Abstract-We present a gradient-based method for rigid registration of a patient preoperative computed tomography (CT) to its intraoperative situation with a few fluoroscopic X-ray images obtained with a tracked C-arm. The method is noninvasive, anatomybased, requires simple user interaction, and includes validation. It is generic and easily customizable for a variety of routine clinical uses in orthopaedic surgery. Gradient-based registration consists of three steps: 1) initial pose estimation; 2) coarse geometry-based registration on bone contours, and; 3) fine gradient projection registration (GPR) on edge pixels. It optimizes speed, accuracy, and robustness. Its novelty resides in using volume gradients to eliminate outliers and foreign objects in the fluoroscopic X-ray images, in speeding up computation, and in achieving higher accuracy. It overcomes the drawbacks of intensity-based methods, which are slow and have a limited convergence range, and of geometry-based methods, which depend on the image segmentation quality. Our simulated, in vitro, and cadaver experiments on a human pelvis CT, dry vertebra, dry femur, fresh lamb hip, and human pelvis under realistic conditions show a mean 0.5-1.7 mm (0.5-2.6 mm maximum) target registration accuracy.Index Terms-Fluoroscopic X-ray to CT registration, gradient based, image registration, 2D/3D rigid registration.
This paper presents a new on-line automatic X-ray fluoroscopic C-arm calibration method for intraoperative use. C-arm calibration is an essential prerequisite for accurate X-ray fluoroscopy-based navigation and image-based registration. Our method utilizes a customdesigned calibration ring with a two-plane pattern of fiducials that attaches to the C-arm image intensifier, and an on-line calibration algorithm. The algorithm is robust, fully automatic, and works with images containing anatomy and surgical instruments which cause fiducial occlusions. It consists of three steps: fiducial localization, distortion correction, and camera calibration. Our experimental results show submillimetric accuracy for calibration and tip localization with occluded fiducials.
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