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Helmut Buchner, Ludwig Adams, Achim Knepper, Rainer Rüger, Gabriel Laborde, Joachim M. Gilsbach, Irene Ludwig, Jürgen Reul and Michael Scherg

electrode markers is shown in Fig. 2 . Fig. 2. Case 19. Magnetic resonance imaging slice showing electrode markers. Magnetic Resonance Imaging Data The MR image data were read into a computer-assisted surgery (CAS) system and displayed in a pseudo 3-D view. 2 This system was developed by the Institute of Measurement Technology at the Technical University of Aachen and is used in basal skull surgery for intraoperative navigation. 2, 15 The CAS system consists of a mechanical arm with six degrees of freedom and a high-resolution incremental digitizer

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John G. Golfinos, Brian C. Fitzpatrick, Lawrence R. Smith and Robert F. Spetzler

-weighted MR imaging to delineate low-grade gliomatous lesions more clearly. More importantly, future improvements will allow updating of the imaged dataset to reflect changes in the position of the brain during resection. Until these improvements are made, the viewing wand system is a useful and reliable implement for intraoperative navigation. References 1. Adams L , Krybus W , Meyer-Ebrecht D , et al : Computer-assisted surgery. IEEE Comput Graph Appl 10 : 43 – 51 , 1990 Adams L, Krybus W, Meyer-Ebrecht D, et al

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Neil L. Dorward, Olaf Alberti, Binti Velani, Frans A. Gerritsen, William F. J. Harkness, Neil D. Kitchen and David G. T. Thomas

Object. This prospective study was conducted to quantify brain shifts during open cranial surgery, to determine correlations between these shifts and image characteristics, and to assess the impact of postimaging brain distortion on neuronavigation.

Methods. During 48 operations, movements of the cortex on opening, the deep tumor margin, and the cortex at completion were measured relative to the preoperative image position with the aid of an image-guidance system. Bone surface offset was used to assess system accuracy and correct for registration errors. Preoperative images were examined for the presence of edema and to determine tumor volume, midline shift, and depth of the lesion below the skin surface. Results were analyzed for all cases together and separately for four tumor groups: 13 meningiomas, 18 gliomas, 11 nonglial intraaxial lesions, and six skull base lesions.

For all 48 cases the mean shift of the cortex after dural opening was 4.6 mm, shift of the deep tumor margin was 5.1 mm, and shift of the cortex at completion was 6.7 mm. Each tumor group displayed unique patterns of shift, with significantly greater shift at depth in meningiomas than gliomas (p = 0.007) and significantly less shift in skull base cases than other groups (p = 0.003). Whereas the preoperative image characteristics correlating with shift of the cortex on opening were the presence of edema and depth of the tumor below skin surface, predictors of shift at depth were the presence of edema, the lesion volume, midline shift, and magnitude of shift of the cortex on opening.

Conclusions. This study quantified intraoperative brain distortion, determined the different behavior of tumors in four pathological groups, and identified preoperative predictors of shift with which the reliability of neuronavigation may be estimated.

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Neil L. Dorward, Olaf Alberti, James D. Palmer, Neil D. Kitchen and David G. T. Thomas

✓ The authors present the results of accuracy measurements, obtained in both laboratory phantom studies and an in vivo assessment, for a technique of frameless stereotaxy. An instrument holder was developed to facilitate stereotactic guidance and enable introduction of frameless methods to traditional frame-based procedures. The accuracy of frameless stereotaxy was assessed for images acquired using 0.5-tesla or 1.5-tesla magnetic resonance (MR) imaging 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 obtained using a frameless stereotactic procedure, and the accuracy of these procedures 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 frameless stereotaxy was 1.1 mm (SD 0.5 mm) and that for MR image—directed procedures was 1.4 mm (SD 0.7 mm). The CT-guided frameless stereotaxy was significantly more accurate than MR image—directed stereotaxy (p = 0.0001). In addition, 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 frameless stereotactically obtained biopsies, all specimens yielded the appropriate diagnosis and no complications ensued. Early postoperative MR images were obtained in 16 of these cases and displacement of the biopsy site from the intraoperative target was determined by fusion of pre- and postoperative image data sets. The mean in vivo linear error of frameless stereotactic 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.

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Neil L. Dorward, Olaf Alberti, Binti Velani, Frans A. Gerritsen, William F. J. Harkness, Neil D. Kitchen and David G. T. Thomas

This prospective study was conducted to quantify brain shifts during open cranial surgery, to determine correlations between these shifts and image characteristics, and to assess the impact of postimaging brain distortion on neuronavigation.

