial, 3-mm axial, or 3-mm helical computerized tomography (CT ) scanning. A clinical series is reported in which biopsy samples were obta ined using a frameless stereotactic procedure, and the accuracy of these pr ocedures was assessed using postoperative MR images and image fusion. The overall mean error of phantom frameless stereotaxy was found to be 1.3 mm (standard deviation [SD] 0.6 mm). The mean error for CT-directed framele ss stereotaxy was 1.1 mm (SD 0.5 mm) and that for MR image-directed procedu res was 1.4 mm (SD 0.7 mm). The CT-guided frameless stereotaxy was signific antly more accurate than MR image-directed stereotaxy (p = 0.0001). In addi tion, 2-mm axial CT-guided stereotaxy was significantly more accurate than 3-mm axial CT-guided stereotaxy (p = 0.025). In the clinical series of 21 f rameless stereotactically obtained biopsies, all specimens yielded the appr opriate diagnosis and no complications ensued. Early postoperative MR image s were obtained in 16 of these cases and displacement of the biopsy site fr om the intraoperative target was determined by fusion of pre- and postopera tive image data sets. The mean in vivo linear error of frameless stereotact ic biopsy sampling was 2.3 mm (SD 1.9 mm). The mean in vivo Euclidean error was 4.8 mm (SD 2 mm). The implications of these accuracy measurements and of error in stereotaxy are discussed.