Radiation Dose, Biology & Risk

Transcription

1 ENGG 167 MEDICAL IMAGING Lecture 2: Sept. 27 Radiation Dosimetry & Risk References: The Essential Physics of Medical Imaging, Bushberg et al, 2 nd ed. Radiation Detection and Measurement, Knoll, 2 nd Ed. Intermediate physics for medicine and biology, Hobbie, 3 rd ed. 1 Radiation Dose, Biology & Risk Today: 1) Dose, Effective Dose, Exposure 2) Natural, Occupational 3) Medical Exposures & Doses 4) Radiation induced risk of cancer 5) Radiation Detectors Next Day: Radiation biology 2 1

2 1) Dose and Kerma Kerma kinetic energy released in matter Units of J/kg, Gray (Gy). Old units 1 rad = 1 cgy Kerma calculation: K = energy fluence x mass transfer attenuation coeff. K = Φ E (µ tr /ρ) Where Φ is the number of photons per unit area E is the energy per photon (µ tr /ρ) is the mass transfer attenuation coefficient, which is the attenuation of photons which are converted to charged particles. Dose is the energy absorbed per unit mass: D = E/ m D = Φ E (µ en /ρ) Where (µ en /ρ) is the mass energy absorption attenuation coefficient Note that for low energy and low Z, (µ tr /ρ) (µ en /ρ) (below 200keV this is approx. true in tissue) Ref: Bushberg 3 1) Equivalent dose The equivalent dose is factors in the radiation weighting factor, w R, which is the relative factor for damage to biological cells: H = D w R Units: Sievert, (Sv). If w R = 1, then 1 Gy = 1 Sv Old unit 1 rem = 1 csv Ref: Bushberg 4 2

3 1) Effective dose Since different organ tissues vary in their sensitivity to radiation, a factor must be included to weight this sensitivity, in order to compare effective doses between organs. The effective dose is the summation of equivalent doses weighted by the organ factor. E(Sv) = w T H T (Sv) Ref: Bushberg 5 1) Exposure Exposure - amount of electrical charge, Q, produced by ionizing E- M radiation per mass, m, of air X = Q/ m Units: R Roentgen = 2.58x10-4 C/kg C/kg is the SI unit. Easily measured with air filled ionization detectors. Ref: Nias 6 3

4 1) Dose relative to Exposure Air dose is defined: D air (mgy) = 8.76 X (R) Which is the SI unit for exposure. Units: rad, Gy (SI) 1 Gy = 100 rads Ref: Nias 7 2) Effective Dose due to Background Radiation Ref: Bushberg 8 4

5 2) Effective Dose Ref: Bushberg 9 2) Radiation Risk natural sources Average annual effective dose assumed to be 1-3 msv Average risk estimate of radiation induced mortality = 4% per Sv (estimated for adult workers, using BEIR V). Ref: Hobbie 10 5

6 2) Radiation dose from the environment Average annual effective dose is 3.6 msv Ref: Bushberg & Nias 11 2) Radiation dose from occupational exposure Average annual effective dose is 3.6 msv Ref: Bushberg 12 6

7 3) Typical medical radiation doses Ref: Hobbie 13 3) Typical radiation doses in medicine Ref: Hobbie 14 7

8 3) Organ doses from diagnostic radiology Ref: Hall 15 3) Effective dose from diagnostic radiology Ref: Hall 16 8

9 4) ICRP recommended dose limits Ref: Nias 17 4) Cancer mortality risk - summary Ref: Hall 18 9

10 4) Calculate Increased risk of dying from cancer for : Mammogram CT scan Doubling Background radiation Smoking 19 5) Radiation Detection 1) Ionization chambers 2) Proportional counters 3) Geiger-Mueller counters 4) Scintillators (and Photomultiplier tubes) 5) Semiconductor diode detectors 6) Thermoluminescent detectors (TLD) 7) Film 8) many many miscellaneous detectors (Knoll) 20 10

11 5.1 Ionization Chambers Ion pairs are formed in gas as a fast charged particle passes through. Under the influence of an electric field, this generates a current. Currents near 10-6 to Amps are typical when used in CW mode. Typical usage would be gamma ray exposure assessment Ionization Chambers Examples survey meters, radiation therapy probes Walls are air equivalent attenuation (aluminum or plastic). Battery powered, provides measure of Exposure to air 22 & Attix 11

