数字化X射线成像系统MTF的刀口测量法

Edge Measurement for Modulation Transfer Function in Digital Radiography X-Ray Imaging System

讨论了数字化X射线成像系统调制传递函数(MTF)的刀口测量法.首先通过数值模拟方法研究了采样率和噪声对准确测量MTF的影响,真实测量中采用多条过采样刀口响应函数(ERF)取平均以压制噪声获取光滑的ERF曲线;然后用指数函数对所获ERF进行分段拟合以确保刀口边缘强度值变化的单调性,将由其获得的MTF值与线对卡结果进行比较,证明了该方法用于成像系统MTF测量的有效性.同时,通过实验分析介绍了刀口法材料、材料厚度、光源曝光电压及曝光时间的选择,最终在本系统测量中选择的刀口材料为1 mm厚铅板,曝光管电压为33 kV,曝光时间为10 s.

The effectiveness of edge measurement for the modulation transfer function （MTF）in the digital radiogra- phy X-ray imaging system has been discussed. Firstly, numerical simulation was performed to study the effect of sampling rate and noise on precise measurement of MTF. Then, a smoother curve of edge respond function （ERF） was obtained by averaging some over-sampled ERFs to suppress the noise. Finally, exponential curve-fitting was applied to different segments to keep the monotonicity of ERF. A great consistency was achieved between the MTF of the test card and that obtained with the proposed method, which verifies the effectiveness of the method for MTF measure- ment in the imaging system. The choice of edge material, the thickness of material, the exposure voltage, as well as the exposure time was analyzed through experiment and at last, a lead sheet with the thickness of 1 mm was utilized under the exposure voltage of 33 kV and the exposure time of 10 s in the proposed measurement.