Anatomy based registration of CT-scan and x-ray fluoroscopy data for intra-operative guidance of a surgical robot

We describe a new method for rigid registration of a pre-operative CT-scan image to a set of intra-operative x-ray fluoroscopic images, for guiding a surgical robot to its trajectory planned from CT. Our goal is to perform the registration, i.e. compute a rotation and translation of one data set with respect to the other to within a prescribed accuracy, based upon bony anatomy only, without external fiducial markers. With respect to previous approaches, the following aspects are new: (1)We correct the geometric distortion in fluoroscopic images and calibrate them directly with respect to the robot by affixing to it a new calibration device designed as a radiolucent rod with embedded metallic markers, and by moving the device along two planes, while radiographs are being acquired at regular intervals. (2) The registration uses an algorithm for computing the best transformation between a set of lines in three space, the (intra-operative) x-ray paths, and a set of points on the surface of the bone (imaged pre-operatively), in a statistically robust fashion, using the Cayley parameterization of a rotation. (3) To find corresponding sets of points to the x-ray paths on the surfaces, our new approach consists of extracting the surface apparent contours for a given viewpoint, as a set of closed three dimensional non planar curves, before registering the apparent contours to x-ray paths. Aside from algorithms, there are a number of major technical difficulties associated with engineering a clinically viable system using anatomy and image based registration. To detect and solve them, we have so far conducted two experiments with the surgical robot in an operating room (OR), using CT and fluoroscopic image data of a cadaver bone, and attempting to faithfully simulate clinical conditions. Such experiments indicate that fluoroscopy based registration is a promising alternative to marker based registration for clinical use with our proposed method.