The main problem connected to cone beam computed tomography (CT) systems for
industrial applications employing 450 kV X-ray tubes is the high amount of scattered
radiation which is added to the primary radiation (signal). This stray radiation leads to
a significant degradation of the image quality. A better understanding of the scattering
and methods to reduce its effects are therefore necessary to improve the image quality.
Several studies have been carried out in the medical field at lower energies, whereas
studies in industrial CT, especially for energies up to 450 kV, are lacking. Moreover,
the studies reported in literature do not consider the scattered radiation generated by
the CT system structure and the walls of the X-ray room (environmental scatter). In
order to investigate the scattering on CT projections a GEANT4-based Monte Carlo
(MC) model was developed. The model, which has been validated against
experimental data, has enabled the calculation of the scattering including the
environmental scatter, the optimization of an anti-scatter grid suitable for the CT
system, and the optimization of the hardware components of the CT system. The
investigation of multiple scattering in the CT projections showed that its contribution
is 2.3 times the one of primary radiation for certain objects. The results of the
environmental scatter showed that it is the major component of the scattering for
aluminum box objects of front size 70 x 70 mm2 and that it strongly depends on the
thickness of the object and therefore on the projection. For that reason, its correction is
one of the key factors for achieving high quality images. The anti-scatter grid
optimized by means of the developed MC model was found to reduce the scatter-toprimary
ratio in the reconstructed images by 20 %. The object and environmental
scatter calculated by means of the simulation were used to improve the scatter
correction algorithm which could be patented by Empa. The results showed that the
cupping effect in the corrected image is strongly reduced. The developed CT
simulation is a powerful tool to optimize the design of the CT system and to evaluate
the contribution of the scattered radiation to the image. Besides, it has offered a basis
for a new scatter correction approach by which it has been possible to achieve images with the same spatial resolution as state-of-the-art well collimated fan-beam CT with a
gain in the reconstruction time of a factor 10. This result has a high economic impact
in non-destructive testing and evaluation, and reverse engineering.
| Identifer | oai:union.ndltd.org:unibo.it/oai:amsdottorato.cib.unibo.it:848 |
| Date | 12 June 2008 |
| Creators | Miceli, Alice <1978> |
| Contributors | Casali, Franco |
| Publisher | Alma Mater Studiorum - Università di Bologna |
| Source Sets | Università di Bologna |
| Language | English |
| Detected Language | English |
| Type | Doctoral Thesis, PeerReviewed |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess |
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