A grandeza produto kerma-área (PKA) é importante para estabelecer níveis de referência em exames de radiologia diagnóstica. Essa grandeza pode ser obtida por meio de medidores do PKA. O uso desses medidores é fundamental para avaliar a dose de radiação em procedimentos radiológicos, além de ser um bom indicador para que os limites de dose na pele do paciente não sejam excedidos. Algumas vezes, esses medidores vêm acoplados a equipamentos de radiação X, o que dificulta sua calibração. Neste trabalho foi desenvolvida uma metodologia de calibração de medidores do PKA. O instrumento utilizado para este fim foi o Patient Dose Calibrator (PDC). Ele foi desenvolvido para ser utilizado como referência na verificação da calibração de medidores do PKA e kerma no ar usados na dosimetria de pacientes e para verificação da consistência e do comportamento de sistemas de controle de exposição automáticos. Por se tratar de um equipamento novo, que, no Brasil, ainda não é utilizado como equipamento de referência para calibração, foi realizado, também o controle de qualidade deste equipamento, com testes de caracterização, a calibração e a avaliação da dependência energética. Após os testes, ficou provado que o PDC pode ser utilizado como instrumento de referência para a calibração a ser realizada in situ, de forma que as características de cada equipamento de radiação X onde os medidores do PKA são utilizados sejam consideradas. A calibração foi, então, realizada com medidores do PKA portáteis e em um equipamento de radiologia intervencionista que possui um medidor do PKA acoplado. Os resultados foram bons e ficou provada a necessidade de calibração desses medidores e a importância da calibração in situ com um medidor de referência. / The quantity kerma area product (PKA) is important to establish reference levels in diagnostic radiology exams. This quantity can be obtained using a PKA meter. The use of such meters is essential to evaluate the radiation dose in radiological procedures and is a good indicator to make sure that the dose limit to the patient\'s skin doesnt exceed. Sometimes, these meters come fixed to X radiation equipment, which makes its calibration difficult. In this work, it was developed a methodology for calibration of PKA meters. The instrument used for this purpose was the Patient Dose Calibrator (PDC). It was developed to be used as a reference to check the calibration of PKA and air kerma meters that are used for dosimetry in patients and to verify the consistency and behavior of systems of automatic exposure control. Because it is a new equipment, which, in Brazil, is not yet used as reference equipment for calibration, it was also performed the quality control of this equipment with characterization tests, the calibration and an evaluation of the energy dependence. After the tests, it was proved that the PDC can be used as a reference instrument and that the calibration must be performed in situ, so that the characteristics of each X-ray equipment, where the PKA meters are used, are considered. The calibration was then performed with portable PKA meters and in an interventional radiology equipment that has a PKA meter fixed. The results were good and it was proved the need for calibration of these meters and the importance of in situ calibration with a reference meter.

A grandeza produto kerma-área (PKA) é importante para estabelecer níveis de referência em exames de radiologia diagnóstica. Essa grandeza pode ser obtida por meio de medidores do PKA. O uso desses medidores é fundamental para avaliar a dose de radiação em procedimentos radiológicos, além de ser um bom indicador para que os limites de dose na pele do paciente não sejam excedidos. Algumas vezes, esses medidores vêm acoplados a equipamentos de radiação X, o que dificulta sua calibração. Neste trabalho foi desenvolvida uma metodologia de calibração de medidores do PKA. O instrumento utilizado para este fim foi o Patient Dose Calibrator (PDC). Ele foi desenvolvido para ser utilizado como referência na verificação da calibração de medidores do PKA e kerma no ar usados na dosimetria de pacientes e para verificação da consistência e do comportamento de sistemas de controle de exposição automáticos. Por se tratar de um equipamento novo, que, no Brasil, ainda não é utilizado como equipamento de referência para calibração, foi realizado, também o controle de qualidade deste equipamento, com testes de caracterização, a calibração e a avaliação da dependência energética. Após os testes, ficou provado que o PDC pode ser utilizado como instrumento de referência para a calibração a ser realizada in situ, de forma que as características de cada equipamento de radiação X onde os medidores do PKA são utilizados sejam consideradas. A calibração foi, então, realizada com medidores do PKA portáteis e em um equipamento de radiologia intervencionista que possui um medidor do PKA acoplado. Os resultados foram bons e ficou provada a necessidade de calibração desses medidores e a importância da calibração in situ com um medidor de referência. / The quantity kerma area product (PKA) is important to establish reference levels in diagnostic radiology exams. This quantity can be obtained using a PKA meter. The use of such meters is essential to evaluate the radiation dose in radiological procedures and is a good indicator to make sure that the dose limit to the patient\'s skin doesnt exceed. Sometimes, these meters come fixed to X radiation equipment, which makes its calibration difficult. In this work, it was developed a methodology for calibration of PKA meters. The instrument used for this purpose was the Patient Dose Calibrator (PDC). It was developed to be used as a reference to check the calibration of PKA and air kerma meters that are used for dosimetry in patients and to verify the consistency and behavior of systems of automatic exposure control. Because it is a new equipment, which, in Brazil, is not yet used as reference equipment for calibration, it was also performed the quality control of this equipment with characterization tests, the calibration and an evaluation of the energy dependence. After the tests, it was proved that the PDC can be used as a reference instrument and that the calibration must be performed in situ, so that the characteristics of each X-ray equipment, where the PKA meters are used, are considered. The calibration was then performed with portable PKA meters and in an interventional radiology equipment that has a PKA meter fixed. The results were good and it was proved the need for calibration of these meters and the importance of in situ calibration with a reference meter.

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.