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Monte Carlo study of the dependence of the KAP-meter calibration coefficient on beam aperture, X-ray tube voltage, and reference plane
Linköping University, Department of Medicine and Health Sciences, Radiation Physics . Linköping University, Faculty of Health Sciences.ORCID iD: 0000-0003-1257-2383
Linköping University, Department of Medicine and Health Sciences, Radiation Physics . Linköping University, Faculty of Health Sciences.
Linköping University, Department of Medicine and Health Sciences, Radiation Physics . Linköping University, Faculty of Health Sciences.ORCID iD: 0000-0003-0209-498X
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.

Place, publisher, year, edition, pages
2007. Vol. 52, no 4, 1157-1170 p.
National Category
Medical and Health Sciences
Identifiers
URN: urn:nbn:se:liu:diva-13034DOI: 10.1088/0031-9155/52/4/020OAI: oai:DiVA.org:liu-13034DiVA: diva2:17698
Available from: 2008-03-13 Created: 2008-03-13 Last updated: 2015-03-20
In thesis
1. Calculation of scatter in cone beam CT: Steps towards a virtual tomograph
Open this publication in new window or tab >>Calculation of scatter in cone beam CT: Steps towards a virtual tomograph
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:nbn:se:liu:diva-11275 (URN)978-91-7393-951-5 (ISBN)
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
2. Calibration of Ionization Chambers for Measuring Air Kerma Integrated over Beam Area in Diagnostic Radiology
Open this publication in new window or tab >>Calibration of Ionization Chambers for Measuring Air Kerma Integrated over Beam Area in Diagnostic Radiology
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The air kerma area product PKA is an important quantity used by hospital physicists in quality assurance and optimization processes in diagnostic radiology and is recommended by national authorities for setting of diagnostic reference levels. PKA can be measured using a transmission ionization chamber (kerma area product (KAP) meter) mounted on the collimator housing. Its signal QKAP must be calibrated to give values of PKA. The objective of this thesis is to analyze the factors influencing the accuracy of the calibration coefficients k= PKA/QKAP and of reported PKA-values.

Due to attenuation and scatter in the KAP-meter and presence of extra-focal radiation, values of PKA depend on the choice of integration area A and the distance of the reference plane from the focal spot yielding values of PKA that may differ by as much as 23% depending on this choice. The two extremes correspond to (1) PKA=PKA,o integrated over the exit surface of the KAP-meter resulting in geometry independent calibration coefficients and (2) PKA=PKA,Anom integrated over the nominal beam area in the patient entrance plane resulting in geometry dependent calibration coefficients.

Three calibration methods are analysed. Method 1 aims at determine PKA,Anom, for clinical use at the patient entrance plane. At standard laboratories, the method is used to calibrate with respect to radiation incident on the KAP-meter. Problems with extra-focal and scattered radiation are then avoided resulting in calibration coefficients with low standard uncertainty (±1.5 %, coverage factor 2). Method 2 was designed in this work to approach determination of PKA,o using thermoluminescent detectors to monitor contributions from extra-focal radiation and account for the heel effect. The uncertainty in derived calibration coefficients was ± 3% (coverage factor 2). Method 3 uses a Master KAP-meter calibrated at a standard laboratory for incident radiation to calibrate clinical KAP-meters. It has potential to become the standard method in the future replacing the tedious method 2 for calibrations aiming at determination of PKA,o.

Commercially available KAP-meters use conducting layers of indium oxide causing a strong energy dependence of their calibration coefficients. This dependence is investigated using Monte Carlo simulations and measurements. It may introduce substantial uncertainties in reported PKA– values since calibration coefficients as obtained from standard laboratories are often available only at one filtration (2.5 mm Al) as function of tube voltage or HVL. This is not sufficient since higher filtrations are commonly used in practice, including filters of Cu. In extreme cases, calibration coefficients for the same value of HVL but using different tube voltages and filtrations can deviate by as much as 30%. If standardised calibration methods are not used and choice of calibration coefficients not carefully chosen with respect to beam quality, the total uncertainty in reported PKA–values may be as large as 40-45%. Conversion of PKA-values to risk related quantities is briefly discussed. The large energy dependence of the conversion coefficients, ε/PKA, for determination of energy imparted,ε, to the patient reduces to a lower energy dependence of calibration coefficients CQ,ε = ε/QKAP for determination of ε from the KAP-meter signal.

Place, publisher, year, edition, pages
Institutionen för medicin och vård, 2006
Series
Linköping University Medical Dissertations, ISSN 0345-0082 ; 970
Keyword
KAP-meter, DAP-meter, PKA, kerma area product, energy dependence, calibration
National Category
Radiology, Nuclear Medicine and Medical Imaging
Identifiers
urn:nbn:se:liu:diva-7848 (URN)91-85643-32-7 (ISBN)
Public defence
2006-12-08, Elsa Brändströmsalen, Södra Entrén, Campus US, Linköpings Universitet, Linköping, 09:00 (English)
Opponent
Supervisors
Available from: 2006-11-30 Created: 2006-11-30 Last updated: 2015-03-20

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Malusek, AleksandrLarsson, J. PeterAlm Carlsson, Gudrun

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