Holdsworth David W, Pollmann Steven I, Nikolov Hristo N, Fahrig Rebecca
Imaging Research Laboratories, Robarts Research Institute, London N6A 5K8, Ontario, Canada.
Med Phys. 2005 Jan;32(1):55-64. doi: 10.1118/1.1827751.
X-ray image intensifier (XRII) geometric distortion reduces the accuracy of image-guided procedures and quantitative image reconstructions. Due to the dependence of this error on the earth's magnetic field, the required correction is angle dependent, and calibration data should ideally be acquired simultaneously with clinical image data, at a specific orientation. We describe a technique to correct XRII geometric image distortion at any angular position during a stereotactic procedure. This approach uses a machined plastic grid, which contains channels that can be filled with iodinated contrast agent and subsequently flushed with water, providing contrast and mask images, respectively, of a geometric calibration grid. The standard image subtraction capabilities of conventional digital subtraction angiography devices can then be used to create a subtraction image of the iodine-filled channels, without any confounding anatomical structure. Grid-line intersection points are used to determine the control points that are required for a global polynomial correction algorithm, creating a correction map that is specific to the current angular position and XRII field of view (FOV). Tests with a clinical C-arm based XRII show that control points can be obtained with a precision of +/-0.053 mm, resulting in geometric correction accuracy of +/-0.152 mm, at a nominal FOV of 40 cm. While the precision and accuracy are both poorer than that achieved with a high-contrast steel-bead grid, the fact that the liquid grid can remain rigidly attached to the XRII during an entire procedure results in the establishment of an absolute detector coordinate system (referenced to the liquid-filled correction grid). The design of the liquid-filled channels allows the required control points to be introduced into the image or removed in about 30 s, avoiding the appearance of obscuring or confounding markers during clinical image acquisition, with a concurrent increase in patient dose of about 8% in the current design. Applications for this technique include stereotactic surgery, radiosurgery, x-ray stereogrammetry, and other image-guided procedures.
X射线影像增强器（XRII）的几何畸变会降低图像引导手术和定量图像重建的准确性。由于这种误差依赖于地球磁场，所需的校正与角度有关，理想情况下，校准数据应在特定方向上与临床图像数据同时获取。我们描述了一种在立体定向手术过程中校正任何角度位置的XRII几何图像畸变的技术。这种方法使用一个加工过的塑料网格，该网格包含可以用碘化造影剂填充并随后用水冲洗的通道，分别提供几何校准网格的对比图像和掩模图像。然后可以使用传统数字减影血管造影设备的标准图像减法功能来创建碘填充通道的减法图像，而不会有任何混淆的解剖结构。网格线交点用于确定全局多项式校正算法所需的控制点，创建特定于当前角度位置和XRII视野（FOV）的校正图。基于临床C型臂的XRII测试表明，在40 cm的标称FOV下，控制点的精度可达±0.053 mm，几何校正精度可达±0.152 mm。虽然精度和准确性都比使用高对比度钢珠网格时要差，但液体网格在整个手术过程中可以牢固地附着在XRII上这一事实导致建立了一个绝对探测器坐标系（参考液体填充的校正网格）。液体填充通道的设计允许在大约30秒内将所需的控制点引入图像或从图像中移除，避免在临床图像采集过程中出现模糊或混淆的标记，当前设计中患者剂量同时增加约8%。该技术的应用包括立体定向手术、放射外科、X射线立体测量以及其他图像引导手术。
