Medical Imaging Prezentacja multimedialna współfinansowana przez Unię Europejską w ramach Europejskiego Funduszu Społecznego w projekcie pt. Innowacyjna dydaktyka bez ograniczeń - zintegrowany rozwój Politechniki Łódzkiej - zarządzanie Uczelnią, nowoczesna oferta edukacyjna i wzmacniania zdolności do zatrudniania osób niepełnosprawnych Politechnika Łódzka, ul. Żeromskiego 116, 90-924 Łódź, tel. (042) 631 28 83 www.kapitalludzki.p.lodz.pl
Radiation quantity & protection 2 Lecture overview: Radiation intensity Radiation exposure & dose Dose measurement The biological effects of radiation Radiation protection & monitoring
Radiation intensity 3 Radiation energy in transit from one location to another Radiation flux: Intensity (energy flux): E particle energy E i energy of given particle, w i weighting factor, N i = w i N
4 Radiation intensity Source activity number N of particles emitted by the source in a unit time. 1 Curie [Ci] = 3.7 10 10 desintegrations/s SI unit: 1 Becquerel [Bq] = 1 desintegration/s 1 Bq = 2.703 10-11 Ci Fraction of radiation flux:
5 Radiation exposure Ionizing radiation > primary ion pairs production -> secondary ions appear as the primary ions deposit their energy by ionizing nearby atoms. The total number of ion pairs is proportional to the energy that the radiation deposits in the medium. The radiation exposure: Q the total charge by radiation beam in the small amount of air mass m 1 Roentgen [R] = 2.58 10-4 Coulomb/kg of air 1 SI unit of exposure = 3.786 10 3 R
Radiation exposure 6 Biological and chemical changes in tissue exposed to ionizing radiation depend upon the energy absorbed by the tissue from the radiation rather than upon the ionization amount produced by the radiation. Thus the air Kinetic Energy Released by Unit Mass (kerma) is used to quantify source strength: 1 Gray [Gy] = 1 Joule/kg of air The conversion between ionization and kerma: 0.00873 Gy 1 R energy (dose) charge (exposure)
7 Radiation dose There are several quantities that use the SI unit of Sievert to describe the radiation effects and damage extent on biological tissues. Mainly, these dose variables reflect: Damage at the cellular level per unit dose by different types of radiation Sensitivity of different body tissues Effects that damage to different tissues have upon the overall health of the organism Impact upon the future generations
Absorbed dose 8 The amount of radiation from a beam (or radioactive source) absorbed in the body depends on the radiation intensity and the atomic density and composition of the absorbing tissue. The tissue related factors and their dependence on the radiation energy are described by an empirical dimensionless factor f. Absorbed dose (Gy) = Exposure f E.g., for for tissues and x rays f is Bone, 20 kev 1.06 Bone, 200 kev 0.76 1 Gy = 100 rad = 100 erg/g
9 RBE dose Physical and chemical effects of irradiation depends not only on the amount of the energy absorbed but also upon the distribution of this energy within the tissue. The relative biologic effectiveness (RBE) describes the efficiency with which a particular type of radiation will evoke a certain biological or chemical effect. RBE = Dose of reference radiation required to produce a particular effect Dose of radiation in question required to produce the same effect Sample RBEsof 60 Co gamma rays (reference radiation medium energy x rays): Lens opacity in mice 0.8 Cataract induction in rats 1 Hatchability of chicken eggs 0.81 The SI unit of RBE dose is Sievert RBE dose (Sv) = Absorbed dose (Gy) RBE
10 Dose equivalent Another approach to quantify the maximum dose levels for different radiations is based on dose equivalent concept. The dose equivalent is a product of absorbed dose and a quality factor QF determined by the linear energy transfer (LET) of the radiation. Dose equivalent (Sv) = Absorbed dose (Gy) QF Average LET in water QF [kev/µm] <100 <3.5 100-200 3.5-7.0 200-650 7.0-23 650-1500 23-53 >1500 53-175 The SI unit of dose equivalent is Sievert
11 Effective dose Other factor describe the influence of radiation of different type and energy (radiation weighting factor). This factor was introduced and estimated after studies of the survivors of the atomic dropped in Hiroshima and Nagasaki in 1945. Effective dose (Sv) = Absorbed dose (Gy) W R Radiation source x rays, gamma rays, W R positrons 1 Neutrons <10 kev 5 10 kev 100 kev 10 100 kev 2 MeV 20 2 MeV 20 MeV 10 Alpha particles 20 The SI unit of effective dose is Sievert
Effective dose 12 In the detailed dosimetry, to define more precisely the effective dose, one might consider tissue weighting factors (WT) along with radiation weighting factor. Effective dose (Sv) = Absorbed dose (Gy) W R sum (W T ) Tissue W T Gonads 0.20 Lungs, colon, stomach 0.12 Liver, breasts, bladder 0.05 Bone surfaces, skin 0.01 Other organs/tissues 0.05
