Practical dry calibration with medium adaptation for fluid-immersed endoscopy

A few endoscopic procedures are performed with a fluid-immersed endoscope. Fetoscopy is one such minimally invasive procedure which allows observation and intervention within the amniotic sac during pregnancy. The fetoscope is inserted through the uterus and is immersed in amniotic fluid. Fluid has a strong influence on the image formation process due to refraction at the interface of the fetoscopic lens which is determined by the optical properties of the amniotic medium. Accurate calibration is critical to vision-based methods for providing image-guided surgery and real-time information from the surgical site [1]. It consists of recording images of a calibration target of known geometric pattern in order to estimate optical properties of a camera. In the case of a fetoscope, calibration should be realised in the amniotic fluid which cannot be practically realised. A fluid-immersed pre-intervention calibration is also impractical for sterilisation purposes. Few computer vision methods address this issue and most of them are not adapted to the wide field of view as well as the severe distortion effect of a fetoscope. We experimentally show a direct link between dry and fluid-immersed camera parameters which can compensate for the optical properties of bodily or amniotic fluid as well as radial distortion effects.