American Association of Physicists in Medicine, 38th Annual Meeting, Philadelphia, Pennsylvania, 21-25 July. Paper TU-C4-04.
A method is described for accurately correcting the spatial distortion produced in an x-ray image intensifier camera imaging system. An image produced in an image intensifier of a rectilinear grid of wires is acquired using a digital charge coupled device (CCD) camera and computer. The wire crossing positions are determined to sub-pixel accuracy and associated with their true coordinates. The true coordinates are determined by analyzing a digitized radiograph of the wire grid. A piecewise affine transformation is found which maps observed spatial position-o their correct address in the image array. Each output pixel point needed is then mapped to its corresponding location in the distorted image. Bilinear interpolation is then used to estimate the gray level at each pixel. After calibration data is analyzed, all subsequent images are corrected using a fast and simple table lookup algorithm. The accuracy and precision of the method has been determined using a test phantom with an array of tantalum beads positioned on a lucite plate. The precision for determining the wire crossing position in the calibration phantom was measured to be 0.04 pixel. The precision for determining the bead positions in the test object was measured to be 0.07 mm. The RMS accuracy for which the beads could be properly registered with respect to there true position was 0.09 mm.