12 5.2 Proportional Counters Gas filled detector introduced in 1940 s Operated in pulsed mode- using amplification by avalanche breakdown in the gas Can be used in very low count situations. Detected count rate is proportional to the number of ion pairs formed in an interaction proportional to deposited energy. 23 & Attix 5.3 Geiger-Mueller Counters Gas filled detector Operated in pulsed mode- but with very high voltage All pulses from a G-M tube have the same amplitude, regardless of the number of original ion pairs initiating the process. Typical pulses are ion pairs in the discharge, producing signals in the range of volts. Little amplification is needed Inexpensive system Large dead time limits their use to a few hundred counts per second

13 5.3 Geiger-Mueller Counters Scintillation Counters Converts kinetic energy of charged particles into proportionate amounts of detectable light via luminescence. Properties work predomiantly through Photoelectric Effect, therefore high Z material desired! transparent to emitted light, refractive index near , short luminescence decay time. Organic and inorganic scintillators exist

14 5.4 Organic Scintillation Counters S 0 to S 1 spacing near 3-4 ev, vibrational spacings near 0.15 ev Examples are anthracene and stilbene crystals Liquid scintillators are used for test samples which can be immersed in the medium applications C 14 dating, tritium sampling. Plastic and thin film scintillators are also used Inorganic Scintillation Counters Crystals of NaI(Tl) where first shown to provide excellent scintillation (sodium iodide with trace amounts of thallium iodide). (note Iodine is high Z good for photoelectric effect (γ,e - ) process) Several alkali halide combinations are now available. Impurities present in the crystal present energy states between valence and conduction bands, which provide scintillation photons. Charged particle interaction promotes valence electrons to conduction. Electrons and holes migrate and interact with activator (impurity) sites. Typical energies are 20 ev for electron-hole pair generation

15 5.4 Inorganic Scintillation Counters Hygroscopic require canning or sealing to prevent water vapor interaction Can be made in many shapes and sizes Photomultiplier tubes detection of light 30 15

16 5.5 microchannel plate PMTs Gamma Ray spectroscopy with scintillators Primary gamma-rays are invisible to detector. Energy transfer from gamma ray into medium, inducing electron excitation. Escape of secondary electrons from detector is small. Initial interaction with medium by gamma rays is from: Photoelectric effect, Compton scattering, Pair production. Secondary effects of electrons are: Bremsstrahlung & characteristic x-rays Pulse intensity is proportionate to the deposited energy

17 6.6 Gamma Ray spectroscopy with scintillators Photoelectric effect + Compton Scatter + Pair Production Spectroscopy with scintillators 34 17

18 5.6 Spectroscopy with scintillators Spectrum of cesium-137 A- incident photon, B-Compton continuum, C-Compton edge, D-backscatter peak, E-barium x-ray photopeak, F-Lead K-shell x-rays from shielding. Ref: Bushberg 36 18

19 5.7 Semiconductor detectors Reverse biased pn photodiodes thin layer n material with p-type silicon - electron-hole pairs are induced in the depletion zone - pre-amp and amplifier required for signal - fast time response ( sec) Ref: Bushberg, Knoll, Attix Thermoluminescent detectors TLDs consist of a small crystalline dielectric containing trace activator centers which provide crystal lattice imperfections excitation of an electron to the conduction band leaves a hole trapping of the electrons and holes in the center stores some energy heating the crystal releases the energy in a phosphorescent photon example of material is Lithium fluoride powder similar atomic number to tissue good for tissue dose measurement. Ref: Attix 38 19

20 5.8 Thermoluminescent detector reading Ref: Attix Film detection Silver bromide (AgBr) grains embedded in gelatin layer. Radiation induced ion pairs convert Ag + to atomic Ag Development converts all remaining Ag + ions to atomic Ag, and bromide is removed leaving a opaque grains of Ag. Those already converted are developed more quickly, and the process is stopped at the appropriate time, leaving the exposed areas darker than the unexposed areas. Ref: Attix 40 20

21 5.9 Film analysis Ref: Attix Film dosimetry Strengths of film high spatial resolution for imaging permanent record of exposure (unlike TLD which is erased when read) high commercial availability thin geometry is convenient SD linearly proportional to dose (100 mr 0.15 OD, upper limit 3R 3 OD) dose-rate independence Disadvantages of film wet chemical processing required energy dependence of response below 300 kev (corrected with a filter) temperature & humidity sensitivity decreasing OD at very high dose levels (reverses Ag + Ag process) Ref: Attix 42 21

22 REVIEW: Radiological quantities Ref: Bushberg 43 22