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Effect of scatter on reconstructed image quality in cone beam CT: evaluation of a scatterreduction optimization function
Linköpings universitet, Institutionen för medicin och hälsa, Medicinsk radiofysik. Linköpings universitet, Hälsouniversitetet.ORCID-id: 0000-0003-1257-2383
Linköpings universitet, Institutionen för systemteknik, Bildbehandling. Linköpings universitet, Tekniska högskolan.ORCID-id: 0000-0002-9072-2204
Linköpings universitet, Institutionen för medicin och hälsa, Medicinsk radiofysik. Linköpings universitet, Hälsouniversitetet.ORCID-id: 0000-0003-3352-8330
Linköpings universitet, Institutionen för medicin och hälsa, Medicinsk radiofysik. Linköpings universitet, Hälsouniversitetet.ORCID-id: 0000-0003-0209-498X
2005 (engelsk)Inngår i: Radiation Protection Dosimetry, ISSN 0144-8420, Vol. 114, nr 1-3, s. 337-340Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

The effect of scatter on reconstructed image quality in conebeam computed tomography was investigated and a function whichcan be used in scatter-reduction optimisation tasks was tested.Projections were calculated using the Monte Carlo method inan axially symmetric cone beam geometry consisting of a pointsource, water phantom and a single row of detector elements.Image reconstruction was performed using the filtered backprojectionmethod. Image quality was assessed by the L2-norm-based differencerelative to a reference image derived from (1) weighted linearattenuation coefficients and (2) projections by primary photons.It was found that the former function was strongly affectedby the beam hardening artefact and did not properly reflectthe amount of scatter but the latter function increased withincreasing beam width, was higher for the larger phantom andexhibited properties which made it a good candidate for scatter-reductionoptimisation tasks using polyenergetic beams.

sted, utgiver, år, opplag, sider
2005. Vol. 114, nr 1-3, s. 337-340
HSV kategori
Identifikatorer
URN: urn:nbn:se:liu:diva-13033DOI: 10.1093/rpd/nch541OAI: oai:DiVA.org:liu-13033DiVA, id: diva2:17697
Tilgjengelig fra: 2008-03-13 Laget: 2008-03-13 Sist oppdatert: 2015-03-20
Inngår i avhandling
1. Calculation of scatter in cone beam CT: Steps towards a virtual tomograph
Åpne denne publikasjonen i ny fane eller vindu >>Calculation of scatter in cone beam CT: Steps towards a virtual tomograph
2008 (engelsk)Doktoravhandling, med artikler (Annet vitenskapelig)
Abstract [en]

Scattered photons—shortly scatter—are generated by interaction processes when photon beams interact with matter. In diagnostic radiology, they deteriorate image quality since they add an undesirable signal that lowers the contrast in projection radiography and causes cupping and streak artefacts in computed tomography (CT). Scatter is one of the most detrimental factors in cone beam CT owing to irradiation geometries using wide beams. It cannot be fully eliminated, nevertheless its amount can be lowered via scatter reduction techniques (air gaps, antiscatter grids, collimators) and its effect on medical images can be suppressed via scatter correction algorithms.

Aim: Develop a tool—a virtual tomograph—that simulates projections and performs image reconstructions similarly to a real CT scanner. Use this tool to evaluate the effect of scatter on projections and reconstructed images in cone beam CT. Propose improvements in CT scanner design and image reconstruction algorithms.

Methods: A software toolkit (CTmod) based on the application development framework ROOT was written to simulate primary and scatter projections using analytic and Monte Carlo methods, respectively. It was used to calculate the amount of scatter in cone beam CT for anthropomorphic voxel phantoms and water cylinders. Configurations with and without bowtie filters, antiscatter grids, and beam hardening corrections were investigated. Filtered back-projection was used to reconstruct images. Automatic threshold segmentation of volumetric CT data of anthropomorphic phantoms with known tissue compositions was tested to evaluate its usability in an iterative image reconstruction algorithm capable of performing scatter correction.

Results: It was found that computer speed was the limiting factor for the deployment of this method in clinical CT scanners. It took several hours to calculate a single projection depending on the complexity of the geometry, number of simulated detector elements, and statistical precision. Data calculated using the CTmod code confirmed the already known facts that the amount of scatter is almost linearly proportional to the beam width, the scatter-to-primary ratio (SPR) can be larger than 1 for body-size objects, and bowtie filters can decrease the SPR in certain regions of projections. Ideal antiscatter grids significantly lowered the amount of scatter. The beneficial effect of classical antiscatter grids in cone beam CT with flat panel imagers was not confirmed by other researchers nevertheless new grid designs are still being tested. A simple formula estimating the effect of scatter on the quality of reconstructed images was suggested and tested.

Conclusions: It was shown that computer simulations could calculate the amount of scatter in diagnostic radiology. The Monte Carlo method was too slow for a routine use in contemporary clinical practice nevertheless it could be used to optimize CT scanner design and, with some enhancements, it could become a part of an image reconstruction algorithm that performs scatter correction.

sted, utgiver, år, opplag, sider
Institutionen för medicin och hälsa, 2008. s. 67
Serie
Linköping University Medical Dissertations, ISSN 0345-0082 ; 1051
HSV kategori
Identifikatorer
urn:nbn:se:liu:diva-11275 (URN)978-91-7393-951-5 (ISBN)
Disputas
2008-04-09, Elsa Brändströmsalen, Campus US, Linköpings universitet, Linköping, 09:00 (engelsk)
Opponent
Veileder
Tilgjengelig fra: 2008-03-13 Laget: 2008-03-13 Sist oppdatert: 2015-03-20

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