Evaluation of image reconstruction algorithms in cone-beam computed tomography technique

Cone-beam computed tomography (CBCT) technique is largely used in medical diagnostic imaging and nondestructive materials testing, especially in cases which require fast times and high accuracy level. In this paper, the pros and cons of Feldkamp-Davis-Kress (FDK) and simultaneous iterative reconstruction technique (SIRT) algorithms used in CBCT technique is studied. The method of simulating CBCT systems is also used to provide richer projection data, which helps the research to evaluate many aspects of algorithms.

Analyses of different propagation models for the estimation of the topside ionosphere and plasmasphere with an Ensemble Kalman Filter

The accuracy and availability of satellite-based applications like GNSS positioning and remote sensing crucially depends on the knowledge of the ionospheric electron density distribution. The tomography of the ionosphere is one of the major tools to provide link specific ionospheric corrections as well as to study and monitor physical processes in the ionosphere and plasmasphere. In this work, we apply an Ensemble Kalman Filter (EnKF) approach for the 4D electron density reconstruction of the topside ionosphere and plasmasphere with the focus on the investigation of different propagation models and compare them with the iterative reconstruction technique SMART+. The STEC measurements of eleven LEO satellites are assimilated into the reconstructions. We conduct a case study on a global grid with altitudes between 430 and 20200 km, for two periods of the year 2015 covering quiet to perturbed ionospheric conditions. Particularly, the performance of the methods to estimate independent STEC and electron density measurements from the three Swarm satellites is analysed. The results indicate that the methods EnKF with Exponential decay as the propagation model and SMART+ perform best, providing in summary the lowest residuals.

Quantifying Morphology and Diffusion Properties of Mesoporous Carbon From High-Fidelity 3D Reconstructions

A reliable quantitative analysis in electron tomography, which depends on the segmentation of the three-dimensional reconstruction, is challenging because of constraints during tilt-series acquisition (missing wedge) and reconstruction artifacts introduced by reconstruction algorithms such as the Simultaneous Iterative Reconstruction Technique (SIRT) and Discrete Algebraic Reconstruction Technique (DART). We have carefully evaluated the fidelity of segmented reconstructions analyzing a disordered mesoporous carbon used as support in catalysis. Using experimental scanning transmission electron microscopy (STEM) tomography data as well as realistic phantoms, we have quantitatively analyzed the effect on the morphological description as well as on diffusion properties (based on a random-walk particle-tracking simulation) to understand the role of porosity in catalysis. The morphological description of the pore structure can be obtained reliably both using SIRT and DART reconstructions even in the presence of a limited missing wedge. However, the measured pore volume is sensitive to the threshold settings, which are difficult to define globally for SIRT reconstructions. This leads to noticeable variations of the diffusion coefficients in the case of SIRT reconstructions, whereas DART reconstructions resulted in more reliable data. In addition, the anisotropy of the determined diffusion properties was evaluated, which was significant in the presence of a limited missing wedge for SIRT and strongly reduced for DART.

Tomographic reconstructions of borehole sections using the radio imaging method at Pyhäsalmi massive sulfide deposit in Finland

We have used the radio imaging method (RIM) to delineate attenuating zones in two borehole sections in the area of the Pyhäsalmi volcanogenic massive sulfide (VMS) copper-zinc deposit located in central Finland. The frequency band (312.5–2500 kHz) is higher and thus provides better resolution and sensitivity to conductive targets than traditional ground-level and borehole electromagnetic (EM) methods. When EM waves are assumed to be propagated along straight rays, the simultaneous iterative reconstruction technique can be used and the decayed amplitudes of the electric field are converted to the attenuation coefficient in dB/m. The straight-ray assumption was, however, not met in this study. The reconstruction results of two borehole sections were compared with time-domain EM (TEM) data and electric logging data. Electric logging reveals the nearby conducive mineralizations, and when compared with RIM data, the continuation of attenuating formations can be better predicted. The intersections interpreted from the TEM data were consistent with the RIM data. However, continuation of the attenuating domains could only be established from RIM data. Low ray densities at the upper and lower edges, violation of the straight-ray assumption, and out-of-plane targets may generate artifacts. In addition, the constructions suffer from smearing in the direction of the raypath. According to the results, we can recover the shape and orientation of attenuating targets in the borehole sections, but the physical properties are underestimated due to the straight-ray assumption. The comparison studies confirmed that RIM is well-suited to estimating subsurface conductivity properties and to predicting the continuation of attenuating domains between the boreholes at the Pyhäsalmi VMS deposit.