Most of these approaches focus on the algorithm used for feature detection of the calibration pattern. Zhang's method has been shown to be fast and computationally stable however, several researchers have developed ways to improve upon it. Zhangs's method gathers multiple images of a calibration pattern of known geometry and estimates the pinhole camera model that best models the projection of the calibration pattern to the captured images. In the computer-assisted surgery literature, most authors use Zhang's method 11 to determine the intrinsic parameters of a given camera. The result is calibration procedures that are highly user-dependent, which is not acceptable for an approach that is intended for wider clinical deployment. Although calibration procedures claiming to be fully automatic 8, 10 are not new, they remain susceptible to variation in data application. However, in most cases, validation and comparison with alternative approaches are performed by the authors without clearly defining the acquisition protocols used. Some have gone further to propose methods that are compatible with the operating theater. Previous studies have investigated the main mathematical, image processing, and technical aspects of camera calibration. This has led us to develop a calibration rig and protocol that enables robust and repeatable calibration by theater staff. Safe clinical practice is dependent on the existence of clear protocols that describe how a procedure should be performed and specific success criteria. These challenges, along with the basic mathematical challenges, mean that although the literature on laparoscope calibration is extensive, we still lack a standard, repeatable methodology for performing this essential operation. The term hand–eye derives from the methods' origin in the robotics literature 8, 9 where the camera forms the robot's eye and the hand refers to the robotic end-effector holding the camera.Ĭalibration of surgical laparoscopic cameras brings further challenges including the requirement to maintain sterility, constraints on the range of laparoscope motion, the need to fit in with clinical workflows, and the requirement to be performed by clinically trained theater staff rather than technical specialists. Finally, for systems that utilize external tracking systems such as electromagnetic or optical systems, 7 it becomes necessary to perform a hand–eye calibration to determine the location and orientation of each camera relative to the reference frame. 5 Where multiple cameras are present, for example, stereo laparoscopes, it may also be necessary to perform an extrinsic calibration to determine the relative locations and orientations of the two lenses to enable stereoscopic reconstruction 6 and reprojection. 4 In the simplest case, camera calibration consists of intrinsic calibration of the laparoscope cameras to determine the cameras' optical characteristics, enabling both the reconstruction of visible surfaces and the projection of virtual objects onto the image plane. 1- 3 The calibration accuracy is a key contributor to the overall system accuracy. Laparoscopic camera calibration is a prerequisite for surgical image guidance systems that utilize model to image registration. The use of the calibration rig results in a statistically significant decrease in calibration error metrics, versus freehand calibration, and represents the preferred approach for use in the operating theater. The use of a ChArUco pattern yielded slightly lower reprojection errors, while a dot grid produced lower reconstruction errors and was more robust under strong global illumination. 12.64 mm, p-value 1e−6) compared with freehand calibration. 10.10 px, p-value 6e−7), and tracked reconstruction (1.38 mm vs. Use of the calibration rig reduced mean errors: reprojection (1.47 mm vs. Stereo reprojection, stereo reconstruction, tracked stereo reprojection, and tracked stereo reconstruction error metrics were used to evaluate calibration quality. The rig was compared against freehand calibration. MethodsĪ custom calibration rig, to enable rapid calibration in a surgical setting, was designed. This work seeks to identify a suitable method for tracked stereo laparoscope calibration within theater. The process must also be able to be undertaken by a nonexpert user in a surgical setting. Accurate camera and hand-eye calibration are essential to ensure high-quality results in image-guided surgery applications.
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