Leclair R J, Johns P C
Ottawa-Carleton Institute for Physics, Department of Physics, Carleton University, Ontario, Canada.
Med Phys. 1998 Jun;25(6):1008-20. doi: 10.1118/1.598279.
Although x-ray scatter is generally regarded as a nuisance that reduces radiographic contrast (C) and the signal-to-noise ratio (SNR) in conventional images, many technologies have been devised to extract useful information from the scattered x rays. A systematic approach, however, for analyzing the potential applications of x-ray scatter imaging has been lacking. Therefore, we have formulated a simple but useful semianalytic model to investigate C and SNR in scatter images. Our model considers the imaging of a target object against a background material of the same dimensions when both are situated within a water phantom. We have selected biological materials (liver, fat, bone, muscle, blood, and brain matter) for which intermolecular form factors for coherent scattering were available. Analytic relationships between C and SNR were derived, and evaluated numerically as the target object thickness (0.01-40 mm) and photon energy (10-200 keV) were systematically varied. The fundamental limits of scatter imaging were assessed via calculations that assumed that all first-order scatter exiting the phantom, over 4 pi steradians, formed the signal. Calculations for a restricted detector solid angle were then performed. For the task of imaging white brain matter versus blood in a 15 cm thick water phantom, the maximum SNR, over all energies, for images based on the detection of all forward scatter within the angular range 2 degrees-12 degrees is greater than that of primary images for target object thicknesses < or = 23 mm. Use of the backscattered x rays within the range 158 degrees-178 degrees to image objects 3 cm below the surface of a 25 cm thick water phantom allows the liver to be distinguished from fat with a SNR superior to that of primary imaging when the objects are < or = 22 mm thick. Our analysis confirms the usefulness of scattered x rays, and provides simple methods for determining the regimes of medical interest in which x-ray scatter imaging could outperform conventional imaging.
尽管X射线散射通常被视为一种会降低传统图像中射线照相对比度（C）和信噪比（SNR）的干扰因素，但人们已经设计出许多技术来从散射的X射线中提取有用信息。然而，一直缺乏一种用于分析X射线散射成像潜在应用的系统方法。因此，我们制定了一个简单但有用的半解析模型来研究散射图像中的C和SNR。我们的模型考虑了目标物体在与水模体中相同尺寸的背景材料相对比下的成像情况，此时两者都位于水模体中。我们选择了有相干散射分子形式因子的生物材料（肝脏、脂肪、骨骼、肌肉、血液和脑物质）。推导了C和SNR之间的解析关系，并在系统改变目标物体厚度（0.01 - 40毫米）和光子能量（10 - 200千电子伏）时进行了数值评估。通过假设从模体射出的所有一阶散射在4π立体角上形成信号的计算来评估散射成像的基本极限。然后对受限探测器立体角进行了计算。对于在15厘米厚的水模体中对白质与血液进行成像的任务，在所有能量下，基于检测2度 - 12度角范围内所有前向散射的图像的最大SNR，对于目标物体厚度小于或等于23毫米时大于原始图像的SNR。使用158度 - 178度范围内的反向散射X射线对25厘米厚水模体表面以下3厘米处的物体进行成像时，当物体厚度小于或等于22毫米时，肝脏与脂肪的区分SNR优于原始成像。我们的分析证实了散射X射线的有用性，并提供了简单方法来确定医学感兴趣的、X射线散射成像可能优于传统成像的范围。
