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Calculation of scatter in cone beam CT: Steps towards a virtual tomograph
Linköping University, Department of Medicine and Health Sciences, Radiation Physics . Linköping University, Faculty of Health Sciences.ORCID iD: 0000-0003-1257-2383
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
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.

Place, publisher, year, edition, pages
Institutionen för medicin och hälsa , 2008. , 67 p.
Series
Linköping University Medical Dissertations, ISSN 0345-0082 ; 1051
National Category
Radiology, Nuclear Medicine and Medical Imaging
Identifiers
URN: urn:nbn:se:liu:diva-11275ISBN: 978-91-7393-951-5 (print)OAI: oai:DiVA.org:liu-11275DiVA: diva2:17701
Public defence
2008-04-09, Elsa Brändströmsalen, Campus US, Linköpings universitet, Linköping, 09:00 (English)
Opponent
Supervisors
Available from: 2008-03-13 Created: 2008-03-13 Last updated: 2015-03-20
List of papers
1. Simulation of scatter in cone beam CT – effects on projection image quality
Open this publication in new window or tab >>Simulation of scatter in cone beam CT – effects on projection image quality
2003 (English)In: Proceedings of SPIE 5030: Medical Imaging 2003: Physics of Medical Imaging, Vol. 5030, 740-751 p.Article in journal (Refereed) Published
Abstract [en]

Cone-beam computed tomography (CT) projections were calculated by the Monte Carlo method for two cylindrical water phantoms of different sizes and for an antropomorphic voxel phantom with and without the presence of an anti-scatter grid. The scatter-to-primary ratio (SPR) was evaluated for each projection and the dependence of the amount of scattered radiation on the phantom size, cone beam size, photon energy, and antiscatter grid was investigated. It was found that the amount of scattered radiation is a slowly varying function of position in the image plane whose values, depending on configuration parameters, may cover a range of several magnitudes. The SPR reflects changes in the amount of primary photons and may reach values around 5 for large phantoms, wide beams and 120 kV spectrum or even higher values for low energy photons.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-13032 (URN)10.1117/12.479940 (DOI)
Available from: 2008-03-13 Created: 2008-03-13 Last updated: 2015-03-20
2. Effect of scatter on reconstructed image quality in cone beam CT: evaluation of a scatterreduction optimization function
Open this publication in new window or tab >>Effect of scatter on reconstructed image quality in cone beam CT: evaluation of a scatterreduction optimization function
2005 (English)In: Radiation Protection Dosimetry, ISSN 0144-8420, Vol. 114, no 1-3, 337-340 p.Article in journal (Refereed) 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.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-13033 (URN)10.1093/rpd/nch541 (DOI)
Available from: 2008-03-13 Created: 2008-03-13 Last updated: 2015-03-20
3. Monte Carlo study of the dependence of the KAP-meter calibration coefficient on beam aperture, X-ray tube voltage, and reference plane
Open this publication in new window or tab >>Monte Carlo study of the dependence of the KAP-meter calibration coefficient on beam aperture, X-ray tube voltage, and reference plane
2007 (English)In: Physics in medicine and biology, ISSN 0031-9155, Vol. 52, no 4, 1157-1170 p.Article in journal (Refereed) Published
Abstract [en]

The Monte Carlo method was used to study the dependence of the calibration coefficient on the tube voltage, beam aperture and reference plane in simplified over-couch geometries modelling VacuTec's type 70157 KAP-meter both with and without an additional filter. The MCNP5 code was used to calculate (i) energy imparted to air cavities of the KAP-meter and (ii) spatial distribution of air collision kerma at entrance and exit planes of the KAP-meter and at a plane close to the patient. From these data, the air kerma area product and calibration coefficient were calculated and their dependence on the tube voltage and beam aperture was analysed. It was found that the variation of the calibration coefficient as a function of tube voltage was up to 40% when the additional filter was used. The additional filter placed closely in front of the KAP-meter decreased the calibration coefficient for the patient plane by about 10% compared to the ideal additional filter. The effect of the beam aperture was small at the patient plane and negligible for the exit plane.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-13034 (URN)10.1088/0031-9155/52/4/020 (DOI)
Available from: 2008-03-13 Created: 2008-03-13 Last updated: 2015-03-20
4. CTmod: a toolkit for Monte Carlo simulation of projections including scatter in computed tomography
Open this publication in new window or tab >>CTmod: a toolkit for Monte Carlo simulation of projections including scatter in computed tomography
2008 (English)In: Computer Methods and Programs in Biomedicine, ISSN 0169-2607, E-ISSN 1872-7565, Vol. 90, no 2, 167-178 p.Article in journal (Refereed) Published
Abstract [en]

The CTmod toolkit is a set of C++ class libraries based on the CERN’s application development framework ROOT. It uses the Monte Carlo method to simulate energy imparted to a CT-scanner detector array. Photons with a given angle–energy distribution are emitted from the X-ray tube approximated by a point source, transported through a phantom, and their contribution to the energy imparted per unit surface area of each detector element is scored. Alternatively, the scored quantity may be the fluence, energy fluence, plane fluence, plane energy fluence, or kerma to air in the center of each detector element. Phantoms are constructed from homogenous solids or voxel arrays via overlapping. Implemented photon interactions (photoelectric effect, coherent scattering, and incoherent scattering) are restricted to the energy range from 10 to 200 keV. Variance reduction techniques include the collision density estimator and survival biasing combined with the Russian roulette. The toolkit has been used to estimate the amount of scatter in cone beam computed tomography and planar radiography.

Place, publisher, year, edition, pages
Elsevier, 2008
Keyword
Monte Carlo, Computed tomography, Cone beam, Scatter
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-13035 (URN)10.1016/j.cmpb.2007.12.005 (DOI)
Note
Original Publication: Alexandr Malusek, Michael Sandborg and Gudrun Alm Carlsson, CTmod: a toolkit for Monte Carlo simulation of projections including scatter in computed tomography, 2008, Computer Methods and Programs in Biomedicine, (90), 2, 167-178. http://dx.doi.org/10.1016/j.cmpb.2007.12.005 Copyright: Elsevier Science B.V., Amsterdam. http://www.elsevier.com/ Available from: 2008-03-13 Created: 2008-03-13 Last updated: 2015-03-20
5. A Monte Carlo Study of the Effect of a Bowtie Filter on the Amount of Scatter in Computed Tomography
Open this publication in new window or tab >>A Monte Carlo Study of the Effect of a Bowtie Filter on the Amount of Scatter in Computed Tomography
Manuscript (Other academic)
Identifiers
urn:nbn:se:liu:diva-13036 (URN)
Available from: 2008-03-13 Created: 2008-03-13 Last updated: 2010-01-13

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