During 48 operations, movements of the cortex on opening, the deep tumor margin, and the cortex at completion were measured relative to the preoperative image position with the aid of an image-guidance system. Bone surface offset was used to assess system accuracy and correct for registration errors. Preoperative images were examined for the presence of edema and to determine tumor volume, midline shift, and depth of the lesion below the skin surface. Results were analyzed for all cases together and separately for four tumor groups: 13 meningiomas, 18 gliomas, 11 nonglial intraaxial lesions, and six skull base lesions.

For all 48 cases the mean shift of the cortex after dural opening was 4.6 mm, shift of the deep tumor margin was 5.1 mm, and shift of the cortex at completion was 6.7 mm. Each tumor group displayed unique patterns of shift, with significantly greater shift at depth in meningiomas than gliomas (p = 0.007) and significantly less shift in skull base cases than other groups (p < 0.003). Whereas the preoperative image characteristics correlating with shift of the cortex on opening were the presence of edema and depth of the tumor below skin surface, predictors of shift at depth were the presence of edema, the lesion volume, midline shift, and magnitude of shift of the cortex on opening.

This study quantified intraoperative brain distortion, determined the different behavior of tumors in four pathological groups, and identified preoperative predictors of shift with which the reliability of neuronavigation may be estimated.

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Marie Bourgeois, Christian Sainte-Rose, Arielle Lellouch-Tubiana, Conor Malucci, Francis Brunelle, Wirginia Maixner, Giuseppe Cinalli, Alain Pierre-Kahn, Dominique Renier, Michel Zerah, Jean-François Hirsch, Francoise Goutières and Jean Aicardi

involved the whole temporal lobe. All operations were performed after induction of general anesthesia, and intraoperative electrocorticography was not performed. Intraoperative localization methods included ultrasonography, stereotaxy, and, more recently, image-guided computer-assisted surgery. 3, 30 Histopathological Studies All resected specimens were examined both macroscopically and histologically, and appropriate staining was performed when tumor was found. Particular attention was paid to specimens previously diagnosed as low-grade gliomas, which were reclassified

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Timothy Ryken, Terrence Julien, Bruce Frankel, Greg Canute, John Haller and Arthur Rosenbaum

Transoral odontoidectomy is often performed in the treatment of cervicomedullary junction disease. The operating microscope is frequently used to improve visualization in this narrow field of view. In the setting of complex anatomy or surgical revision the authors hypothesized that combining frameless stereotactic technique with intraoperative microscopy would improve the ability to visualize and identify intraoperative anatomy. In addition they believed that the ability to visualize the targeted region directly in the operating microscope "image injection" would be of particular interest in this setting, provided that sufficient accuracy for use could be obtained in the registration process. The authors assessed the efficacy of this approach in a cadaveric model and obtained sufficient accuracy to warrant use in the operating room. This technique was applied in the surgical management of a 56-year-old woman with rheumatoid arthritis who had undergone a previous decompressive transoral procedure. Subsequently she suffered progressive deterioration and was found to have residual bony compression of the anterior cervicomedullary junction. The authors performed decompressive surgery and obtained satisfactory results by using the image-injected technique, and the patient experienced subsequent clinical improvement. The authors conclude that the image-injected frameless stereotactic technique is of potential benefit, particularly in the narrow window of approach of the transoral odontoidectomy.

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appreciate the interest of Dr. Benzel in our study and wish to thank him very much for his comments. Our provided strategy may seem a bit confusing, but principally it is a mathematical description. Moreover, it is always difficult to explain mathematics with words. That is why we described our technique via a very simple mathematical equation. It is also more practical to obtain the measures from direct images and to calculate the result in the operating room. We believe that computer-assisted surgery is the only reliable way to access real-time correct screw placement

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Sabri Cem Açıkbaş and Mehmet Recai Tuncer

still requires surgical skill and experience. The reported incidence of screw misplacement ranges from 28.1 to 39.9% in clinical studies in which the screw positions were verified using intraoperative fluoroscopy. 8, 9, 14, 16 A low incidence of screw malpositioning of 4.3%, under clinical conditions, has also been reported when computer-assisted surgery has been performed. 9 Despite the inherent risks of inserting transpedicular screws, permanent nerve root injuries related to pedicle screw insertion have occurred less frequently (range 0.1–2.0%) than might be

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Reto J. Bale, Johannes Burtscher, Wilhelm Eisner, Alois A. Obwegeser, Michael Rieger, Reinhart A. Sweeney, Andreas Dessl, Salvatore M. Giacomuzzi, Klaus Twerdy and Werner Jaschke

for computer-assisted surgery. This is especially important for young children who are not candidates for rigid head fixation. A recently published study 12 confirms submillimetric repositioning accuracy of the VBH mouthpiece itself with respect to the patient's head. In addition, the MR-compatible mouthpiece and the registration rods can be equipped with different types of fiducial markers for accurate fusion of CT, MR, PET, and SPECT images. We found the availability of accurate reference points for image fusion to be an especially promising feature, which is