Radiation Detection. Tom Lewellen, PhD

Transcription

1 Radiation Detection Tom Lewellen, PhD Nuclear Medicine Basic Science Lectures basic-science-resident-lectures September 2011 Fall 2011, Copyright UW Imaging Research Laboratory

2 Main points of last week s lecture: Charged particles Short ranged in tissue ~ mm for betas (predictable, continuously slowing path) ~ µm for alphas (more sporadic path) Interactions Types: Excitation, Ionization, Bremsstrahlung Linear energy transfer (LET) - nearly continual energy transfer Bragg ionization peak (LET peaks as particle slows down) Photons Relatively long ranged (~cm) Local energy deposition - photon deposits much or all of their energy each interaction Interactions Types: Rayleigh, Photoelectric, Compton, Pair Production Compton - dominant process in tissue-equivalent materials for Nuc. Med. energies Beam hardening - polychromatic photon beam Buildup factors - narrow vs. wide beam attenuation Secondary ionization - useful for photon detection Now we shall discuss How interaction of radiation can lead to detection Fall 2011, Copyright UW Imaging Research Laboratory

3 Types of radiation relevant to Nuclear Medicine Particle! Symbol! Mass (MeV/c 2 )! Charge Electron! e-, β -!! 0.511!!! -1 Positron! e+, β+!! 0.511!!! +1 Alpha!! α!! 3700!!! +2 Photon! γ! no rest mass!! none

4 α Particle Range in Matter mono-energetic Loses energy in a more or less continuous slowing down process as it travels through matter. The distance it travels (range) depend only upon its initial energy and its average energy loss rate in the medium. The range for an α particle emitted in tissue is on the order of µm s. α µm s

5 β Particle Range in Matter continuous energy spectrum β particle ranges vary from one electron to the next, even for βs of the same energy in the same material. This is due to different types of scattering events the β encounters (i.e., scattering events, bremsstrahlung-producing collisions, etc.). The β range is often given as the maximum distance the most energetic β can travel in the medium. The range for β particles emitted in tissue is on the order of mm s. β ± mm s -

6 Interactions of Photons with Matter Exponential Penetration: N=N 0 e - λx Photoelectric effect! photon is absorbed! Compton scattering cm s! part of the energy of the photon is absorbed! scattered photon continues on with lower energy N 0 λ N x Pair production! positron-electron pair is created! requires photons above MeV! Coherent (Rayleigh) scattering! photon deflected with very little energy loss! only significant at low photon energies (<50 kev)

7 Basic Radiation Detector System incoming radiation Pulse or Current Amplify & condition Analog -todigital stored to disk

8 Basic Radiation Detector Systems What do you want to know about the radiation? Energy? Position (where did it come from)? How many / how much? Important properties of radiation detectors (depends on application) Energy resolution Spatial resolution Sensitivity Counting Speed

9 Pulse Mode versus Current Mode Pulse mode Detect individual photons Required for NM imaging applications Current mode Measures average rates of photon flux Avoids dead-time losses

10 Types of Radiation Detectors detection modes / functionality Counters Number of interactions Pulse mode Spectrometers Number and energy of interactions Pulse mode Dosimeters Net amount of energy deposited Current mode Imaging Systems CT = current mode NM = pulse mode

11 Types of Radiation Detectors physical composition Gas-filled detectors Solid-state (semiconductor) detectors Organic scintillators (liquid & plastic) Inorganic scintillators scintillators operate with a photo-sensor (i.e. another detector)

12 Gas-filled Detectors Ionizing event in air requires about 34 ev From: Physics in Nuclear Medicine (Sorenson and Phelps)

13 Gas-filled detectors (operates in three ranges) Geiger-Muller counters Proportional counters Ionization chambers Radiation survey meters Dosimeters (dose calibrator) From: Radiation Detection and Measurement (Knoll, GF)

14 Ionization Chambers Ionization chamber region From: Physics in Nuclear Medicine (Sorenson and Phelps) ATOMLAB 200 Dose Calibrator No amplification No dead-time Signal = liberated charge Settings for different isotopes Calibrations

15 Geiger-Muller counters No energy info Long dead-time Thin window probe From: Physics in Nuclear Medicine (Sorenson and Phelps)

