Grating based X-ray imaging technology is a coherent imaging technique that bears tremendous potential in three-dimensional tomographic imaging of weak absorption contrast specimens. Three kinds of contrast information including absorption, phase and scattering can be retrieved separately based on a single set of raw projections. However, the grating based X-ray imaging with the conventional phase-retrieval method using the conventional phase-stepping approach and filtered back projection (FBP) reconstruction algorithm require large amounts of raw data, so that long exposure time and large amounts of radiation dose is accepted by the sample. According to the traditional grating based X-ray imaging system, we propose a low dose, fast, multi-contrast CT reconstruction approach based on the iterative reconstruction algorithm that optimizes dose e?ciency but does not share the main limitations of other reported methods. Prior to reconstruction, firstly, the projections are acquired with the phase stepping approach and multi-contrast projections are retrieved from the raw data by conventional retrieval algorithm. Then the rotational variable differential phase projections are converted to rotational invariable projections by means of decomposing the differential phase projections into the rotational invariable projections in two mutually perpendicular derivative directions via the transformation of coordinates. Finally, the absorption, phase and scattering information are reconstructed with the iterative reconstruction algorithm and the phase is retrieved based on the fast Fourier transform (FFT). We validated and assessed the phase reconstruction approach with a numerical simulation on a phase Shepp-Logan phantom. The experiment was performed at the X-ray imaging and biomedical application beam line (BL-13W) in the Shanghai Synchrotron Radiation Facility (SSRF) where 20 keV X-ray from a Si(111) monochromator is emitted. The X-ray interferometer was positioned at 34 m from the Wiggler source. The images were recorded with a scintillator/lens-coupled CCD camera with 2048 pixel × 2048 pixel resolution and an effective pixel size of 9 μm. The numerical tests and the experimental results demonstrate that, for the small radiation dose deposited in the sample, the iterative reconstruction algorithm provides phase reconstructions of better quality and higher signal to noise ratio than the conventional FBP reconstruction algorithm, and also provides the multi-contrast 3D images, including absorption image, phase image and scattering image. This development is of particular interest for applications where the samples need inspecting under low dose and high speed conditions, and will play an important role in the nondestructive and quantitative imaging in the industry, biomedical and medical diagnosis fields.%基于光栅干涉仪的X射线成像技术可以同时获得样品内部的吸收信息、相位信息和散射信息,既保持了传统X射线衰减成像的优点,又拥有相衬成像和散射成像的优势.然而基于传统CT重建算法的X射线光栅成像需要采集大量完整的原始投影数据,数据采集时间过长从而使得物体接受很大的辐射剂量,难以在实际中应用.提出基于传统代数迭代重建算法的光栅成像技术.该方法利用现有X射线光栅成像系统采集少量原始投影数据,基于传统代数迭代重建算法,对旋转变化的相位数据进行CT重构,同时基于傅里叶变换的方法对微分相位数据进行相位恢复.模拟和实验结果表明,基于少量或不完备的原始投影数据,该方法能够准确重构成像对象的吸收、相位和散射三维信息,同时还能对微分相位切片进行高信噪比的相位恢复,得到样品折射率实部衰减率,为X射线光栅成像技术在工业、生物和医学诊断等领域的应用提供理论和技术支撑.
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