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Ionization chambers for measuring air kerma integrated over beam area: Deviations in calibration values using simplified calibration methods
Linköpings universitet, Institutionen för medicin och hälsa, Medicinsk radiofysik. Linköpings universitet, Hälsouniversitetet.
Linköpings universitet, Institutionen för medicin och hälsa, Medicinsk radiofysik. Linköpings universitet, Hälsouniversitetet. Östergötlands Läns Landsting, Kirurgi- och onkologicentrum, Radiofysikavdelningen.
Linköpings universitet, Institutionen för medicin och hälsa, Medicinsk radiofysik. Linköpings universitet, Hälsouniversitetet. Östergötlands Läns Landsting, Kirurgi- och onkologicentrum, Radiofysikavdelningen.ORCID-id: 0000-0003-0209-498X
1998 (Engelska)Ingår i: Physics in Medicine and Biology, ISSN 0031-9155, Vol. 43, nr 3, s. 599-607Artikel i tidskrift (Refereegranskat) 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).

Ort, förlag, år, upplaga, sidor
1998. Vol. 43, nr 3, s. 599-607
Nationell ämneskategori
Medicin och hälsovetenskap
Identifikatorer
URN: urn:nbn:se:liu:diva-14176DOI: 10.1088/0031-9155/43/3/011OAI: oai:DiVA.org:liu-14176DiVA, id: diva2:22788
Tillgänglig från: 2006-11-30 Skapad: 2006-11-30 Senast uppdaterad: 2015-03-20
Ingår i avhandling
1. Calibration of Ionization Chambers for Measuring Air Kerma Integrated over Beam Area in Diagnostic Radiology
Öppna denna publikation i ny flik eller fönster >>Calibration of Ionization Chambers for Measuring Air Kerma Integrated over Beam Area in Diagnostic Radiology
2006 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
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.

Ort, förlag, år, upplaga, sidor
Institutionen för medicin och vård, 2006
Serie
Linköping University Medical Dissertations, ISSN 0345-0082 ; 970
Nyckelord
KAP-meter, DAP-meter, PKA, kerma area product, energy dependence, calibration
Nationell ämneskategori
Radiologi och bildbehandling
Identifikatorer
urn:nbn:se:liu:diva-7848 (URN)91-85643-32-7 (ISBN)
Disputation
2006-12-08, Elsa Brändströmsalen, Södra Entrén, Campus US, Linköpings Universitet, Linköping, 09:00 (Engelska)
Opponent
Handledare
Tillgänglig från: 2006-11-30 Skapad: 2006-11-30 Senast uppdaterad: 2015-03-20

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Larsson, PeterPersliden, JanAlm Carlsson, Gudrun

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Medicin och hälsovetenskap

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