Image Quality Analysis of a Laminographic Multiplicative Algebraic Reconstruction Algorithm compared to Filtered Shift Averaging

Ionizing radiation imaging techniques are widely used in the industrial area for inspection of components and equipment. Among these techniques, Laminography has been used in situations where there is an asymmetry of the object of interest or in situations where it is not possible to perform a complete rotation of the object or of the X-ray tube and detector around the object for Computed Tomography (CT). Typically, Laminography uses reconstruction algorithms producing volumetric information (3D solid) from a certain number of projections. Several reconstruction algorithms are described in the literature, namely: Tomosynthesis, modified Filtered Backprojection, Algebraic Reconstruction Technique (ART), Simultaneous Iterative Reconstruction Technique (SIRT), Simultaneous Algebraic Reconstruction Technique (SART), Maximum Likelihood and Statistical Reconstruction. In this work, an analytical algorithm, called "Filtered Shifted Average" (FSA) and another iterative algorithm, called "Multiplicative Algebraic Reconstruction Technique" (MART) were optimized and used to perform coplanar translational laminographic reconstructions. Several aspects of image quality, such as analysis of Modulation Transfer Function (MTF), Contrast-to-Noise ratio (CNR), Noise and Detectability were measured to characterize and evaluate the relative performance between the FSA and MART techniques. All laminographic image volumes were obtained using the Coplanar Translational Laminography (CTL) arrangement. Several image acquisitions were made for an increasing number of projections (from 25 up to few thousands) in order to have a greater range of analysis and conclusions about the two algorithms used in this study. The results show that there is no significant dependence of the MTF10% value regarding the tested reconstruction algorithms and the number of projections. The MTF analysis is a suitable tool to describe the different filter properties of FSA and the optimized MART reconstruction. Furthermore, both CNR and the 1/Noise analysis showed higher values for all acquisition conditions with the MART algorithm in comparison with FSA. Regarding detectability, the differences between the algorithms were more accentuated for the condition with a low number of projections. The CTL was applied as one unidirectional scan and as a combination of two orthogonal planar scans, called cross coplanar translational laminography (C-CTL). Finally, there was a significant improvement in the contrast modulation for the images obtained with C-CTL, applying both algorithms for CTL. The choice and correct use of an iterative algorithm (MART) yields several advantages regarding the image quality, despite requiring more computational resources than the analytical algorithm (FSA).