16 Dosimeter - Film Badge A) Film pack B) Black (opaque) envelope C) Film D) Plastic film badge F) Teflon filter G) Lead filter H) Copper filter I) Aluminum filter J) Open window Dose calibrator From: The Essential Physics of Medical Imaging (Bushberg, et al) Fall 2011, Copyright UW Imaging Research Laboratory

17 Pocket Dosimeter Dose calibrator From: The Essential Physics of Medical Imaging (Bushberg, et al) Fall 2011, Copyright UW Imaging Research Laboratory

18 Semiconductor Detectors Works on same principle as gas-filled detectors (i.e., production of electron-hole pairs in semiconductor material) Only ~3 ev required for ionization (~34 ev, air) Usually needs to be cooled (thermal noise) Usually requires very high purity materials or introduction of compensating impurities that donate electrons to fill electron traps caused by other impurities

19 Semiconductor Detectors CdZnTe detectors - can operate at room temperature - starting to show up in some dedicated cardiac gamma cameras

20 Organic Liquid Scintillators (liquid scintillator cocktail) Organic solvent - must dissolve scintillator material and radioactive sample Primary scintillator (p-terphenyl and PPO) Secondary solute (wave-shifter) Additives (e.g., solubilizers) Effective for measuring beta particles (e.g., H-3, C-14).

21 Inorganic Scintillators (physical characteristics) Absorption of radiation lifts electrons from valence to conduction band Impurities (activators) create energy levels within the band gap permitting visible light scintillations

22 Inorganic Scintillators (physical characteristics) NaI(Tl) BGO LSO(Ce) GSO(Ce) relevant detector property Density (gm/cm3) Effective Atomic Number Attenuation Coefficient 511 kev, cm -1 ) Light Output (photons/mev) 40K ~8K ~30K ~20K Decay Time 230 ns 300 ns 12 ns 60 ns 40 ns sensitivity energy & spatial resol. counting speed Wavelength 410 nm 480 nm 420 nm 430 nm Index of Refraction Hygroscopy yes no no no Rugged no yes yes no photo-sensor matching manufacturing / cost

23 photo-sensor needed with scintillators Photomultiplier Tube (PMT) - most common photo-sensor currently in use for Nuclear medicine From: Physics in Nuclear Medicine (Sorenson and Phelps)

24 Sample Spectroscopy System Hardware incoming highenergy gamma ray converted to 1000s of visible photons ~20% converted to electrons electron multiplication becomes electric signal larger current or voltage more electrons more scintillation photons higher gamma energy deposited in crystal From: The Essential Physics of Medical Imaging (Bushberg, et al)

25 Silicon Photomultipliers (Geiger-mode APDs) Fall 2011, Copyright UW Imaging Research Laboratory

26 GM-APDs Arrays at the UW Zecotek Photonics Type 3-N 8x8 array with 3.3 mm square elements SensL 4x4 array with 3 mm square elements Fall 2011, Copyright UW Imaging Research Laboratory

27 Interactions of Photons with a Spectrometer A. Photoelectric B. Compton + Photoelectric C. Compton D. Photoelectric with characteristic x-ray escape E. Compton scattered photon from lead shield F. Characteristic x-ray from lead shield From: The Essential Physics of Medical Imaging (Bushberg, et al)

28 Sample Spectroscopy System Output Ideal Energy Spectrum counting mode From: The Essential Physics of Medical Imaging (Bushberg, et al) From: Physics in Nuclear Medicine (Sorenson and Phelps) Remember - you are looking at the energy deposited in the detector!

29 Energy Resolution Realistic Energy Spectrum From: Physics in Nuclear Medicine (Sorenson and Phelps)

30 Sample Spectrum (Cs-137) Detection efficiency (32 kev vs. 662 kev) A. Photopeak B. Compton continuum C. Compton edge D.! Backscatter peak E.! Barium x-ray photopeak F.! Lead x-rays From: The Essential Physics of Medical Imaging (Bushberg, et al)

31 Sample Spectrum (Tc-99m) A. Photopeak B. Photoelectric with iodine K-shell x-ray escape C. Absorption of lead x- rays from shield From: The Essential Physics of Medical Imaging (Bushberg, et al)

