Improving neurosurgical visualisation: Assessment of the thermal stability of a new visualisation device for neurosurgery

Abstract

Background Current neurosurgical visualisation systems, operating microscopes as well as exoscopic systems, display problems that make their performance suboptimal. Operating microscopes severely restrict ergonomics, occasionally leading to pain or injuries. Existing exoscope systems have diverse limitations, therefore not yet being able to completely replace the neurosurgical operating microscope. A plan was made to develop a new and improved visualisation system, tackling the problems encountered with standard operating microscopes, as well as existing exoscope systems. This will be done by developing technology that integrates three important techniques which hold the potential to improve neurosurgical outcome, i.e. surgical visualisation, navigation technology, and 3D X-ray imaging. The rising temperature in this system might cause unwanted deformations which lead to inaccuracies in the visualisation. Therefore the objective of this thesis is to firstly develop a virtual model of this system and secondly, to evaluate the thermal stability through simulations, in order to ensure that the system's accuracy is adequate despite potential deformations due to increased temperature. Hence the research question: "Is the thermal stability of the new exoscope concept sufficient for adequate accuracy?'' Method To ensure the new visualisation system possesses all essential features required for improving and possibly replacing the current visualisation systems, its requirements are summarised. This set the foundation of the new system's design. The best concept is elaborated upon after evaluating different design concepts, meaning that different combinations of materials, frames, and mounts were designed. To evaluate which set-up best performs in regard to stability, simulations were carried out with 3D models of the concepts. These simulations were performed in COMSOL Multiphysics, a programme that can calculate heat transfer and displacement throughout the system, mapping out how susceptible the concept is to increasing temperature. Results A total of 23 simulations were carried out in COMSOL Multiphysics. The first five aimed to provide insight into important material properties, which was used in the next stage to select four different materials: anodised aluminium alloy 6061, stainless steel 304, invar, and silicon nitride. The next simulations evaluated the different combinations of frames, mounts and materials, concluding that the design made out of invar, displayed the least deformations. This selection was made based on displacements found in the zoom and the navigation lenses. These would eventually translate into a certain image shift, reducing the system's accuracy. After finding the most stable design, the absolute image shift was calculated from the displacements. The shift reached a maximum of approximately 0.10 mm for the navigation lens and 0.18 mm for the zoom lens. Conclusion Despite the increasing temperature and other possible effects as gravity, the image shift remained within acceptable limits. This means the virtual concept has thermal stability high enough to provide adequate accuracy during surgery. The new exoscope system is therefore feasible in terms of stability, meaning that the integration of the navigation and the 3D X-ray systems could be a valuable improvement to the current neurosurgical visualisation systems.