Active Contours in Three Dimensions

iv " Ready comprehension is often a knee-jerk response and the most dangerous form of understanding. It blinks an opaque screen over your ability to learn. Be warned. Understand nothing. All comprehension is temporary. " [8] v ABSTRACT To find a shape in an image, a technique called snakes or active contours can be used. An active contour is a curve that moves towards the sought-for shape in a way controlled by internal forces-such as rigidity and elasticity-and an image force. The image force should attract the contour to certain features, such as edges, in the image. This is done by creating an attractor image, which defines how strongly each point in the image should attract the contour. In this thesis the extension to contours (surfaces) in three dimensional images is studied. Methods of representation of the contour and computation of the internal forces are treated. Also, a new way of creating the attractor image, using the orientation tensor to detect planar structure in 3D images, is studied. The new method is not generally superior to those already existing, but still has its uses in specific applications. During the project, it turned out that the main problem of active contours in 3D images was instability due to strong internal forces overriding the influence of the attractor image. The problem was solved satisfactory by projecting the elasticity force on the contour's tangent plane, which was approximated efficiently using sphere-fitting.