Ogura T, Fujita H
Department of Radiology, Cancer Institute Hospital, Tokyo, Japan.
Med Phys. 1994 Mar;21(3):463-9. doi: 10.1118/1.597387.
Spatial resolution properties of a 2048 x 2048 matrix image intensifier (II) TV based real-time digital radiography (DR) system were investigated. The presampling modulation transfer function (MTF), which includes the unsharpness of the detector and the blurring effect of the sampling aperture, was measured by a slit method. Also determined was the limiting resolution using a bar pattern. It was found that, the spatial resolution in this system is substantially improved relative to previous II-TV based DR systems, and is comparable or even superior to that in a computed radiography system depending on the imaging plate employed. However, the presampling MTF for 2048 x 2048 matrix is comparable to that for 1024 x 1024 matrix in this DR system, because the same sampling aperture is assumed to be used for both matrices. Even under these conditions obtained the improved clinical image since the Nyquist frequency is doubled for 2048 x 2048 matrix. It should be noted that the resolution property in the II-TV based DR system depends on the signal current from the TV image pickup tube (level of image pixel value) and also on the size of the iris in the TV lens. We must pay attention to these parameters when acquiring clinical images and evaluating the resolution properties. Finally, it was demonstrated that smaller focus than that used in a screen-film system can be employed in the DR system due to its high sensitivity, so the geometric unsharpness can be reduced. Therefore, the image resolution of our system was found to be superior to a screen-film system at some geometric magnification factors.
对基于2048×2048矩阵影像增强器（II）电视的实时数字射线照相（DR）系统的空间分辨率特性进行了研究。采用狭缝法测量了预采样调制传递函数（MTF），该函数包括探测器的不清晰度和采样孔径的模糊效应。还使用条形图案确定了极限分辨率。结果发现，该系统的空间分辨率相对于以前基于II-TV的DR系统有了显著提高，并且根据所使用的成像板，与计算机射线照相系统的空间分辨率相当甚至更优。然而，在该DR系统中，2048×2048矩阵的预采样MTF与1024×1024矩阵的预采样MTF相当，因为假定两个矩阵使用相同的采样孔径。即使在这些条件下，由于2048×2048矩阵的奈奎斯特频率加倍，仍能获得改进的临床图像。应当指出，基于II-TV的DR系统中的分辨率特性取决于来自电视图像拾取管的信号电流（图像像素值水平）以及电视镜头中光圈的大小。在获取临床图像和评估分辨率特性时，我们必须注意这些参数。最后，结果表明，由于DR系统具有高灵敏度，因此可以采用比屏-片系统更小的焦点，从而可以降低几何不清晰度。因此，发现在某些几何放大倍数下，我们系统的图像分辨率优于屏-片系统。
