混叠效应对数字化X线影像的影响及其分析  

Analysis of the influence of aliasing effect on the digital X ray images

作　　者：牛延涛[1] 刘振生[1] 王革新[1] 赵波[1] 郝晖[1] 燕树林[1] 

机构地区：[1]首都医科大学附属北京同仁医院放射科,北京100730

出　　处：《中华放射医学与防护杂志》2007年第6期593-595,共3页Chinese Journal of Radiological Medicine and Protection

基　　金：北京市自然科学基金资助项目（7073090）

摘　　要：目的探讨数字X线摄影中混叠效应的产生原因及其对影像质量的影响。方法使用KodakCR900系统分别对矩形波测试卡和固定滤线栅成像。测试卡铅条在平行于激光扫描方向和呈45°角的放置方式下分别成像，观察两种影像对分辨率测试卡的显示情况。使滤线栅铅条的方向在平行和垂直于激光扫描方向时分别成像，在显示器上使用不同的放大率对滤线栅影像进行观察，并比较两种影像外观的差异。结果测试卡影像上，3．93线对／mm以上频率的铅条影无法相互分辨，因此尼奎斯特频率为3．93LP／mm。但在测试卡45。放置的影像中，频率为4．86LP／mm的铅条仍能相互分辨。滤线栅铅条平行于成像板的激光扫描方向时，最终影像中显示出明显的条纹影。在显示器上使用不同的放大率观察时，栅条形成的条纹影宽度和方向都发生明显变化。铅条与激光扫描方向垂直时影像中的条纹影显著减弱。结论CR应用中混叠效应会对数字影像产生不利影响，应根据设备的极限分辨率选择适当的固定滤线栅，栅频率应大于尼奎斯特频率。在摄影中，滤线栅的铅条方向应与成像板的激光扫描方向相垂直。在显示器上观察固定滤线栅摄影影像时，建议使用原始图像的整数倍的放大率。Objective To investigate the causes and eliminating methods of aliasing effect in digital radiography. Methods Stationary grid and rectangular wave test phantom were imaged on Kodak CR900 system. Lead strips of phantom were parallel to laser scanning direction or with an angle of 45 degrees when they were exposed on imaging plate. The representation ability for resolution test phantom in two types of imges were observed. Grid was imaged when its lead strips are parallel to or perpendicular to laser scanning direction. Two images were observed and contrasted on monitor using various magnifying rate. Results In phantom images, the lead bars below the frequency of 3.93 linepairs per mm could be discriminated, and it is Nyquist frequency of this system. But the lead bars with the frequency of 4.86 linepairs per mm could even been distinguished in the image of test phantom with an angle of 45 degrees. When grid lead bars were parallel to imaging plate scanning direction, resulting images displayed visable streak artifacts. The display degree has marked difference when grid strips were parallel or perpendicular to laser scanning direction. Streaks were not clear when the image was displayed as true size on monitor, but there widths changed in a large range as zoom in or zoom out. At the same time, the directions of streaks changed. Conclusions Optimum stationary grids should be selected in clinical practice according to limited resolution of CR system because aliasing effect would cause disadvantageous influence, and grid frequency should be greater than Nyquist frequency. Grid strip direction should be perpendicular to laser scanner direction in clinics to avoid streak artifacts. There are notable affection to image seeming on monitor when using different magnifying rate, and using integral times of real image size were suggested.

关 键 词：数字化X线摄影 影像质量 混叠效应 

分 类 号：R686[医药卫生—骨科学]