Dosimetric Consideration in Diagnostic Radiology

Transcription

1 Dosimetric Consideration in Diagnostic Radiology Prof. Ng Kwan-Hoong Department of Biomedical Imaging University of Malaya Radiation Dosimetry Workshop, March 2014

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3 Why do we measure dose? Optimization - obtaining radiological images with lowest amount of radiation requisite Assessment of radiation detriment as a factor of patient-outcome efficacy (risk assessment) Development of reference levels As an indicator of radiology QA 3

4 Why do we measure dose? Dose quantities outside the patient s body Dose quantities to estimate risks of skin injuries and effects that have threshold Dose quantities to estimate stochastic risks 4

5 Quantities & units Exposure and exposure rate (R and R/s) Absorbed dose and kerma (Gy) Equivalent dose H (Sv) Effective Dose (Sv) Related dosimetry quantities (Entrance Surface Dose, Kerma Area Product) 5

6 Radiation Quantities Used to describe an x-ray beam: Quantities to express total amount of radiation Quantities to express radiation at a specific point Total radiation Total photons Integral dose Radiation at a specific point Photon fluence Absorbed dose Kerma Dose equivalent 6

7 Exposure, X Exposure is a dosimetric quantity for ionizing electromagnetic radiation, based on the ability of the radiation to produce ionization in air. This quantity is only defined for electromagnetic radiation producing interactions in air. 7

8 Exposure, X Before interacting with the patient (direct beam) or with the staff (scattered radiation), X rays interact with air The quantity exposure gives an indication of the capacity of X rays to produce a certain effect in air The effect in tissue will be, in general, proportional to this effect in air 8

9 Exposure, X The exposure is the absolute value of the total charge of the ions of one sign produced in air when all the electrons liberated by photons per unit mass of air are completely stopped in air. X = dq dm 9

10 Exposure, X The SI unit of exposure is Coulomb per kilogram [C kg -1 ] The old special unit of exposure was Roentgen [R] 1 R = 2.58 x 10-4 C kg -1 1 C kg -1 = 3876 R 10

11 Exposure rate, X/t Exposure rate (and later, dose rate) is the exposure produced per unit of time. The SI unit of exposure rate is the [C/kg] per second or (in old units) [R/s]. In radiation protection it is common to indicate these rate values per hour (e.g. R/h). 11

12 Absorbed dose, D 12 Energy absorbed per unit mass of the medium Unit Gray (Gy) 1 Gy is defined as the absorption of 1 joule of energy in one kilogram of material Entrance surface dose includes the back-scatter from the patient ESD D * 1.4

13 Absorbed dose, D & kerma The kerma (kinetic energy released in a material) K = de trans dm where de trans is the sum of the initial kinetic energies of all charged ionizing particles liberated by uncharged ionizing particles in a material of mass dm The SI unit of kerma is the J kg-1, gray (Gy). In diagnostic radiology, kerma and D are equal. 13

14 Absorbed dose in soft tissue & in air Values of absorbed dose to tissue will vary by a few percent depending on the exact composition of the medium that is taken to represent soft tissue. The following value is usually used for 80 kv and 2.5 mm Al of filtration : Dose in soft tissue = 1.06 x Dose in air 14

15 Equivalent Dose, H T the product of the absorbed dose (D) and the radiation weighting factor (W R ), which reflects the relative biological effectiveness of the radiation Unit Sievert (Sv) 15

16 16 ICRP 2007

17 Effective Dose, E sum of the tissue equivalent doses ( HT ) where each of the dose multiplied by the appropriate tissue weighting factor (W T ) for tissue (T) unit: Joule per kilogram (J kg -1 ) which is termed the Sievert (Sv) E = å T W T H T 17

18 Absorption & biological effects Different tissues have different sensitivities to ionizing radiation Same amount of radiation will have different effect in different tissues The higher the weighting factor, the more sensitive the tissue 18

19 ICRP 2007 Tissue w T Breast, Bone marrow, colon, Lung, stomach, remainder tissues 0.12 Gonads 0.08 Bladder, Esophagus, liver, thyroid Bone surface, brain, salivary gland, skin

20 ABSORBED DOSE (Gy) Energy imparted by radiation to unit mass of tissue Multiply by Radiation Weighting Factor, W R EQUIVALENT DOSE (Sv) Absorbed dose weighted for harmfulness of different soures of radiation Multiply by Tissue Weighting Factor, W T EFFECTIVE DOSE (Sv) Equivalent dose weighted for susceptibility to harm of different tissues COLLECTIVE EFFECTIVE DOSE (man-sv) 20 Effective dose to a group from a source of radiation System of radiation quantities and units

21 Radiation Quantities used in Clinical Dosimetry Source Quantities Field Quantities Patient Dose Quantities 22

22 SOURCE QUANTITIES Tube current Exposure time Applied potential Filtration FIELD QUANTITIES Photon fluence Energy fluence Exposure Absorbed dose in air Air Kerma Dose-area product Energy imparted PATIENT DOSE QUANTITIES Entrance surface area (with or without backscatter) Depth dose Organ Dose Exit surface dose 23 Wall et. al. 1988

23 Air KERMA Absorbed dose X-ray Tube Primary X- ray Beam Detector Detector 24 Phantom Image Receptor KHNg

24 Absorbed dose Absorbed dose (air kerma) in X ray field can be measured with Ionization chambers Semiconductor dosimeters Thermoluminescent dosimeters (TLD) Optically stimulated luminescent dosimeter (OSLD) 25

25 Chest radiography Dose area product 100 mgycm 2 Integral dose 1 mj Effective dose 20 Sv 120 kvp 12 mas Image Receptor 5 Gy Gonad dose 2 Gy Entrance skin dose 0.1 mgy 26 Huda - Review...