Dose measurement 13 A radiation dose absorbed by the medium describes the energy absorbed during the radiation exposure. A dosimeter used to measure radiation dose in soft tissue should absorb the amount of energy equal to that absorbed by the same mass of the tissue (tissue equivalent dosimeter). Calorimetric dosimetry Majority of the radiation energy is degraded to heat. A thermistor can be applied to measure the temperature difference. insulator Dose s T s specific heat absorber - graphite
Dose measurement 14 Semiconductor dosimetry Based on a silicon diode. When the dosimeter is irradiated, charged particles are set free which allows a current to flow. Diodes can be operated with and without bias. In the photovoltaic mode (without bias), the generated voltage is proportional to the dose rate. Instead of diode, the MOSFET transistor can be applied. Johnson Darko, Radiation Dosimeters, www.physics.queensu.ca/~phys495/radiation%20dosimeters.pdf
15 Dose measurement Many more approaches: Photographic dosimetry, Scintillation dosimetry, Thermoluminescencedosimetry, Free Air Ionizing Chambers,. Early approach to dosimetry The degree of patient s hand redness
The Biological Effects of Radiation 16 primary photoelectron
17 The Biological Effects of Radiation Stage 1 the atomic level The most important factor is the density with which the ionization is deposited along the track of incident particle. Ionizing radiation > primary ion pairs production (highly energetic) -> cascade of short range photoelectrons with smaller energies. The overall energy is described by LET, depending on radiation energy. Heavy charged particles (protons, alphas) -> high LET -> dense concentration of photoelectrons in small region -> highly damaging X, γ rays -> longer mean path (many centimeters) -> low LET -> energy is distributed over the entire path throw the body Stage 1 duration time 10-6 s
18 The Biological Effects of Radiation Stage 2 chemical interactions The chemical disruption is caused by the interaction of photoelectron shower and the tissue molecules. Direct effect: photoelectron hits the molecule -> chemical bond is broken or the molecule is excited -> chemical bond disruption Indirect effect: in water molecules, electron ejection -> ion + free electron formation -> trapped by other molecule -> negative molecular water ion -> disassociation into an ion and free radical extremely chemically reactive molecule -> chemical bond with almost all other atoms -> formation of molecules with different chemistry -> cell damaging or death These effects are emphasized in the presence of oxygen: result of reaction with radical H*: water -> toxic hydrogen peroxide Stage 2 duration time 10-3 s
The Biological Effects of Radiation 19 Stage 3 cellular changes Three classes of event may appear as a result of molecular changes: cell death (result of total dislocation of the internal metabolism) cell survive, but its ability to reproduce is impaired/destroyed cell survive and reproduce, but with altered metabolism -> mutations Cell damage can be fully or partially repaired by the natural metabolic processes. In radiotherapy, natural repair is both a hindrance to the effective destruction of tumors and the means by which the healthy surrounding tissues can be kept viable. Stage 3 duration time 60 s days, weeks, years
Radiation protection 20 Patient protection Accordingly to the International Commission of Radiological Protection all exposure to radiation should be guided by the three tenets: 1. The benefits of any radiation examination should outweigh the likely detriment 2. As Low As Reasonably Achievable principle should be adopted this relates to radiation dose 3. Only in exceptional circumstances the dose limits defined by ICRP should be ever exceeded standard diagnostic imaging: dose of tens of mili-siverts > carcinogenesis risk = (50 100) 10 6 this risk is equivalent to: smoking 75 cigarettes/day or travelling 4000 km by car There is no lower exposure threshold for long term effects!
Radiation protection 21 Typical Effective Doses produced by x ray and nuclear medical investigations related to equivalent number of chest x rays.
Radiation protection 22 Personnel protection Three factors in effective protection from radiation exposure 1. Distance from the source (intensity 1/distance 2 ) 2. Time spent in the radiation field (as short as possible) 3. Shielding (heavy beam stops like lead-shielded screens, heavy lead aprons) Personal exposure monitoring -> average and peak exposure -> compulsory film badges -> total exposure regulation by the radiation protection administration
Radiation protection 23
Medical Imaging Prezentacja multimedialna współfinansowana przez Unię Europejską w ramach Europejskiego Funduszu Społecznego w projekcie pt. Innowacyjna dydaktyka bez ograniczeń - zintegrowany rozwój Politechniki Łódzkiej - zarządzanie Uczelnią, nowoczesna oferta edukacyjna i wzmacniania zdolności do zatrudniania osób niepełnosprawnych Politechnika Łódzka, ul. Żeromskiego 116, 90-924 Łódź, tel. (042) 631 28 83 www.kapitalludzki.p.lodz.pl