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Calibration of Ionization Chambers for Measuring Air Kerma Integrated over Beam Area in Diagnostic Radiology
Linköping University, Department of Medicine and Health Sciences, Radiation Physics . Linköping University, Faculty of Health Sciences.
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 [en]
KAP-meter, DAP-meter, PKA, kerma area product, energy dependence, calibration
National Category
Radiology, Nuclear Medicine and Medical Imaging
Identifiers
URN: urn:nbn:se:liu:diva-7848ISBN: 91-85643-32-7 (print)OAI: oai:DiVA.org:liu-7848DiVA: diva2:22791
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
List of papers
1. Transmission ionization chambers for measurements of air collision kerma integrated over beam area. Factors limiting the accuracy of calibration
Open this publication in new window or tab >>Transmission ionization chambers for measurements of air collision kerma integrated over beam area. Factors limiting the accuracy of calibration
1996 (English)In: Physics in Medicine and Biology, ISSN 0031-9155, Vol. 41, no 11, 2381-2398 p.Article in journal (Refereed) Published
Abstract [en]

Kerma - area product meters (KAP meters) are frequently used in diagnostic radiology to measure the integral of air-collision kerma over an area perpendicular to the x-ray beam. In this work, a precise method for calibrating a KAP meter to measure is described and calibration factors determined for a broad range of tube potentials (40 - 200 kV). The integral is determined using a large number of TL dosimeters spread over and outside the nominal field area defined as the area within 50% of maximum . The method is compared to a simplified calibration method which approximates the integral by multiplying the kerma in the centre of the field by the nominal field area . While the calibration factor using the precise method is independent of field area and distance from the source, that using the simplified method depends on both. This can be accounted for by field inhomogeneities caused by the heel effect, extrafocal radiation and scattered radiation from the KAP meter. The deviations between the calibration factors were as large as for collimator apertures of and distances from the source of 50 - 160 cm. The uncertainty in the calibration factor using the precise method was carefully evaluated and the expanded relative uncertainty estimated to be with a confidence level of 95%.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-14174 (URN)10.1088/0031-9155/41/11/010 (DOI)
Available from: 2006-11-30 Created: 2006-11-30 Last updated: 2015-03-20
2. Evaluation of the uncertainties in KAP-meter calibrations
Open this publication in new window or tab >>Evaluation of the uncertainties in KAP-meter calibrations
1996 (English)Report (Other academic)
Abstract [en]

This report was prepared in order to give more details to the uncertainty evaluation of the Kerma area product meter calibration procedure described in the paper:

Larsson J P Persliden J Sandborg S and Alm Carlsson G 1996 Transmission ionization chambers for measurements of air collision kerma integrated over beam area. Factors limiting the accuracy of calibration. Phys. Med. Biol. 41 2381-2398.

Figures and equations referred to in this report will be found in the paperabove.

For convenience, however, the equations in the paper that are used in the uncertainty analysis are retyped on the next two sides, see text in section 2.7. in the paper for further details. The numbering of the equations are kept as in the paper.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 1996. 14 p.
Series
Report / Department of Radiology, Linköping University, 1990-1997, ISSN 1102-1799 ; 82
National Category
Radiology, Nuclear Medicine and Medical Imaging
Identifiers
urn:nbn:se:liu:diva-14175 (URN)LIU-RAD-R-082 (ISRN)91-7871-814-7 (ISBN)
Available from: 2006-11-30 Created: 2006-11-30 Last updated: 2015-03-20Bibliographically approved
3. Ionization chambers for measuring air kerma integrated over beam area: Deviations in calibration values using simplified calibration methods
Open this publication in new window or tab >>Ionization chambers for measuring air kerma integrated over beam area: Deviations in calibration values using simplified calibration methods
1998 (English)In: Physics in Medicine and Biology, ISSN 0031-9155, Vol. 43, no 3, 599-607 p.Article in journal (Refereed) Published
Abstract [en]

Calibrations of kerma-area product meters (KAP meters) are often performed using simplified methods. The accuracy thus obtained can be insufficient, especially when the KAP meters are used for optimizing radiological procedures. The deviations between the best available calibration factor (k) and the simplified calibration factor were measured at different clinical x-ray installations. Depending on the type of x-ray installation and calibration method, the quotient ranged from 0.83 to 1.19, reflecting the error made in practice using these methods. A simple alternative calibration method based on comparison with a KAP meter calibrated by the best available method is described. Depending on tube potential and the stability of the electrometers, the uncertainty in the calibration factor derived with this method was between 3.8% and 5.6% (at 95% confidence level).

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-14176 (URN)10.1088/0031-9155/43/3/011 (DOI)
Available from: 2006-11-30 Created: 2006-11-30 Last updated: 2015-03-20
4. 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
5. Energy dependence in KAP-meter calibration coefficients: Dependence on calibration method, type of KAP-meter, and added filter close to the KAP-meter
Open this publication in new window or tab >>Energy dependence in KAP-meter calibration coefficients: Dependence on calibration method, type of KAP-meter, and added filter close to the KAP-meter
2006 (English)In: Physics in Medicine and Biology, ISSN 0031-9155Article in journal (Refereed) Submitted
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-14178 (URN)
Available from: 2006-11-30 Created: 2006-11-30 Last updated: 2015-03-20

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