Visualization Techniques for Virtual Endoscopy

Virtual Endoscopy is a technique to explore hollow organs and anatomical cavities using 3D medical imaging and computer graphics. Virtual Endoscopy turns out to be a promising technique to improve, or even in some procedures substitute, real endoscopy. A real endoscopy is invasive and usually implies some risk for the patient. In the last decades, much research has been done in this field. This thesis reflects a piece of this research concentrated on investigating visualization techniques for virtual endoscopy. Most common methods in virtual endoscopy simulate the behavior of a real endoscope. In the first part of this thesis, we concentrate on studying these methods and present a general framework for such a system. We develop a prototype according to this framework. A central path of the organ is used to move the camera through the organ. We improve an existing algorithm to find the central path of the organ. We describe two new techniques to accelerate high quality perspective volume rendering. The first method is a new space leaping acceleration technique for ray casting. The second method uses hardware acceleration (i.e., VolumePro) of orthographic volume rendering to generate perspective volume rendering. Simulating a real endoscopy is not the most suitable visualization technique in many endoscopy procedures. A real endoscopy is restricted due to physical limitations that a virtual endoscopy does not have. The second part of the thesis presents two techniques which virtually unfold the colon to inspect its surface and detect polyps. We concentrate on the colon although these techniques could be used with other organs too. The first technique, locally unfolds the colon and generates an animation sequence from consecutive unfolded regions. The images are generated with a projection technique that allows the physician to visualize most of the surface, and to easily recognize polyps that in an endoscopic view would be hidden by folds or would be hard to localize. The drawback of the local colon unfolding is that the physician has to inspect a video to be able to visualize the whole surface. The goal of the second technique, nonlinear colon unfolding, is to enable the physician to inspect and get as much information as possible of the inner organ surface at a first glance. It obtains a single image of the complete unfolded colon. In this way, the problematic areas can be identified quickly and inspected later in more detail. Every technique have been tested with several data sets to show their feasibility. Although, these tests are not enough to use the methods in the clinical environment, the results show their potential to achieve such an state.