MRI scans, with subsequent fusion of the two image sets on the navigation workstation, was described. The CT scan allowed accurate patient-to-image registration on the basis of bony landmarks. The MRI scan allowed detailed evaluation of soft tissue and bony distortion by the tumor. A registration error of less than 2 mm was obtained in each of the 10 cases.

The workstation facilitates selection of the optimal skull base approach for maximal resection of the lesion. Sure et al. [11] found it particularly useful in deciding whether a pterional or an orbitozygomatic approach would give the better access to a given parasellar lesion. In posterior fossa surgery, it allows precise definition of keyhole craniotomies in relationship to the underlying dural venous sinuses. Neuronavigation also allows a clear pre-operative indication of how much tumor can be excised safely, depending on the proximity of the tumor to important structures.

Intraoperatively, early identification of important anatomical landmarks, such as the clinoid processes in anterior fossa surgery, and the petrous apex, the arch of the atlas, the vertebral artery, the occipital condyle and the bone cells in the posterior fossa, is useful. Major vessels and critical neural structures can be identified early during intratumoral decompression, facilitating their preservation when displaced by, or encased within, the tumor mass. It allows the neurosurgeon to calculate the position of the instrument within a large tumor cavity devoid of landmarks. A better understanding of the topography of complex anatomical structures is afforded; this is relevant, for example, when the petrous portion of the carotid artery is being protected during drilling of invaded or eroded petrous temporal bone. Bony infiltration by skull base meningiomas is sometimes not grossly identifiable, even under the microscope, and image guidance then allows a more complete tumor resection.

In one of the few studies that also takes account of the overall cost effectiveness of image guidance, Elias et al. [12] described the role of CT-based neuronavigation in transphe-noidal surgery for pituitary tumors. The maintenance of an appropriate midline trajectory is vital in transsphenoidal surgery. Intracranial entries into the anterior fossa floor and through the clivus have been reported. Whereas intraoperative fluoroscopy only provides sagittal guidance, the neuronavigation system gives constant 3D information and allows adjustments in both the sagittal and the coronal planes during the approach to the sella. Attachment of reflective markers to ring curettes and their calibration within the navigation system allow the neurosurgeon to identify the sellar boundaries and to delineate the cavernous sinus and the carotid artery on each side. The system, however, cannot reliably demonstrate soft and potentially mobile tissues within the sella once excision of the adenoma has begun. Use of the image-guidance system to dynamically assist in, or confirm, the complete removal of the adenoma necessitates the incorporation of some form of intraoperative image updating.

The usefulness of such a system in defining and confirming the midline trajectory is particularly evident in re-operations. Disruption of important midline landmarks, such as the vomer, the anterior nasal spine and the rostrum of the sphenoid, renders such operations hazardous. Image guidance allows the neurosurgeon to approach the sella with increased confidence and goes a long way to increase the safety of the procedure.

In an effort to decrease the additional cost ($318 per procedure) and the time requirement in the setting up and registration of the system (mean of an extra 12 minutes per procedure), the same neurosurgical unit has evaluated frameless fluoroscopy-guided trans-sphenoidal surgery using the FluoroNav Virtual Fluoroscopy System (Medtronic Sofamor Danek Inc., Memphis, TN) [13]. A dynamic reference arc was attached to the headholder fixed to the patient's cranium. A calibration device containing multiple light-emitting diodes was attached to a standard C-arm fluoroscope. This device supplies information regarding the relationship of the C-arm and the reference arc to the computer. Frontal and lateral videofluoro-scopic images were then obtained, calibrated and stored in the computer. With a referenced probe, the surgeon was then able to refer to the stored images and visualize its position in real time. The principal advantages of this system are: (1) radiation exposure to the patient is reduced (fluoroscopy is only used once at the beginning of the procedure, and there is no pre-operative CT scan); (2) the fluoroscope can be removed from the theater before draping the patient, cutting down on radiology costs;

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