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DOSIMETRY FOR MEDICAL APPLICATION OF IONIZING RADIATIONS: Calibration requirements and clinical applications Olivera Ciraj-Bjelac, Milojko Kovacevic, Danijela.

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Presentation on theme: "DOSIMETRY FOR MEDICAL APPLICATION OF IONIZING RADIATIONS: Calibration requirements and clinical applications Olivera Ciraj-Bjelac, Milojko Kovacevic, Danijela."— Presentation transcript:

1 DOSIMETRY FOR MEDICAL APPLICATION OF IONIZING RADIATIONS: Calibration requirements and clinical applications Olivera Ciraj-Bjelac, Milojko Kovacevic, Danijela Arandjic, Djordje Lazarevic Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Department Laboratory for Radiation Measurements Belgrade, Serbia Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

2 Content  Global trends in medical exposures  Dosimetric quantities and units  Dosimetry in diagnostic radiology Metrology and calibration requirements Clinical application Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

3  Medical exposure contributes 99% of man-made radiation exposure to humans  The concept of risk is used to quantify possible detrimental effects Medical exposure to ionizing radiation Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory Total dose from man-made sources of radiation> 0.61 mSv Medical: 0.6 mSv (> 99.97%) Source: United Nations Scientific Committee for Effect of Atomic Radiation (UNSCEAR), 2010

4 Dose? Medical exposure to ionizing radiation The role of dosimetry is to determine the amount of radiation received by a person from the radiological examination

5 Dosimetry in diagnostic radiology  Patient dose assessment  Establishment of Diagnostic Reference Levels (DRL), optimisation of protection  Assessment of x-ray equipment performance  Standards of good practice  Assessment of radiation detriment Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

6 Global trend  3,6 billion radiological examinations in the period  Increase of 50% compared to previous decade  Significant increase of CT practice: Examination frequency Dose per examination  Interventional procedures Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

7  Depends on the examination type  Variations for the same type of procedure mSv  2 mSv  100CxR 5-20 mSv CxR Dose to patient 50 chest radiographies= annual natural background radiation dose Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

8 Problems  Dose for the same examination type varies up to 2 orders of magnitude  Increased utilization of high-dose procedures CT Interventional procedures  Increase of probability for stochastic effects, in particular in the case of the repeated examinations  Possible radiation injuries in high-dose procedures Effects Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

9 Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

10 Radiation injuries 10 Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory ICRP 85

11 Basic metrology elements  International Measurements System (IMS)  Framework for dosimetry in diagnostic radiology  Consistency in radiation dosimetry Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

12 International Measurements System (IMS)  Bureau International des Poids et Mesures (BIPM)  National Primary Standard Dosimetry Laboratories (PSDL)  Secondary Standards Dosimetry Laboratories (SSDL)  Users performing measurements Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

13 Traceability chain Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

14 Metrology and traceability  Dosimeters used to determine doses received by individuals  Measurements need to be traceable though an unbroken chain of comparisons to national and international standards  Traceability is needed to ensure accuracy and reliability  Legal and economic implications Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

15 Role of the SSDL  The prime function: to provide a service in metrology  Designated by the competent national authorities  SSDL-Secondary standards, calibrated against the primary standards of laboratories participating in the IMS Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

16 Journal of the ICRU Vol 5 No 2 (2005) Report 74 Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

17 Dosimetric quantities in units in diagnostic radiology  Basic dosimetric quantity: Air kerma Easy to measure  Calibration: Dosimeters calibrated in terms of air kerma  Clinical application: Quantities derived from air kerma for different imaging modalities Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

18 Dosimetric quantities  Basic dosimetric quantities  Application specific dosimetric quantities  Quantities for risk assessment  Conversion coefficient for tissue and organ dose assessment Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

19 Energy fluence Energy fluence Unit:J/m 2 Kerma Kerma Unit:J/kg, Gy Absorbed dose Absorbed dose Unit:J/kg, Gy Basic dosimetric quantities Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

20 Basic dosimetric quantities  Charged-particle equilibrium  Absence of bremsstrahlung losses Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

21 Application specific dosimetric quantities QuantitySymbolUnitEquation Incident air kermaKiGy Entrance -surface air kerma KeGy Air-kerma area product P KA Gym 2 Air-kerma length product P KL Gym X-ray tube outputY(d)Gy/As Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

