Accuracy assessment of a clinical biplane fluoroscope for three-dimensional measurements and targeting

There is considerable interest in using a fluoroscope for accurate three-dimensional measurements for diagnosis and therapy delivery. In this manuscript, we describe the necessary processing used to accurately find three-dimensional points in the field of view of a fluoroscopic imager and experimental results that show the system is capable of submillimetric accuracy. The image produced by a fluoroscope is spatially distorted -- a radial distortion results from the curved geometry of the xray detector and a rotation and translation are caused by an interaction ofthe electrons in the image intensifier tube with the local magnetic fields. Tests indicate that these distortions are significant (on the order of 1 cm) and affect the accuracy with which one can measure objects and distances in the images. By attaching a dense grid of radiopaque beads to the surface of the intensifier, it is possible to measure the amount of distortion present in the final image by comparing the bead positions in the image with the physical position of the beads on the grid. Furthermore, warping parameters can be derived from the bead locations and used to correct the distortion. By tessellating the image into triangular regions and applying a bilinear warping technique, we have been able to dramatically reduce distortion present in the image. Results indicate that after warping, the bead spacing is correct with a median error of 0.03 mm and a maximum error of 0.65 mm; the standard deviation of the distance error is 0.25 mm. Using pairs of images from the fluoroscopes, triangulation techniques are used to find target points in three dimensions assuming a nine-parameter, pinhole camera model. The parameters include the source-to-intensifier distance (SID), the image center (ut, va), the translation of the x-ray source (s, s, si), and the rotation of the fluoroscope about a world coordinate system (O,,y). The world coordinate system is set up using a calibration object that consists of radiopaque beads embedded in a Delrin cylinder. The beads are arranged such that they lie along a helical path; this shape is chosen to help avoid overlap of the beads in each projection image. By placing the calibration object within the field of view, the nine parameters of the model can be determined for each fluoroscope in the biplane system. In vitro experiments were performed using a Philips' Biplane Poly Diagnost I fluoroscope in clinical use at Johns Hopkins Hospital. The system has an SID of approximately 1 m and a 36 cm intensifier diameter. Initial results indicate that the points in space can be found with a high degree of accuracy (within 0.5 mm error) using the fluoroscope. In conclusion, it is possible to use a clinical biplanar fluoroscope for accurately finding three-dimensional points in space which is useful for making anatomical measurements and targeting for therapy delivery. Keywords: Fluoroscope, Computer Vision, Local Optimization, Distortion Correction, Accuracy, Three-dimensional, Measurement.