评价衍射增强成像空间分辨率的方法

衍射增强成像是X射线成像领域的前沿科技。相比于传统的吸收成像，衍射增强成像能大幅提高图像的衬度，尤其是对于由C、H、O、N等低原子序数元素构成的物体，这一特性使得衍射增强成像在医学诊断方面具有突出的应用价值。目前还没有系统评价衍射增强成像空间分辨率的方法。本文通过构建成像系统的调制传递函数模型，推导出衍射增强成像的空间分辨率计算公式，从而对衍射增强实验平台的整体性能进行综合评价。调制传递函数综合考虑了摇摆曲线几何、CCD像素尺寸、闪烁体荧光弥散效应对系统空间分辨率的固有影响，并详细分析了系统调制传递函数各因素对空间分辨率的影响规律，为衍射增强实验平台的物理设计及设备选型提供理论依据。

Method of Evaluating Spatial Resolution of Diffraction Enhanced Imaging

Diffraction enhanced imaging (DEI) is one of the most advanced technologies that differs from traditional radiography in generating image contrast and will play an important role in the field of X-ray medical diagnostic imaging. Compared with conventional absorbing imaging, DEI can greatly improve the detail and get higher contrast and sensitivity with the same radiation dose, especially for objects composed of low atomic number elements such as C, H, O and N. It makes the DEI have a prominent potential value in medicine. Classical X-ray imaging only produces absorbing contrast which is based on absorbing coefficient difference for various materials. But DEI has multiple contrast, such as absorbing contrast, extinction contrast and refraction contrast. Due to the analyzer crystal Si(111) converting refraction angle into changing of X-ray intensity distribution, refraction contrast is better for the image quality fundamentally. Spatial resolution is an extremely important parameter that characterizes overall performance of image system, but there is no theory to objectively evaluate spatial resolution of DEI until now, so it’s necessary to present a method to evaluate the spatial resolution of DEI prototype. Modulate transfer function (MTF) is an objective method for identifying the spatial resolution of imaging system. The theory model of spatial resolution of DEI system was established by calculating the MTF and the response of each factor to the spatial resolution was analyzed and explained. The theory model takes into account the inherent influence of the geometric factors by geometry optic approximation (including non-linear conversion when analyzer crystal converts refraction angle into X-ray intensity changing and limit of maximum diffraction angle). The image degenerates in quality during acquisition because of CCD pixel size and scintillator’s fluorescence dispersion effect caused by comprehensive influence of transportation of fluorescent photon and secondary electron. The geometric distortion is the primary factor contributing to image degeneration, and fluorescence dispersion effect also has some impacts on image spatial resolution. The analysis of the MTF provides the theoretical basis for the physical design and equipment selection of the DEI experiment platform.