Computed tomography from X-rays: old 2-D results, new 3-D problems

We consider old results on 2-D computerized tomography methods and their relevance to new fully 3-D problems. We examine the 2-D filtered back-projection method (FBP) from several perspectives to better understand how it works. Based on that understanding, we question whether stable, reliable reconstruction algorithms can be found for wide-detector cone-beam 3-D machines with their resulting large-slant beams. We use a numerical method of "point response function fitting" to compute convolution kernels H(u, v) (for use with back-projection) for a model slant-beam problem. These kernels exhibit disturbing growing and spreading oscillations which would greatly amplify errors in the projection data

Potential of X-ray computed tomography for 3D anatomical analysis and microdensitometrical assessment in wood research with focus on wood modification

Studying structure and chemistry of wood and wood-based materials is the backbone of all wood research and many techniques are at hand to do so. A very valuable modality is X-ray computed tomography (CT), able to non-destructively probe the three-dimensional (3D) structure and composition. In this paper, we elaborate on the use of Nanowood, a flexible multi-resolution X-ray CT set-up developed at UGCT, the Ghent University Centre for X-ray Tomography. The technique has been used successfully in many different fields of wood science. It is illustrated how 3D structural and microdensitometrical data can be obtained using different scan set-ups and protocols. Its potential for the analysis of modified wood is exemplified, e.g. for the assessment of wood treated with hydrophobing agents, localisation of modification agents, pathway analysis related to functional tissues, dimensional changes due to thermal treatment, etc. Furthermore, monitoring of transient processes is a promising field of activity too

Wire tomography in the H-1NF heliac for investigation of fine structure of magnetic islands

Electron beam wire tomography in the H-1NF heliac enables high resolution mapping of vacuum flux surfaces with minimal disruption of the plasma operations schedule. Recent experimental results have proven this technique to be a highly accurate and high resolution method for mapping vacuum magnetic islands. Islands of width as small as delta approximately 8 mm have been measured, providing estimates of the internal rotational transform of the island. Point-to-point comparison of the mapping results with computer tracing, in conjunction with an image warping technique, enables systematic exploration of magnetic islands and surfaces of interest. Recent development of a fast mapping technique significantly reduced the mapping time and made this technique suitable for mapping at higher magnetic fields. This article presents recent experimental results and associated techniques.with support from the Australian Research Council Grant No. DP0344361

Regions of Backprojection and Comet Tail Artifacts for π-Line Reconstruction Formulas in Tomography

We explore two characteristic features of x-ray computed tomography inversion formulas in two and three dimensions that are dependent on π-lines. In such formulas the data from a given source position contribute only to the reconstruction of ƒ(x) for x in a certain region, called the region of backprojection. The second characteristic is a certain small artifact in the reconstruction called a comet tail artifact. We propose that the comet tail artifact is closely related to the boundary of the region of backprojection and make this relationship precise, developing a general theory of the region of backprojection, its boundary, and the location of the artifact in helical and fan-beam tomography. This theory is applied to a number of specific examples and confirmed by numerical experiments. Furthermore it is demonstrated that a strong comet tail artifact appears in numerical reconstructions from misaligned fan-beam data. A numerical method for using the artifact to find the correct alignment is suggested. Key words. computed tomography, π-lines, helicalKeywords: π-lines, Helical tomography, Fan-beam tomography, Computed tomograph