9/26/200611ENGG 167 MEDICAL IMAGINGLecture 2: Sept. 27Radiation Dosimetry & RiskReferences: The Essential Physics of Medical Imaging, Bushberg et al, 2nded.Radiation Detection and Measurement, Knoll, 2ndEd.Intermediate physics for medicine and biology, Hobbie, 3rded.2Radiation Dose, Biology & RiskToday:1) Dose, Effective Dose, Exposure2) Natural, Occupational 3) Medical Exposures & Doses4) Radiation induced risk of cancer5) Radiation DetectorsNext Day: Radiation biology9/26/2006231) Dose and KermaRef: BushbergKerma – kinetic energy released in matter Units of J/kg, Gray (Gy). Old units 1 rad = 1 cGyKerma calculation: K = energy fluence x mass transfer attenuation coeff.K = Φ E (µtr/ρ)Where Φ is the number of photons per unit areaE is the energy per photon(µtr/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/∆mD = Φ E (µen/ρ)Where (µen/ρ) is the mass energy absorption attenuation coefficientNote that for low energy and low Z, (µtr/ρ) ≅ (µen/ρ)(below 200keV this is approx. true in tissue)41) Equivalent doseRef: BushbergThe equivalent dose is factors in the radiation weighting factor, wR, which is the relative factor for damage to biological cells:H = D wRUnits: Sievert, (Sv). If wR= 1, then 1 Gy = 1 SvOld unit Æ 1 rem = 1 cSv9/26/2006351) Effective doseRef: BushbergSince 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) = ∑ wTHT(Sv)61) ExposureRef: NiasExposure - amount of electrical charge, ∆Q, produced by ionizing E-M radiation per mass, ∆m, of airX = ∆Q/∆mUnits: R – Roentgen = 2.58x10-4C/kgC/kg is the SI unit.Easily measured with air filled ionization detectors.9/26/2006471) Dose relative to ExposureRef: NiasAir dose is defined:Dair(mGy) = 8.76 X (R)Which is the SI unit for exposure.Units: rad, Gy (SI)1 Gy = 100 rads82) Effective Dose due to Background Radiation Ref: Bushberg9/26/2006592) Effective DoseRef: Bushberg102) Radiation Risk – natural sourcesRef: HobbieAverage annual effective dose assumed to be 1-3 mSvAverage risk estimate of radiation induced mortality = 4% per Sv(estimated for adult workers, using BEIR V).9/26/20066112) Radiation dose from the environmentRef: Bushberg & NiasAverage annual effective dose is 3.6 mSv122) Radiation dose from occupational exposureRef: BushbergAverage annual effective dose is 3.6 mSv9/26/20067133) Typical medical radiation doses Ref: Hobbie143) Typical radiation doses in medicine Ref: Hobbie9/26/20068153) Organ doses from diagnostic radiologyRef: Hall163) Effective dose from diagnostic radiologyRef: Hall9/26/20069174) ICRP recommended dose limitsRef: Nias184) Cancer mortality risk - summary Ref: Hall9/26/200610194) Calculate Increased risk of dying from cancer for :MammogramCT scanDoubling Background radiationSmoking205) Radiation DetectionRef: Knoll1) Ionization chambers2) Proportional counters3) Geiger-Mueller counters4) Scintillators (and Photomultiplier tubes)5) Semiconductor diode detectors6) Thermoluminescent detectors (TLD)7) Film 8) many many miscellaneous detectors (Knoll)…9/26/200611215.1 Ionization ChambersRef: KnollIon 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 10-6to 10-14Amps are typical when used in CW mode.Typical usage would be gamma ray exposure assessment.225.1 Ionization ChambersRef: Knoll & AttixWalls are air equivalent attenuation (aluminum or plastic).Battery powered, provides measure of Exposure to airExamples – survey meters, radiation therapy probes9/26/200612235.2 Proportional CountersRef: Knoll & Attix•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.245.3 Geiger-Mueller CountersRef: Knoll• 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 109-1010ion 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.9/26/200613255.3 Geiger-Mueller CountersRef: Knoll265.4 Scintillation CountersRef: Knoll•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 1.5-2.0, short luminescence decay time.•Organic and inorganic scintillators exist.9/26/200614275.4 Organic Scintillation CountersRef: Knoll•S0to S1spacing 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 C14dating, tritium sampling.•Plastic and thin film scintillators are also used.285.4 Inorganic Scintillation CountersRef: Knoll• 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.9/26/200615295.4 Inorganic Scintillation CountersRef: KnollHygroscopic – require canning or sealing to prevent water vapor interactionCan be made in many shapes and sizes305.5 Photomultiplier tubes – detection of light Ref: Knoll9/26/200616315.5 microchannel plate PMTsRef: Knoll325.6 Gamma Ray spectroscopy