Digital Infrared Navigator For Surgery

Advanced navigators for surgery enable a surgeon to look through the anatomy of the patient, to see the instruement guided into the anatomy, and to plan the path of the instruement, so that the invasion is reduced and the accuracy of operation is improved. Consequently, surgical navigation has become a hot spot of application and research in medical field. Drived by the domestic demand, the thesis is intended to research into key techniques involved in navigators for surgery.In order to get sufficient information and find correct orientation of the research, a review on the technology is made. After introduction of concepts, advantages and development related to navigator for surgery, the necessity to research and develop surgical navigator is clear; through comparison of different development level in the world and in our country, the urgency of developping domestic surgical navigator with completely independent intellectual property rights is addressed. According to our condition and advantages, the main contents and objective of the research are determined. The major results of the thesis are as follows.1. A method to calibrate camera using the Center Line of planar homography matrix is proposed. The most promising surgical navigators so far use optical localizer whose principle is stereo, in which camera calibration is a key step. In practice of calibrating stereo system, some problems with the existing camera calibration methods are encountered. To solve these problems, a method using the Center Line of planar homography matrix is proposed. Some useful properties of the Center Line are presented and derived, based on which the calibration method is described. The properties of the Center Line are also used to discuss the problem on taking effective calibration images to obtain accurate calibration result and improve performance of optical navigator for surgery. Experiments validate the method. 2. A model of lens distortion is proposed. Lens distortion is one of the main factors affecting accuracy of camera calibration and stereo computation, and distortion model is an important issue in distortion correction. While calibrating cameras used in surgical navigator, it is noticed that the parameters in the traditional distortion model are not independent from one another. Based on analysis of lens distortion sources and the relation among different distortion components, a new model of lens distortion is proposed with theoretical derivation and experimental validation.3. A method to estimate initial value of lens distortion is proposed. According to the requirement for the measuring volume and the distance from the stereo rig to the instruement, wide-angle lenses with short focal length are used. Consequently, lens distortion can not be neglected. At the initializing stage of existing methods, the distortion center is assumed to be the image center, while all disdortion coefficients are assumes to be zeros. When a lens has visible distortion, the iterative process of optimization may converge to local minima. So it is desirable to estimate distortion parameters more accurately. The method is derived theoretically and validated experimentally.4. A direct method to reconstruct the scene point from nonparallel stereo is proposed. The stereo rigs used in surgical navigators are usually configured with their camera coordinate systems being not parallel to each other. In this case, the conventional reconstruction method based on the standard parallel stereo configuration cannot be used directly. Existing methods fall into two classes. One back-projects the projecting rays into space and finds their intersection. Problems with the method are initial value estimation and local minima. The other rectifies arbitrary stereo image pairs to make them satisfy standard parallel stereo and then employs the conventional algorithm. The main disadvantages of the later method are time consuming and not taking noise into account, which will affect real-time and accuracy of instruement tracking. In the thesis, a direct method to reconstruct a scene point from nonparallel stereo is proposed, bypassing the process of rectifying the images or iteratively optimizing. Theoretical derivation and experiment results from both simulated data and real images validate the method. Based on the above results of research, a prototype of digital infrared navigator for surgery is developped, which includes optical system designing, the hardware constructing, program coding, system calibrating, testing, accuracy evaluating and navigation simulating. The experiment results show that the main performance of the prototype has satisfied the aimed requirement, which validates the methods proposed above.