TY - JOUR A1 - Borg, Leise A1 - Jørgensen, Jakob Sauer A1 - Frikel, Jürgen A1 - Sporring, Jon T1 - Reduction of variable-truncation artifacts from beam occlusion during in situ x-ray tomography JF - Measurement Science and Technology N2 - Many in situ x-ray tomography studies require experimental rigs which may partially occlude the beam and cause parts of the projection data to be missing. In a study of fluid flow in porous chalk using a percolation cell with four metal bars drastic streak artifacts arise in the filtered backprojection (FBP) reconstruction at certain orientations. Projections with non-trivial variable truncation caused by the metal bars are the source of these variable-truncation artifacts. To understand the artifacts a mathematical model of variable-truncation data as a function of metal bar radius and distance to sample is derived and verified numerically and with experimental data. The model accurately describes the arising variable-truncation artifacts across simulated variations of the experimental setup. Three variable-truncation artifact-reduction methods are proposed, all aimed at addressing sinogram discontinuities that are shown to be the source of the streaks. The 'reduction to limited angle' (RLA) method simply keeps only non-truncated projections; the 'detector-directed smoothing' (DDS) method smooths the discontinuities; while the 'reflexive boundary condition' (RBC) method enforces a zero derivative at the discontinuities. Experimental results using both simulated and real data show that the proposed methods effectively reduce variable- truncation artifacts. The RBC method is found to provide the best artifact reduction and preservation of image features using both visual and quantitative assessment. The analysis and artifact-reduction methods are designed in context of FBP reconstruction motivated by computational efficiency practical for large, real synchrotron data. While a specific variable- truncation case is considered, the proposed methods can be applied to general data cut-offs arising in different in situ x-ray tomography experiments. KW - computed tomography KW - COMPUTED-TOMOGRAPHY KW - CT KW - filtered backprojection KW - IMAGE-RECONSTRUCTION KW - in situ measurements KW - sinogram discontinuities KW - variable-truncation artifacts Y1 - 2017 U6 - https://doi.org/10.1088/1361-6501/aa8c27 VL - 28 IS - 12 PB - IOP Publishing ER - TY - CHAP A1 - Frikel, Jürgen A1 - Borg, Leise A1 - Jørgensen, Jakob Sauer A1 - Quinto, Eric Todd ED - Burger, Martin ED - Hahn, Bernadette ED - Quinto, Eric Todd T1 - Singular artifacts in incomplete data x-ray tomography T2 - Tomographic Inverse Problems: Theory and Applications; 27.01. - 02.02.2019 Y1 - 2019 U6 - https://doi.org/10.4171/OWR/2019/4 SN - 1660-8941 SP - 295 EP - 297 ER - TY - INPR A1 - Borg, Leise A1 - Frikel, Jürgen A1 - Jørgensen, Jakob Sauer A1 - Quinto, Eric Todd T1 - Theorems that Characterize Artifacts for Arbitrary Limited X-ray CT Data N2 - This article provides a mathematical classification of artifacts from arbitrary incom-plete X-ray tomography data when using the classical filtered backprojection algorithm. Usingmicrolocal analysis, we prove that all artifacts arise from points at the boundary of the data set.Our results show that, depending on the geometry of the data set boundary, two types of artifactscan arise: object-dependent and object-independent artifacts. The object-dependent artifacts aregenerated by singularities of the object being scanned and these artifacts can extend all along lines.This is a generalization of the streak artifacts observed in limited angle CT. The article also char-acterizes two new phenomena: the object-independent artifacts are caused only by the geometryof the data set boundary; they occur along lines if the boundary of the data set is not smooth andalong curves if the boundary of the data set is smooth. In addition to the geometric descriptionof artifacts, the article also provides characterizations of their strength in Sobolev scale in certaincases. Moreover, numerical reconstructions from simulated and real data are presented illustratingour theorems.This work is motivated by a reconstruction we present from a synchrotron data set in whichartifacts along lines appeared that were independent of the object.The results of this article apply to a wide range of well-known incomplete data problems, in-cluding limited angle CT and region of interest tomography, as well as to unconventional x-ray CTimaging setups. Some of those problems are explicitly addressed in this article, theoretically and numerically. Y1 - 2017 UR - https://www.researchgate.net/publication/318360078_Theorems_that_Characterize_Artifacts_for_Arbitrary_Limited_X-ray_CT_Data ET - version 6 ER - TY - CHAP A1 - Borg, Leise A1 - Jørgensen, Jakob Sauer A1 - Frikel, Jürgen A1 - Quinto, Eric Todd A1 - Sporring, Jon T1 - Reducing artifacts from varying projection truncations T2 - 3rd International Conference on Tomography of Materials and Structures, Lund, Sweden, 26-30 June 2017, ICTMS2017-65-1 N2 - We study samples with full and partial occlusion causing streak artifacts, and propose two mod-ifications of filtered backprojection for artifact removal. Data is obtained by the SPring-8 synchrotron using a monochromatic parallel-beam scan [1]. Thresholding in the sinogram segments the metal, resulting in edges on which we apply 1) a smooth transition, or 2) a Dirichlet boundary condition. KW - X-ray tomography KW - projection truncation KW - streak artifacts KW - boundary conditions Y1 - 2017 UR - https://meetingorganizer.copernicus.org/ICTMS2017/ICTMS2017-65-1.pdf ER -