Fahrig R, Holdsworth D W
Department of Medical Biophysics, University of Western Ontario and the J. P. Robarts Research Institute, London, Canada.
Med Phys. 2000 Jan;27(1):30-8. doi: 10.1118/1.598854.
The image quality of 3D reconstructions produced using a C-arm mounted XRII depends on precise determination of the geometric parameters that describe the detector system in the laboratory frame of reference. We have designed a simplified calibration system that depends on images of a metal sphere, acquired during rotation of the gantry through 200 degrees. Angle-dependent shift corrections are obtained, accounting for nonideal motion in two directions: perpendicular to the axis of rotation and tangential to the circular trajectory (tau), and parallel to the axis of rotation (xi). Projection images are corrected prior to reconstruction using a simple shift-interpolation algorithm. We show that the motion of the gantry is highly reproducible during acquisitions within one day (mean standard deviation in tau and xi is 0.11 mm and 0.08 mm, respectively), and over 21 months (mean standard deviation in tau and xi is 0.10 mm and 0.06 mm, respectively). Reconstruction of a small-bead phantom demonstrates uniformity of the correction algorithm over the full volume of the reconstruction [standard deviation of full-width-half-maximum of the beads is approximately 0.25 pixels (0.13 mm) over the volume of reconstruction]. Our approach provides a simple correction technique that can be applied when trajectory deviations are significant relative to the pixel size of the detector but small relative to the detector field of view, and when the fan angle of the acquisition geometry is small (<20 degrees). A comparison with other calibration techniques in the literature is provided.
使用安装在C型臂上的X射线影像增强器（XRII）生成的三维重建图像质量，取决于在实验室参考系中描述探测器系统的几何参数的精确测定。我们设计了一种简化的校准系统，该系统依赖于在机架旋转200度过程中获取的金属球图像。获得了与角度相关的偏移校正，考虑了两个方向上的非理想运动：垂直于旋转轴并与圆形轨迹相切（τ），以及平行于旋转轴（ξ）。在重建之前，使用简单的偏移插值算法对投影图像进行校正。我们表明，在一天内的采集过程中，机架的运动具有高度可重复性（τ和ξ方向的平均标准偏差分别为0.11毫米和0.08毫米），并且在21个月内也是如此（τ和ξ方向的平均标准偏差分别为0.10毫米和0.06毫米）。对小珠子模型的重建表明，校正算法在重建的整个体积上具有均匀性[在重建体积上，珠子半高全宽的标准偏差约为0.25像素（0.13毫米）]。我们的方法提供了一种简单的校正技术，当轨迹偏差相对于探测器的像素尺寸较大但相对于探测器视场较小时，以及当采集几何结构的扇形角较小时（<20度），均可应用。文中还与文献中的其他校准技术进行了比较。
