Purpose: Although computer- and image-guided surgical procedures are an improvement of frame-guided stereotaxy, many navigation systems still require rigid fixation of the patient's head throughout the operation. This study describes the clinical application of a technical modification that enables cranial navigation with "free head mobility" using CT and MR images as well as the calculated 3-D reconstruction models.
Material and methods: A sensor-based electromagnetic neuronavigation system was expanded to allow the localization and position monitoring of several sensors within an electromagnetic field. One of these sensors was attached to a dental splint as an additional reference (DRF = dynamic reference frame). Thus, it was possible to determine the position of the sensor-guiding surgical instruments and to record the slightest movement of the cranium as well. This information was then used to continuously adapt the position of the imaging plane and the resultant calculated 3-D reconstructions to the actual position of the cranium.
Results: The clinical application of the DRF was tested for different neurosurgical procedures. They included image-guided biopsies and endoscopic interventions using MRI data, transnasal accesses to the base of the skull using CT data and surgical removal of multilocular metastases using data from both imaging modalities. Intracranial target reference points as well as those on the skull were found with a high accuracy to the initial measurement position after arbitrary movement of the patient's head. Thus, navigation was also possible without rigid fixation of the head because of the continuous adaptation of the imaging data on the change in position of the patient's head.
Conclusion: Based on these first test results, a high clinical potential for DRF application in cranial navigation is to be expected. The aim of DRF is to dispense with the rigid fixation of the patient's head. This increases the application scope of image-guided navigation procedures to include, for example, any bioptic or endoscopic intervention, in which rigid pin fixation of the cranium is not required or desired. For all other procedures, continuous position monitoring by DRF ensures automatic correction of imaging data with mechanical alteration of the head position.