Radiation Detection and Measurement - radsafe.com

1 Radiation Detection andMeasurementRadiation Detect & Measure, summer 2005 ( RSM)Types of Radiation relevant toNuclear MedicineParticleSymbolMassChargeElectron e-, -511 keV/c2 -1 Positrone+, +511 keV/c2 +1 Alpha 3700 MeV/c2 +2 Photon variable energy noneRadiation Detect & Measure, summer 2005 ( RSM)Charged particle ranges( particles)Loses energy in a more or less continuous slowing downprocess as it travels through distance it travels (range) depend only upon its initialenergy and its average energy loss rate in the range for an particle emitted in tissue is on the orderof m Detect & Measure, summer 2005 ( RSM)Charged particle ranges( particles)Electrons or particles have ranges that are quitevariable from one electron to the next, even for electronsof exactly the same energy in a specific is because of the different types of scattering eventsthe particle can encounter ( , scattering events,bremsstrahlung-producing collisions, etc.)

2 The range is often given as the maximum distance themost energetic can travel in the range for particles emitted in tissue is on the orderof mm Detect & Measure, summer 2005 ( RSM)Interactions of high energy photons withmatterPhotoelectric effectphoton is absorbedCompton scatteringpart of the energy of the photon is absorbedscattered photon continues on with lower energyPair productionpositron-electron pair is createdrequires photons above MeVCoherent (Rayleigh) scatteringphoton deflected with very little energy lossonly significant at low photon energies (<50 keV) Radiation Detect & Measure, summer 2005 ( RSM)Summary of interaction of chargedparticles and photons with matterCharged particles have a very short range in tissue~ mm for beta particles~ m for alpha particlesAlphas have a predictable, continuously slowing pathBetas have a more random pathPhotons have a longer rangeinteract very infrequently, depositing much or all oftheir energy in each interaction.

3 Range ~ scatter is the dominant process in tissueequivalent materials for the energy range ofphotons for Nuclear Medicine imagingRadiation Detect & Measure, summer 2005 ( RSM)Basic Radiation detectorRadiation Detect & Measure, summer 2005 ( RSM) Pulse Current StoredRadiation detectors used inNuclear MedicineRadiation Detect & Measure, summer 2005 ( RSM)Types of Radiation detectors Counters Number of interactions Pulse mode Spectrometers Number and energy of interactions Pulse mode Dosimeters Net amount of energy deposited Current modeRadiation Detect & Measure, summer 2005 ( RSM)Pulse mode versus current mode Pulse mode Detect individual photons Required for most imaging applications Current mode Measures average rates of photon flux Avoids dead-time lossesRadiation Detect & Measure, summer 2005 ( RSM)Interaction Rate and Dead-timeRadiation Detect & Measure, summer 2005 ( RSM)paralyzablenon-paralyzableFrom: The Essential Physics of Medical Imaging (Bushberg, et al)Types of Radiation detectors Gas-filled detectors Solid-state (semiconductor) detectors Organic liquid scintillators Inorganic scintillatorsRadiation Detect & Measure, summer 2005 ( RSM)Gas-filled DetectorsIonizing event in airrequires about 34 eVFrom.

4 Physics in Nuclear Medicine (Sorenson and Phelps) Radiation Detect & Measure, summer 2005 ( RSM)Gas-filled detectors(operates in three ranges) Geiger-Muller counters Proportional counters Ionization chambers Radiation survey meters Dosimeters (dose calibrator) Radiation Detect & Measure, summer 2005 ( RSM)From: Physics in Nuclear Medicine (Sorenson and Phelps)Ionization ChambersFrom: Physics in Nuclear Medicine (Sorenson and Phelps) Radiation Detect & Measure, summer 2005 ( RSM)ATOMLAB 200 Dose CalibratorNo amplificationNo dead-timeSignal = liberated chargeSettings for different isotopesCalibrationsGeiger-Muller countersFrom: Physics in Nuclear Medicine (Sorenson and Phelps) Radiation Detect & Measure, summer 2005 ( RSM)No energy infoLong dead-timeThin window probeSemiconductor detectors Works on same principle as gas-filled detectors( , production of electron-hole pairs insemiconductor material) Only ~3 eV required for ionization (~34 eV, air) Usually needs to be cooled (thermal noise) Usually requires very high purity materials orintroduction of compensating impurities thatdonate electrons to fill electron traps caused byother impuritiesRadiation Detect & Measure, summer 2005 ( RSM)Semiconductor detectors CdZnTe detectors - can operate atroom temperatureRadiation Detect & Measure, summer 2005 ( RSM)Organic liquid scintillators(liquid scintillator cocktail)

5 Organic solvent - must dissolve scintillator material andradioactive sample Primary scintillator (p-terphenyl and PPO) Secondary solute (wave-shifter) Additives ( , solubilizers) Effective for measuring beta particles ( , H-3, C-14). Radiation Detect & Measure, summer 2005 ( RSM)Inorganic scintillators(physical characteristics)Absorption of Radiation lifts electrons from valence toconduction bandImpurities (activators) create energy levels within theband gap permitting visible light scintillationsRadiation Detect & Measure, summer 2005 ( RSM)Inorganic scintillators(physical characteristics)NaI(Tl)BGOLSO(Ce)GSO(Ce) Density (gm/cm3) Number51756659 AttenuationCoefficient(@ 511 keV, cm-1) Output(photons/Mev)40K~8K~30K~20 KDecay Time230 ns300 ns12 ns60 ns40 nsWavelength410 nm480 nm420 nm430 nmIndex of Detect & Measure, summer 2005 ( RSM)Sample Spectroscopy SystemRadiation Detect & Measure, summer 2005 ( RSM)From: Physics in Nuclear Medicine (Sorenson and Phelps)From.

