This research is aimed at studying the advantages of an x-ray phase-contrast tomosynthesis prototype by using
phantoms. A prototype system is assembled with a micro-focus x-ray source, a rotating stage and a computed
radiography detector mounted on an optical rail. A custom designed bubble wrap phantom is used in experiments.
Angular projection images are acquired from -20° to +20° with 2° interval. The in-plane slices are reconstructed. The feature area on the phantom is observed. The prototype system provides an intrinsic way to investigate the potential and imaging quality of a phase-contrast tomosynthesis imaging method. As the result, phase-contrast tomosynthesis imaging method is demonstrated for its advantages in avoiding structure noise and overlapping issues by comparing the results acquired by computed radiography and phase-contrast radiography.
The objectives of this project are to intrinsically and quantitatively investigate the advantages of a phase-contrast (PC)
tomosynthesis prototype in removing the superimposed structure noise and the effects on image qualities by an off-center
shift of the object along the tube-sweep direction.
Experimentally, phantoms are assembled with standard resolution patterns. Phase-contrast tomosynthesis images are
acquired and reconstructed. In order to study the effects of the limited angular projections on the in-plane resolutions, all
the images were reconstructed in the 2×2 binning mode only, as we will study the phase contrast effects elsewhere. The
in-plane slices reconstructed from each of the experiments examined and the in-plane limiting resolutions are determined.
For comparison, the resolution patterns and phantoms are also imaged by single projections. Under single x-ray
projection, with only one resolution pattern, the limiting resolution of the system is 8 lp/mm; with 2 resolution patterns
superimposed, the image of the resolution patterns is blurred for distinguishing line pairs. The PC tomosynthesis in-plane
images show that the limiting resolution of the system is 7 lp/mm. For objects with a shift along the horizontal axis (the
tube-sweep direction) by 0.8 inch, the spatial resolution is degraded to 4 lp/mm and blur occurs.
As is expected, the PC tomosynthesis prototype studied in this project reveals superimposed fine structures of the object;
the effect induced by the object's off-center shift is determined in a quantified way. The in-plane resolution of this
system can be further improved by optimizing the system alignment, and the reconstruction algorithms.
As a typical 3D image algorithm, Feldkamp-David-Kress reconstruction algorithm is a filtered backprojection very
similar to the 2D algorithm. Recent years, it has been widely used for its easy implementation and acceptable
reconstruction precision for small cone-beam angle. For big cone-beam angles, however, the reconstruction exactness of
horizontal planes will dramatically decrease with the distance from the central plane increases. Therefore, for improving
the reconstruction precision in vertical direction, a similar reciprocal Gaussian function as the weighted factor was
introduced into FDK algorithm in the article. The validity of the improved FDK algorithm was verified and evaluated
through both the computer numerical simulation and the phantom model experiment. The quantitative analysis for the
reconstruction results demonstrated that the reconstruction image using the improved FDK algorithm could primely
revise the original reconstruction images and restore well closely to the tomography image of the prototype object.
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