The Virtual Operating Field—How Image Guidance Could Become Integral to Skull Base Surgery

Abstract

Purpose: Image guidance in skull base surgery no longer is a novelty. Still, the technology has not yet evolved into an omnipresent instrument like, for example, the microscope, although it does have an almost similar potential considering the constant desire for seeing beyond tissue barriers ahead in the surgical path in an ever-variable, individual anatomical environment. The premises for successful and meaningful implementation of virtual reality in image guidance for skull base procedures were investigated with different navigation systems over the past 6 years. Method: Primary volumetric imaging data from more than 200 patients have been acquired using MR, CT, and CT angiography. Pathologies included lesions in the anterior, middle, and posterior skull base. The data were transferred to the workstation of infrared-based image-guidance systems for creation of a virtual operating field (VOF) with translucent surface modulation. During surgery, the target registration error for anatomical landmarks was assessed and the VOF was compared to the individual anatomy of the surgical situs. Results: Technologically it is possible to reconstruct from preoperative imaging data a virtual, true three-dimensional model of the operating field with translucent surface modulation with the option to virtually “fly through” to the target structure. This model can be enhanced by CT-MRI image fusion and by adding functional characteristics, obtained from functional imaging or neurophysiological studies, to the morphology. Complex anatomical structures like the bone surface, the tortuous course of cranial vessels, or the outline of the paranasal sinuses can be easily visualized in such a model and recognized by the surgeon in one glance. Comprehension is greatly facilitated as compared with routine mental reconstruction of triaxial images. On a routine basis, it is not yet feasible to simulate depth in a stereoscopic version of such a VOF. Due to the limitations in spatial resolution with current imaging systems, viewing very small objects in the image (like most of the cranial nerves and small vessels) was rarely possible. Conclusion: There is a clear discrepancy between today's radiological routines and the challenges of surgical practice described in what could be called “the two-dimensional dilemma”. It seems obvious that skull base surgery, with its dependence on spatial visual orientation, would be among the specialties that will take the helm leading medicine into a new era of three-dimensional image guidance. Supported by sound anatomical knowledge, creation of a VOF for a surgical approach in an individual patient offers a deja vue experience that can truly enhance the capabilities of a surgical team. In addition, application of this technique in image-guided procedures aids to target or to avoid hidden anatomical structures. Once stereoscopic images can easily be injected into the microscope and overlaid onto the real operating field in view of the surgeon, image guidance is likely to become an integral part of most skull base procedures.