32 Sample Spectrum (In-111) source detector From: Physics in Nuclear Medicine (Sorenson and Phelps)

33 Effects of Pulse Pileup (count rate) From: Physics in Nuclear Medicine (Sorenson and Phelps)

34 Interaction Rate and Dead-time Measured rate dead time True rate paralyzable time non-paralyzable = recorded events From: The Essential Physics of Medical Imaging (Bushberg, et al)

35 Calibrations Energy calibration (imaging systems/spectroscopy) Adjust energy windows around a known photopeak Often done with long-lives isotopes for convenience! Cs-137: Eγ= 662 kev (close to PET 511 kev), T 1/2 =30yr! Co-57: Eγ= 122 kev (close to Tc99m 140 kev ), T 1/2 =272d Dose calibration (dose calibrator) Measure activity of know reference samples (e.g., Cs-137 and Co-57) Linearity measured by repeated measurements of a decaying source (e.g., Tc-99m)

36 Raphex Question D58. The window setting used for Tc-99m is set with the center at 140 kev with a width of +/-14 kev i.e., 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.

37 Raphex Answer D58. The window setting used for Tc-99m is set with the center at 140 kev with a width of +/-14 kev i.e., 20%. 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 g amma ray e nergy deposited in the crystal. These electrons result in the release of visible ligh t 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.

38 The count rate for a 1 µci source is measured as 25 kcps by a well counter. Assuming no corrections are applied, the measured count rate for a 10 µci source will be: a. 250 kcps Question b. Less than 250 kcps c. Greater than 250 kcps Because of deadtime effects Fall 2011, Copyright UW Imaging Research Laboratory

39 How many peaks would you expect for a 99m-Tc sample placed outside a well counter? 1 1 Question What about inside a well counter? Is your answer dose dependent? At higher doses you will get distortion of the photopeak and a high end tail on the energy spectra due to pileup. See slide 31 from lecture. Fall 2011, Copyright UW Imaging Research Laboratory

40 How many peaks would you expect for a 68-Ge sample placed outside a well counter? What about inside a well counter? Outside, 1 Inside, 2 Question 68-Ge is a positron emitter (e.g., PET) Fall 2011, Copyright UW Imaging Research Laboratory

41 Question Of the following, the most efficient detector for x-rays is: a. Geiger counter b. NaI(Tl) detector c. Single channel analyzer d. Ionization chamber e. Pocket (self-reading) dosimeter NaI(Tl) is an inorganic scintillator and is much more efficient at detecting x-rays than gas filled detectors. From: The Essential Physics of Medical Imaging (Bushberg, et al) Fall 2011, Copyright UW Imaging Research Laboratory

42 Question Gas multiplication occurs in: a. Geiger-Mueller counters b. Scintillation detectors c. Semiconductor detectors d. Ionization chambers e. Dose calibrators From: The Essential Physics of Medical Imaging (Bushberg, et al) Fall 2011, Copyright UW Imaging Research Laboratory

43 Question (True or False) In a photomultiplier tube, the photocathode is at a positive voltage with respect to the first dynode. False Small changes to the voltage applied to an ionization chamber have a large effect upon the charge collected from each interaction with ionizing radiation. False From: The Essential Physics of Medical Imaging (Bushberg, et al) Fall 2011, Copyright UW Imaging Research Laboratory

44 Question (True or False) A 1 MeV beta particle produces a pulse of the same amplitude in a G-M detector as a 200 kev beta particle. True From: The Essential Physics of Medical Imaging (Bushberg, et al) Fall 2011, Copyright UW Imaging Research Laboratory

45 Question Which detector system is most appropriate and accurate for the measurement of a pure beta source: a. Ionization chamber b. Geiger Muller tube c. NaI(Tl) well scintillation counter d. Thermoluminescent dosimeter e. Liquid scintillation counter From: Raphex Fall 2011, Copyright UW Imaging Research Laboratory

46 Question A pulse height analyzer (PHA) window can be used to: a. Identify the energy of a radionuclide b. Reject Compton scattered photons c. Separate a mixture of radionuclides d. Alter the sensitivity or resolution of the system e. All of the above From: Raphex Fall 2011, Copyright UW Imaging Research Laboratory