Guidance from registered images has been used extensively in conjunction with endoscopic neurosurgery. Unlike frame-based stereotaxy, it allows free-hand movement of the endoscope with real-time feedback of its tip position. The versatility of the software is such that a 'tool file' is available for each of the various instruments introduced through the sheath. A change of instrument only necessitates a change in the active tool file on the computer workstation. The pre-calibrated parameters and length of the instrument then allow representation of that instrument in the multiplanar views on the screen.
In a study by Schroeder et al. , the principal usefulness of guidance from registered images was in the selection of an ideal entry point and trajectory to the lesion with minimal injury to the fornices and eloquent brain. Most procedures were then performed under direct endoscopic visual control. The pre-operative trajectory planning was very useful when the ventricular system was small and when the posterior third ventricle had to be approached through a small foramen of Monro. In the management of arachnoid cysts, a trajectory penetrating as many of the septae as possible, as well as ensuring an optimal fenestration point, would be planned. In such situations, as well as in multi-loculated hydrocephalus, there are few, if any, anatomical landmarks, and there is a real risk of disorientation, particularly if the membranes are thick . The value of image guidance in the maintenance of orientation was clear. In the endoscopic resection of colloid cysts, guidance from registered images facilitated the maintenance of a trajectory leading to the most lateral and anterior aspect of the foramen of Monro, without injury to the fornices or the caudate nucleus; this would, in turn, allow visualization of the roof of the third ventricle, rendering complete dissection of the cyst from its base easier.
The authors did not find pre-operative image guidance useful for endoscopic third ventriculostomy . In this procedure the ventricles tend to be large, and a high degree of endoscope maneuverability is possible. Clear intraventric-ular landmarks, such as the thalamo-striate vein, the foramen of Monro and the choroid plexus, determine the orientation. The basilar artery can often be seen through the thinned-out floor of the third ventricle, and its position can be scrutinized from the mid-sagittal preoperative MRI. The optimal position of the stoma is determined via visual information through the endoscope.
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