Radiation Units and Dose Quantities PDF

Exposure • • Exposure is a dosimetric quantity for ionizing radiation, based on the ability of the radiation to produce ionization in air. This quantity is only defined for radiation producing interactions in air. IAEA 2: Radiation units and dose quantities 0 Exposure • The exposure is the ab...

Exposure • • Exposure is a dosimetric quantity for ionizing radiation, based on the ability of the radiation to produce ionization in air. This quantity is only defined for radiation producing interactions in air. IAEA 2: Radiation units and dose quantities 0 Exposure • 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 IAEA 2: Radiation units and dose quantities 0 Exposure: X • • • • Ionising chapter The SI unit of exposure is Coulomb per kilogram [C kg-1] The former special unit of exposure was Roentgen [R] 1 R = 2.58 x 10-4 C kg-1 C kg-1 = 3876 R IAEA 2: Radiation units and dose quantities 0 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 R/s. In radiation protection it is common to indicate these rate values “per hour” (e.g. R/h). IAEA 2: Radiation units and dose quantities 0 IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 2: Radiation units and dose quantities Topic 2: Absorbed dose and KERMA air IAEA in or minds or time International Atomic Energy Agency Absorbed dose, D • • • • The absorbed dose D, is the energy absorbed per unit mass. This quantity is defined for all ionizing radiation (not only for electromagnetic radiation, as in the case of the “exposure”), and for any material. D = dE/dm. The SI unit of D is the Gray [Gy]. 1 Gy = 1 J/kg. The former unit was the “rad”. 1 Gy = 100 rad. IAEA 2: Radiation units and dose quantities 0 Relation between absorbed dose and exposure • • • • • It is possible to calculate the absorbed dose in a material if the exposure is known D [Gy]. = f . X [C kg-1] f = conversion coefficient depending on medium The absorbed energy in a quantity of air exposed to 1 [C kg-1] of X Rays is 0.869 [Gy] f(air) = 0.869 IAEA 2: Radiation units and dose quantities 0 IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 2: Radiation units and dose quantities Topic 3: Mean Absorbed Dose in a tissue IAEA International Atomic Energy Agency Mean absorbed dose in a tissue or organ • t The mean absorbed dose in a tissue or organ DT is the energy deposited in the organ divided by the mass of that organ. Dose tissue. IAEA 2: Radiation units and dose quantities 0 Ratio of absorbed dose in soft tissue to that 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: Dose in soft tissue = 1.06 Dose in air IAEA 2: Radiation units and dose quantities 0 IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 2: Radiation units and dose quantities Topic 4: Equivalent dose H IAEA International Atomic Energy Agency Equivalent dose: H • • • The equivalent dose H is the absorbed dose multiplied by a dimensionless radiation weighting factor, wR which expresses the biological effectiveness of a given type of radiation To avoid confusion with the absorbed dose, the SI unit of equivalent dose is called the sievert (Sv). The old unit was the “rem” Absorbed dose and equivalent dose are differ t things 1 Sv = 100 rem IAEA 2: Radiation units and dose quantities 0 Radiation weighting factor, wR • • • • For most of the radiation used in medicine (X Rays, , e-) wR is = 1, so the absorbed dose and the equivalent dose are numerically equal The exceptions are: alpha particles (wR = 20) neutrons (wR = 5 - 20). IAEA 2: Radiation units and dose quantities 0 IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 2: Radiation units and dose quantities Topic 5: Effective Dose x IAEA International Atomic Energy Agency Detriment • • Radiation exposure of the different organs and tissues in the body results in different probabilities of harm and different severity The combination of probability and severity of harm is called “detriment”. IAEA 2: Radiation units and dose quantities 0 Tissue weighting factor • To reflect the combined detriment from stochastic effects due to the equivalent doses in all the organs and tissues of the body, the equivalent dose in each organ and tissue is multiplied by a tissue weighting factor, WT, and the results are summed over the whole body to give the effective dose E IAEA 2: Radiation units and dose quantities 0 Effective dose, E • • • • E = T wT.HT E: effective dose r wT: weighting factor for organ or tissue T HT: equivalent dose in organ or tissue T IAEA 2: Radiation units and dose quantities 0 IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 2: Radiation units and dose quantities Topic 6: Related dosimetry quantities (surface and depth dose, backscatter factor…..) IAEA International Atomic Energy Agency Entrance surface dose (ESD) • • • Absorbed dose is a property of the absorbing medium as well as the radiation field, and the exact composition of the medium should be clearly stated. Usually ESD refers to soft tissue (muscle) or water Absorbed dose in muscle is related to absorbed dose in air by the ratio of the mass energy coefficients IAEA 2: Radiation units and dose quantities 0 Dose area product (I) • • • plane The dose-area product (DAP) quantity is defined E as the dose in air in a plane, integrated over the area of interest The DAP (cGy·cm2) is constant with distance since the cross section of the beam is a quadratic function which cancels the inverse quadratic dependence on dose This is true neglecting absorption and scattering of radiation in air and even for X Ray housing near the couch table IAEA 2: Radiation units and dose quantities 0 Inverse square law 3 2 1 3 D 4 1 2 6 5 4 9 8 7 2D 3D IAEA 2: Radiation units and dose quantities 0 DAP-meter (Diamentor ®) IAEA 2: Radiation units and dose quantities 0 Dose area product (II) • • • It is always necessary to calibrate and to check the transmission chamber for the X Ray installation in use In some European countries, it is compulsory that new equipment is equipped with an integrated ionization transmission chamber or with automatic calculation methods It is convenient, in this case, also to check the read-out as some systems overestimate the real DAP value IAEA 2: Radiation units and dose quantities 0 IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 2: Radiation units and dose quantities Topic 7: Specific dosimetry quantities (Mammography, CT,…) IAEA International Atomic Energy Agency The average glandular dose (AGD) • • must know the name The Average Glandular Dose (AGD) is the dosimetry quantity generally recommended for risk assessment The use of AGD is recommended by the ICRP, the British Institute of Physical Sciences in Medicine, the NCRP, the BSS and the Netherlands Commission on Radiation Dosimetry (NCS) IAEA 2: Radiation units and dose quantities 0 The average glandular dose AGD (mammography) • • • The AGD cannot be measured directly but it is derived from measurements with the standard phantom for the actual technique set-up of the mammographic equipment The Entrance Surface Air Kerma (ESAK) free-in-air (i.e., without backscatter) has become the most frequently used quantity for patient dosimetry in mammography For other purposes (compliance with reference dose level) one may refer to ESD which includes backscatter IAEA 2: Radiation units and dose quantities 0 The ESAK (mammography) • • • • • ESAK can be determined by: a TLD dosimeter calibrated in terms of air kerma free-inair at a HVL as close as possible to 0.4 mm Al with a standard phantom a TLD dosimeter calibrated in terms of air kerma free-inair at a HVL as close as possible to 0.4 mm Al stuck to the patient skin (appropriate backscatter factor should be applied to Entrance Surface Dose measured with the TLD to express ESAK) Note: due to low kV used the TLD is seen on the image a radiation dosimeter with a dynamic range covering at least 0.5 to 100 mGy (better than 10% accuracy) IAEA 2: Radiation units and dose quantities 0 Dosimetric quantity for C.T. • CTDI (Computed Tomography Dose Index) • DLP (Dose-Length Product) • MSAD (Multiple Scan Average Dose) IAEA 2: Radiation units and dose quantities 0