The Research On Photometric Stereo Reconstruction Applied To 3D Endoscopy

In the medical diagnosis,although color video endoscope has wide field of inspections of the gastrointestinal tract's surface,it still exists many important pathological changes which are not observed or classified correctly,which leads to decrease lesion detection.Many clinically important lesions of the gastrointestinal tract have unique surface topographies.For some lesions,this topographical contrast is more pronounced than their coloration.However,tissue coloration is the primary source of contrast in conventional endoscopic cameras.It may be possible to increase lesion detectability accuracy greatly by introducing Photometric Stereo algorithm and making tiny modifications of the conventionsl endoscope to acquire both conventional color-based images and surface topographical data.In this paper,a three-dimensional reconstruction technique of Photometric Stereo for endoscope is studied.The research work mainly includes the following three parts.(1)The Photometric Stereo algorithm for endoscope is designed and implemented.The algorithm is implemented by MATLAB and the basic flow of the algorithm is as follows:Firstly,the direction of the light sources is calibrated by the high-light black ball;then the normal vector of each point on the target image is calculated by the direction of the calibrated light source,and transformed into a gradient.Finally,the three-dimensional reconstruction of the surface of the object is made by using the gradient-field reconstruction method of the Poisson equation.The comparison between the Poisson equation and the least squares method shows that the reconstruction accuracy of the Poisson equation method is much higher.(2)We construct a simulation endoscope system based on Photometric Stereo,including hardware design of light source and software design of 3D reconstruction algorithm.Using the CMOS camera to collect photometric images;using four LED point light source as light source,they are evenly distributed to the center of the CMOS camera,the radius of 7mm circumference,LED circuit control can be individually opened and closed;the 3D reconstruction software is designed by MATLAB,which has good human-computer interaction ability,and can output the 3D reconstruction result by inputting a group of different LED lighting images.(3)The three-dimensional reconstruction performance was tested and analyzed on the simulation system.The three-dimensional reconstruction of the continuous curved gypsum hemisphere,the stepped surface step bubble and the gears with very fine features are carried out in the range of 25-55mm.The results show that the system can reconstruct their three-dimensional surface well.The measurement accuracy of the simulation system is about 1mm.Through the experimental analysis,this paper validates the feasibility of three-dimensional endoscope measurement system based on Photometric Stereo.Especially in detail reconstruction,3D reconstruction based on Photometric Stereo is more advantageous for endoscopy.