Easy and stable bronchoscope camera calibration technique for bronchoscope navigation system

This paper presents an easy and stable bronchoscope camera calibration technique for bronchoscope navigation system. A bronchoscope navigation system is strongly expected to be developed to make bronchoscopic examinations safer and more effective. In a bronchoscope navigation system, virtual bronchoscopic images are generated from a 3D CT image taken prior to an examination to register a patient's body and his/her CT image. It is absolutely indispensable to know correct intrinsic camera parameters such as focal length, aspect ratio, and the projection center of the camera for the generation of virtual bronchoscopic images. In the case of a bronchoscope, however, it is very complicated to obtain these camera parameters by calibration techniques applied to conventional cameras, since a bronchoscope camera has heavy barrel-type lens distortion. Also image resolution is quite low. Therefore, we propose an easy and stable bronchoscope camera calibration technique that does not require any special devices. In this method, a planar calibration pattern is captured at many different angles by moving the bronchoscope camera freely. Then we automatically detect feature points for camera calibration from captured images. Finally, intrinsic camera parameters are estimated from these extracted feature points by applying Zhang's calibration technique. We applied the proposed method to a conventional bronchoscope camera. The experimental results showed that reprojection error using estimated camera parameters was about 0.7 pixels. Also stable estimation was achieved by the proposed method.