22 Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

23 Application specific dosimetric quantities: computed tomography Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

24 QuantitySymbolUnitEquation CT air-kerma index (free in air) Ca,100Gy CT air-kerma index (in standard phantom) C PMMA, 100Gy Weighted CT air kerma index CwGy Normalized weighted CT air kerma index nCwGy/As Air-kerma length product P KL Gym Application specific dosimetric quantities: computed tomography Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

25 Quantities describing risk  Organ and tissue dose  Equivalent dose  Effective dose  Dose-conversion coefficients for assessment of organ and tissue doses Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

26 The Use of Effective Dose (E)  E is a risk-related quantity and should only be used in the low-dose range  Primary use: to demonstrate compliance with dose limits in regulation, for prospective planning of radioprotection  Not for: detailed retrospective dose and risk assessments after exposure of individuals epidemiological studies, neither in accidents. In the last cases: organ doses are needed ! ICRP 103, ICRP 105 Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

27 Effective Dose in Medical Exposure  The relevant quantity for planning the exposure of patients and risk-benefit assessments is the equivalent dose or the absorbed dose to irradiated tissues.  The assessment and interpretation of E is very problematic when organs and tissues receive only partial exposure or a very heterogeneous exposure (x-ray diagnostics)  E can be of value for comparing doses from different diagnostic procedures similar procedures in different hospitals and countries different technologies for the same medical examination. ICRP 103, ICRP 105 Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

28 Dosimeters in diagnostic radiology Tube voltage keV, various A/F combinations, various modalities Ionization chambers Accurate Good energy dependence Design for different application (cylindrical, parallel-plate, different volumes..) Semiconductor dosimeters Compact Energy dependant Others TLD OSL Film (radiochromic) Scintillation (kVp meters) Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory PSDL/SSDLuser

29 Dosimetry standards in diagnostic radiology  IEC 61674: Dosimeters with ionization chambers and/or semi-conductor detectors as used in X-ray diagnostic imaging Diagnostic dosimeter: detector and measuring assembly  IEC 60580: Dose area product meters Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

30 Requirements for dosemeters User  IEC  Ionization chambers  Semiconductor detectors SSDL  Ionization chamber of reference class Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

31 Calibrations in diagnostic radiology  Air kerma: Radiography and mammography  Kerma-length product Dosimeters in CT  Kerma-area product Radiography and fluoroscopy  PPV: kVp meters  Frequency: according to national regulations Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

32 Calibration in diagnostic radiology Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory  SSDL with relevant measurement capabilities  General requirements: beam qualities, tube voltage and filtration measurements  Dosimeter of reference class (with electrometer) Calibrated Quality control Traceability for all beam qualities  Auxiliary equipment: electrometers, thermometers, barometers…  Environmental conditions

33 Equipment Dosimetry  Ionization chambers  Position system  HV supply for monitor and reference class ionization chamber  Electrometer Radiation source  X-ray generator, kVp, kVp  Ripple less than 10% for radiography and less than 4% for mammography  Beam qualities according IEC  “Shutter” mechanism  Filters and attenuators  Tube voltage meter (ppv, ±1.5%) Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

34 Application Type of chamber Range tube voltage (kV) Intrinsic uncertainty (k=2) Maximum variation of response (%) Range of air kerma rate Unatte- nuated beam Attenuated beam General radiography cylindrical or plane parallel ±2.6 1 mGy/s- 500 mGy/s 10 μGy/s- 5 mGy/s Fluoroscopy cylindrical or plane parallel ± μGy/s- 100 μGy/s Mammogra- phy plane parallel ± μGy/s- 10 mGy/s CTcylindrical ± mGy/s- 50 mGy/s Dental radiography cylindrical or plane parallel ±2.6 1 μGy/s- 10 mGy/s Reference class dosemeter Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

35 Specification of the x-ray beam  Spectrum  X-ray beam quality: First half-value layer (HVL 1 ) Second half-value layer (HVL 2 ) Homogeneity coefficient: Tube voltage Total filtration Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

36 Radiation beam qualities (IEC 61267) Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory Radiation qualityRadiation originPhantom materialApplication RQR Unfiltered beam emerging from x-ray assembly No phantom General radiography, fluoroscopy, dental radiology RQA Radiation beam from an added filter Aluminium Measurements behind the patient (on the image intensifier) RQT Radiation beam from an added filter Copper CT applications (free in air) RQR-M Unfiltered beam emerging from x-ray assembly No phantom Mammography (free in air) RQA-M Radiation beam from an added filter Aluminium Measurements behind the patient

37 Typical calibration set up X-ray tube window Focalspot Shutter Apertures Additional filtration Monitor chamber Test point Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

