Grating-based X-ray Phase-contrast Tomography Methods

描述(申请人提供)：生物软组织主要由轻元素组成，其成分几乎均匀，密度变化很小。传统的基于衰减的X射线成像不能为这类材料提供足够的对比度。在诊断能量范围内，X射线相移截面比软组织中X射线衰减截面大三个数量级。因此，X射线位相对比成像对软组织的细微特征特别是微结构很敏感，并为分析各种正常和疾病情况提供了优越的对比度。X射线位相衬度成像方法在生物医学应用中面临挑战。基于分析仪的相衬成像需要单色X射线和高精度晶体，仅限于同步辐射设备。基于传播的成像存在微焦点X射线管的低光子通量问题。基于光栅的位相衬度成像是近年来的一项突破。然而，这一范式转换的两个主要障碍是(1)小周期、高纵横比的大面积光栅和(2)数据采集所需的较长时间。从技术上讲，要制作大尺寸的光栅是相当困难的，特别是在X射线能量很高的情况下。从理论上讲，从点X射线源出发，模拟X射线通过大栅格的传输是极其复杂的。在这个项目中，我们将建立两个使人信服的创新：(1)内部阶段 用于精确的感兴趣区域(ROI)重建的对比层析成像和(2)用于加速数据采集和最小化辐射剂量的无相步的少视角相位对比重建。这些创新的协同结合将定义X射线相衬断层成像的新前沿。虽然传统的观点认为，基于栅格的相衬断层成像必须使用足够大的栅格来覆盖对象并完全捕获投影，但我们的主要创新思想是从理论上以相对较小的栅格收集的截断数据来精确重建ROI。强调指出，基于栅格的相衬内部重建采用截断差分投影，而典型的内部重建采用截断直接投影。这个项目的另一个新想法是利用重新加权的L1范数进行较少视角的图像重建。本项目的总体目标是建立基于x射线光栅的内部 层析成像理论，发展了相关的无相位步进的少视角重建方法和系统，促进了其在生物医学上的应用。所提出的技术将在数值模拟和体模实验中得到表征，并应用于动物模型的肌肉骨骼成像。该项目完成后，建议的基于光栅的系统将达到30？m的分辨率，缩短扫描时间，并在直径3厘米的ROI上减少辐射剂量，在对比度分辨率方面优于Micro-CT，但提供与传统基于光栅的相衬断层成像相当的ROI图像质量。

DESCRIPTION (provided by applicant): Biological soft tissue consists mainly of light elements, and its composition is nearly uniform with little density variation. Traditional attenuation-based x-ray imaging cannot provide sufficient contrast for this type of materials. The cross-section of x-ray phase shift is three orders of magnitude greater than that of x-ray attenuation in soft tissue over the diagnostic energy range. Hence, x-ray phase-contrast imaging is sensitive to subtle features especially micro-structures of soft tissue and offers superior contrast for analyses of various normal and diseased conditions. X-ray phase-contrast imaging approaches face challenges in biomedical applications. Analyzer-based phase- contrast imaging requires monochromatic x-rays and high-precision crystals, being limited to the synchrotron radiation facility. Propagation-based imaging suffers from a low photon flux of a micro-focus x-ray tube. Grating-based phase-contrast imaging is a recent breakthrough. However, two main obstacles for this paradigm shift are (1) the large-area gratings of small periods and high aspects and (2) the long time needed for data acquisition. Technically, it is rather difficult to make large gratings especially when x-ray energy is high. Theoretically, it is extremely complicated to model the propagation of x-rays through large gratings from a point x-ray source. In this project, we will establish two enabling innovations that are (1) interior phase contrast tomography for accurate region of interest (ROI) reconstruction and (2) few-view phase-contrast reconstruction without phase-stepping for accelerated data acquisition and minimized radiation dose. The synergistic combination of these innovations will define a new frontier of x-ray phase-contrast tomography. Although the conventional wisdom is that grating-based phase-contrast tomography must use sufficiently large gratings to cover an object and capture projections completely, our main innovative thinking is to target theoretically exact reconstruction over an ROI from truncated data collected with relatively small gratings. It is underlined that the grating-based phase-contrast interior reconstruction takes truncated differential projections, while the typical interior reconstruction assumes truncated direct projections. Another new idea for this project is to utilize the reweighted L1 norm for fewer-view image reconstruction. The overall goal of this project is to establish x-ray-grating-based interior tomography theory, develop the associated few-view reconstruction methods and system without phase stepping, and promote its biomedical applications. The proposed technology will be characterized in numerical simulation and phantom experiments, and applied for musculoskeletal imaging in an animal model. Upon the completion of this project, the proposed grating-based system will have achieved 30¿m resolution, shortened scanning time, and reduced radiation dose over a 3cm- diameter ROI, outperforming micro-CT in terms of contrast resolution yet delivering comparable ROI image quality relative to that of conventional grating-based phase-contrast tomography.