1.School of Physics, Zhengzhou University, Zhengzhou 450000, China 2.Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China 3.School of Nuclear Sciences and Technology, University of Chinese Academy of Sciences, Beijing 100049, China
Fund Project:Project supported by the National Key R&D Program of China (Grant No. 2019YFA0404900), the National Natural Science Foundation of China (Grant No. 11875303), and the Key Program of the International Partnership of Bureau of International Cooperation Chinese Academy of Sciences (Grant No. 113462KYSB20160036)
Received Date:12 April 2021
Accepted Date:06 May 2021
Available Online:07 June 2021
Published Online:20 September 2021
Abstract:High energy electron radiography (HEER) proposed first for real-time high spatial and temporal resolution diagnosis of warm dense matter (WDM) and inertial confinement fusion (ICF) has proved experimentally feasible for mesoscale sciences diagnosis. Until now, the spatial resolution of the images close to 1 μm has been reached experimentally which is better than that of X-rays and neutron radiography. However, traditional HEER obtains two-dimensional images which cannot accurately present the three-dimensional structure of the sample. To further improve the capability of HEER to diagnose and obtain the internal information of samples, three-dimensional high energy electron radiography (TDHEER) was put forward by combining HEER with three-dimensional (3D) reconstruction tomography technology. The validity and usage of the TDHEER method have been confirmed through simulation of the fully 3D diagnostic of static mesoscale sample. This paper focuses mainly on the experimental demonstration of the 3D high energy electron radiography. The feasibility of TDHEER is for the first time confirmed by the results achieved with different 3D reconstruction algorithms. The 3D reconstruction algorithms, analytical algorithm-filtered back projection (FBP), iterative algorithms-algebraic reconstruction technique (ART), and simultaneous algebraic reconstruction technique (SART) are used here. In this experiment, the less projected data are used, so it takes the less time to obtain two-dimensional (2D) HEER images and the reconstruction. In order to spend the time as little as possible and obtain the satisfactory quality of reconstruction result, there are three groups of projected image sets, 180, 36 and 18, acquired in our experiment. When all three algorithms are adopted in 180 projected images, the reconstructed images show that all three algorithms FBP, ART and SART are feasible for TDHEER. The different reconstructed slice images of the sample in X-, Y-, and Z- direction clearly show the detailed structure of the sample. The images reconstructed by ART and SART algorithm are equivalent. Comparing with ART and SART, the reconstruction results by FBP can show more details, but there are some artifacts. Because the 36 2D HEER images fail to satisfy the Nyquist sampling theory, the analytic algorithm FBP is not used. Taking the result of FBP reconstructed by 180 images as a standard reference to compare the result of ART with the results of SART, the images reconstructed by the SART algorithm are closer to the original images. Testing 18 images, the results of the ART and SART both have lots of artifacts but the SART algorithm spends less time in reconstruction. As fewer projected images are used, more artifacts are found in the reconstructed images. Therefore, it is advantageous to combine the SART algorithm with 36 HEER projected images, which obtains high-quality reconstruction images and spends less time. The feasibility of TDHEER is confirmed experimentally for the first time and all three dimensions of the sample structures are obtained. Of the three different 3D reconstruction algorithms, the SART algorithm is the most suitable for reconstructing the few-view images. The TDHEER technology will extend HEER’s application fields, especially for mesoscale sciences. Keywords:high energy electron radiography/ three-dimensional reconstruction algorithm/ three-dimensional high energy electron radiography/ mesoscale sciences
解析重建算法FBP能够克服常规反投影的局限性, 当FBP被应用于180幅2D HEER图像的全角度投影重建时, 其重建结果要优于稀疏角度投影重建[20]. 因此采用由HEER平台收集的样本180幅HEER图像(从0o到179o, 间隔1o)用于三维重建. 最基础的迭代重建算法是ART, 当使用稀疏投影数据时, 诸如ART之类的迭代算法的重建结果要优于FBP[28], 但ART要比FBP花费更多的重建时间. 当使用与ART相同的参数时, 采用SART可获得与ART大致相同的图像质量但其重建时长在FBP和ART之间[29]. 在重建过程中, 调整ART和SART的松弛因子、迭代次数和迭代初始值, 以获得良好的重建结果. FBP, ART和SART算法重建的样品分成X, Y和Z三个方向, 每个方向中共有236个重建切片, 切片大小为6.24 cm × 6.24 cm. 在X方向上重建的从X –到X +的第95, 134和145层的切片如图5所示. 在Y方向上重建的从Y –到Y +的第89, 124和150层的切片如图6所示. 在Z方向上重建的从Z –到Z +的第52, 120和155层的切片如图7所示. 图 5 从X–到X+在第95, 134和145层使用不同算法重建的切片的结果 (a)?(c) FBP; (d)?(f) ART; (g)?(i) SART Figure5. Results of reconstructed slices with different algorithms, at the 95th, 134th and 145th layers from X– to X+: (a)?(c) FBP; (d)?(f) ART; (g)?(i) SART.
图 6 在Y–到Y+的第89, 124和150层使用不同算法重建的切片的结果 (a)?(c) FBP; (d)?(f) ART; (g)?(i) SART Figure6. Results of reconstructed slices with different algorithms, at the 89th, 124th and 150th layers from Y– to Y+: (a)?(c) FBP; (d)?(f) ART; (g)?(i) SART.
图 7 从Z–到Z+在第52, 122和155层使用不同算法重建的切片的结果 (a)?(c) FBP; (d)?(f) ART; (g)?(i) SART Figure7. Results of reconstructed slices with different algorithms, at the 52nd, 122th and 155th layers from Z– to Z+: (a)?(c) FBP; (d)?(f) ART; (g)?(i) SART.
根据图5—7中的重建图像可知, 三种算法FBP, ART和SART都可用于高能电子三维成像. 与ART和SART相比, FBP的重建结果可以显示更多细节, 但存在一些伪影. 分析FBP, ART和SART重建的Y方向上的第124层切片中第151列的像素灰度分布图, 如图8(b)所示, 即图8(a)中红线位置的像素灰度分布. 根据曲线走势分布表明, ART和SART算法重建的图像是等效的. 图 8 (a) FBP重建的从Y–到Y +第124个切片图像; (b) 是(a)中红线的像素灰度分布图 Figure8. (a) The 124th slice image from Y– to Y+ reconstructed by FBP; (b) the pixels grayscale of the red line of panel (a).