38 Calibration procedures  Procedures before calibration (acclimatization, positioning, stabilization…)  Calibration procedures (methods, number of measurements, interval between measurements…corrections…  Procedures following calibration (uncertainty budget, certificate…) Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

39 Dosimetry formalism  Air kerma:  Reference conditions: set of influencing quantities  Influencing condition: quantities that are not subject of mesusremst but have an impact on the result  Air density correction:  Beam quality correction: Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

40 Calibrations of dosemeters for CT  Traditionally, irradiation of the whole volume  Contras: Information on chamber response only Size on active volume only assumed Far from real situation Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

41 Calibration for CT: air kerma length product  Cylindrical chamber, 100 mm  Non-uniform irradiation  Uniform response  RQT 9 (120 kVp, HVL: 8.5 mm Al) Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory Focal spot Monitor chamber ApertureIonization chamber w dada drdr

42  In laboratory (SSDL)  Field calibration Film 10 cm Ref. chamber Calibration for fluoroscopy: air kerma area product Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory Cekerevac at al, Poster B3

43 Calibration in terms of practical peak voltage  X-ray tube voltage measurements  Practical Peak Voltage (ppv):  Property of the whole exposure cycle  Related to image contrast  Invasive or non-invasive measurements  Voltage divider Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

44  Uncertainty of the reference standard  Uncertainty of user’s instrument  Uncertainty due to calibration set up  Uncertainty of the evaluation procedure Uncertainty budget Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

45 Uncertainty budget  Air kerma: ± 2.7 %  Air kerma length product: ± 3.0 %  Air kerma area product: ± 15 %  Non-invasive tube voltage measuring devices: 2.5 % Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

46  Goal: minimal uncertainty  Assurance and control of traceability  Quality manual: technical details, methods, traceability, uncertainty budget, QC, safety….  Continuous improvements and reviews  External peer review/audit Quality Management System Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

47 Patient dose assessment Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

48 Clinical dosimetry  Direct measurement on patients or phantoms  Indirect measurements on patients or phantoms Output of the X-ray tube, scaled for exposure and geometry Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

49 Dosimetric quantites Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory BSF KAP Ke Quantities a)incident air kerma, entrance surface air kerma and kerma- area product (radiography); b)kerma-area product and entrance surface air kerma rate (fluoroscopy); c)incident and entrance surface air kerma (mammography); and d)kerma-length product (computed tomography)

50 Patients and phantoms Patient  Real situation Phantoms  Objects that simulate real patients in terms of interaction of radiation with matter  Easy to perform  Standardized Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

51 Radiography Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

52 Fluoroscopy Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

53 Mammography Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

54 Computed tomography Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

55 Patient dose levels Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

56 Uncertainty of clinical dose assessment  Radiographers taking the x-ray images  Determining tube output  Calculation of individual patient doses  Determining dose to an average patient  Use of k=2 for expression of uncertainty of dose assessment  Typically >10% and close to 25%* *if correction for beam quality and for individual patient is not applied Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

57 Form measurements towards risk assessment  Conversion coefficients Conversion of measured quantity into organ doses and effective dose Ratio of the dose to a specified tissue or effective dose divided by the normalization quantity Measured using phantoms or calculated using computer models Voxel phantoms based on images of human anatomy Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

58 Organ dose assesment Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

59 ICRU 74 Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

60 Optimization of protection Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

61 Application of DRLs  Values of measured quantities above which some specified action or decision should be taken Values must be specified Action must be specified  DRLs will be intended for use as a convenient test for identifying situations where the levels of patient dose are unusually high. Quantities that are easily measured! Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

62 Diagnostic Reference Levels Doses to patients from radiographic and fluoroscopic X-ray imaging procedures in the UK—2005 review. HPA RPD-029, HPA; Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

63 Re-cap  Diagnostic radiology is major contribution to total dose from man-made sources of radiation  Dose measurements: population dose assessment, optimization of practice  Application-specific dosimetric quantities (patients, phantoms)  Calibration of dosimeters in the conditions that are similar to the clinical environment, in terms of air kerma kerma-area product kerma-length product Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory

64  Olivera Ciraj-Bjelac, PhD, research associate, dosimetry and radiation physics  Milojko Kovacevic, MSc, Head of MDL, radiation physicist  Danijela Arandjic, MSc, PhD student, dosimetry and radiation physics  Djordje Lazarevic, MSc, PhD student, dosimetry and radiation physics  Dragana Divnic, technician  Milos Jovanovic, technician  Nikola Blagojevic, technician Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory


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