6 The Essential Physics of Medical Imaging (Bushberg, et al)Photomultiplier tubeRadiation Detectors, Summer 2005 ( RSM)From: Physics in Nuclear Medicine (Sorenson and Phelps)Energy ResolutionRadiation Detect & Measure, summer 2005 ( RSM)From: Physics in Nuclear Medicine (Sorenson and Phelps)Interactions of Photons witha SpectrometerRadiation Detect & Measure, summer 2005 ( RSM) + withcharacteristic x-ray scattered photonfrom lead x-ray from leadshieldFrom: The Essential Physics of Medical Imaging (Bushberg, et al)Sample Spectrum (Cs-137) Radiation Detect & Measure, summer 2005 ( RSM) x-ray x-raysDetection efficiency(32 keV vs. 662 keV)From: The Essential Physics of Medical Imaging (Bushberg, et al)Sample Spectrum (Tc-99m) Radiation Detect & Measure, summer 2005 ( RSM) withiodine K-shell of lead x-rays from shieldNote absence ofCompton continuumWhy?

7 From: The Essential Physics of Medical Imaging (Bushberg, et al)Sample Spectrum (In-111) Radiation Detect & Measure, summer 2005 ( RSM)sourcedetectorFrom: Physics in Nuclear Medicine (Sorenson and Phelps)Effects of Pulse PileupRadiation Detect & Measure, summer 2005 ( RSM)From: Physics in Nuclear Medicine (Sorenson and Phelps) Radiation Detect & Measure, summer 2005 ( RSM)Calibrations Energy calibration Adjust energy windows around a known photopeak Often done with Cs-137 and Co-57 Dose calibration (dose calibrator) Measure activity of know reference samples ( ,Cs-137 and Co-57) Linearity measured by repeated measurements of adecaying source ( , Tc-99m)What piece of equipment would you used tomeasure the activity of a pure beta emitter? Radiation Detect & Measure, summer 2005 ( RSM)From: The Essential Physics of Medical Imaging (Bushberg, et al)Raphex QuestionRadiation Detect & Measure, summer 2005 ( RSM)D58.

8 The window setting used for Tc-99m is set with the center at 140 keV with a width of +/-14 keV , 20%. The reason for this is: A. The energy spread is a consequence of the statistical broadening when amplifying the initial energy deposition event in the NaI(Tl) crystal. B. The 140 keV gamma ray emission of Tc-99m is not truly monoenergetic but the center of a spectrum of emissions. C. The higher and lower Gaussian tails are a consequence of compton scattering within the patient. D. The result of additional scattered photons generated in the collimator. E. A consequence of patient motion during scanning. Raphex AnswerRadiation Detect & Measure, summer 2005 ( RSM)D58. The window setting used for Tc-99m is set with the center at 140 keV with a width of +/-14 keV , 20%.

9 The reason for this is: A. Photons, which impinge upon the crystal, lose energy by Compton scattering and the photoelectric effect. Both processes convert the gamma ray energy into electron energy. On average approximately one electron hole pair is produced per 30 eV of gamma ray energy deposited in the crystal. These electrons result in the release of visible light when trapped in the crystal. These light quanta are collected and amplified by photomultiplier tubes. The statistical fluctuation in the number of light quanta collected and their amplification is what causes the spread in the detected energy peak, even when most of the Tc-99m photons deposit exactly 140 keV in the NaI(Tl) crystal. Radiation Detect & Measure, summer 2005 ( RSM)Counting StatisticsRadiation Detect & Measure, summer 2005 ( RSM)Sources of Error Systematic errors Consistently get the same error Random errors Radiation emission and Detection arerandom processes Blunder operator errorRadiation Detect & Measure, summer 2005 ( RSM)Measures of Central Tendency Mean Average value Median Middlemost Measurement (or value) Less affected by outliersExample: 8, 14, 5, 9, 12 Mean = = 9 Radiation Detect & Measure, summer 2005 ( RSM)Measures of Variability Variance Measure of variability.

10 Standard deviation Square root of varianceRadiation Detect & Measure, summer 2005 ( RSM)Poisson PDF Radioactive decay and Detection arePoisson random processes Variance Variance = mean = pN = x Standard deviation Standard deviation = variance = pNRadiation Detect & Measure, summer 2005 ( RSM)Confidence Probabilitythat mean iswithin interval(%)Interval aboutmeasurementRadiation Detect & Measure, summer 2005 ( RSM)Raphex QuestionD70. How many counts must be collected in an instrument with zero background to obtain an error limit of 1% with a confidence interval of 95%? A. 1000 B. 3162 C. 10,000 D. 40,000 E. 100,000 Radiation Detect & Measure, summer 2005 ( RSM)Raphex AnswerD70. How many counts must be collected in an instrument with zero background to obtain an error limit of 1% with a confidence interval of 95%?