Fall 2011, Copyright UW Imaging Research Laboratory Radiation Detection Tom Lewellen, PhD [email protected]Nuclear Medicine Basic Science Lectures http://www.rad.washington.edu/research/our-research/groups/irl/education/ basic-science-resident-lectures September 2011 Tuesday, September 27, 11
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Radiation Detection and Measurement - University of …depts.washington.edu/imreslab/2011 Lectures/radiation_detection... · Inorganic Scintillators (physical characteristics) NaI(Tl)
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Fall 2011, Copyright UW Imaging Research Laboratory
Fall 2011, Copyright UW Imaging Research Laboratory
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
Tuesday, September 27, 11
Types of radiation relevant to Nuclear Medicine
Particle! Symbol! Mass (MeV/c2) ! Charge
Electron! e-, β -! ! 0.511 !! ! -1
Positron! e+, β+!! 0.511 !! ! +1
Alpha!! α! ! 3700 ! ! ! +2
Photon! γ! no rest mass! ! none
Tuesday, September 27, 11
• 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.
• 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 Particle Range in Mattercontinuous energy spectrum
mm’s-
β±
Tuesday, September 27, 11
Interactions of Photons with MatterExponential Penetration: N=N0e-λx
Photoelectric effect! photon is absorbed!Compton scattering! part of the energy of the photon is absorbed! scattered photon continues on with lower energy
Photomultiplier Tube (PMT) - most common photo-sensor currently in use for Nuclear medicine
From: Physics in Nuclear Medicine (Sorenson and Phelps)
photo-sensor needed with scintillators
Tuesday, September 27, 11
Sample Spectroscopy SystemHardware
From: The Essential Physics of Medical Imaging (Bushberg, et al)
incoming high-energy gamma ray
converted to 1000s of visible photons
~20% converted to electrons
electron multiplicationbecomes electric signal larger current or
voltage
more electrons
more scintillation photons
higher gamma energy deposited in
crystal
Tuesday, September 27, 11
Silicon Photomultipliers(Geiger-mode APDs)
Fall 2011, Copyright UW Imaging Research Laboratory
Tuesday, September 27, 11
GM-APDs Arrays at the UW
Zecotek PhotonicsType 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
Tuesday, September 27, 11
Interactions of Photons with a Spectrometer
A. PhotoelectricB. Compton + PhotoelectricC. ComptonD. Photoelectric with characteristic
x-ray escapeE. Compton scattered photon from
lead shieldF. Characteristic x-ray from lead
shield
From: The Essential Physics of Medical Imaging (Bushberg, et al)
Tuesday, September 27, 11
Sample Spectroscopy SystemOutput
From: Physics in Nuclear Medicine (Sorenson and Phelps)
From: The Essential Physics of Medical Imaging (Bushberg, et al)
counting mode
Ideal Energy Spectrum
Remember - you are looking at the energy deposited in the detector!
Tuesday, September 27, 11
Energy Resolution
From: Physics in Nuclear Medicine (Sorenson and Phelps)
Realistic Energy Spectrum
Tuesday, September 27, 11
Sample Spectrum (Cs-137)
A. PhotopeakB. Compton continuumC. Compton edge
D.! Backscatter peakE.! Barium x-ray photopeakF.! Lead x-rays
Detection efficiency (32 keV vs. 662 keV)
From: The Essential Physics of Medical Imaging (Bushberg, et al)
Tuesday, September 27, 11
Sample Spectrum (Tc-99m)
A. PhotopeakB. 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)
Tuesday, September 27, 11
Sample Spectrum (In-111)
source detector
From: Physics in Nuclear Medicine (Sorenson and Phelps)
Tuesday, September 27, 11
Effects of Pulse Pileup (count rate)
From: Physics in Nuclear Medicine (Sorenson and Phelps)
Tuesday, September 27, 11
Interaction Rate and Dead-time
paralyzable non-paralyzable
From: The Essential Physics of Medical Imaging (Bushberg, et al)
True rate
Mea
sure
d ra
te
time
deadtime
= recorded events
Tuesday, September 27, 11
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), T1/2=30yr! Co-57: Eγ= 122 keV (close to Tc99m 140 keV ), T1/2=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)
Tuesday, September 27, 11
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.
Tuesday, September 27, 11
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.
Tuesday, September 27, 11
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 kcpsb. Less than 250 kcpsc. Greater than 250 kcps
Because of deadtime effects
Question
Fall 2011, Copyright UW Imaging Research Laboratory
Tuesday, September 27, 11
How many peaks would you expect for a99m-Tc sample placed outside a well counter?
1
1
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.
What about inside a well counter?
Is your answer dose dependent?
Question
Fall 2011, Copyright UW Imaging Research Laboratory
Tuesday, September 27, 11
How many peaks would you expect for a 68-Ge sample placed outside a well counter? What about inside a well counter?
Outside, 1Inside, 2
68-Ge is a positron emitter (e.g., PET)
Question
Fall 2011, Copyright UW Imaging Research Laboratory
Tuesday, September 27, 11
Of the following, the most efficient detector for x-rays is:
From: The Essential Physics of Medical Imaging (Bushberg, et al)
NaI(Tl) is an inorganic scintillator and is much more efficient at detecting x-rays
than gas filled detectors.
a. Geiger counterb. NaI(Tl) detectorc. Single channel analyzerd. Ionization chambere. Pocket (self-reading) dosimeter
Question
Fall 2011, Copyright UW Imaging Research Laboratory
Tuesday, September 27, 11
Gas multiplication occurs in:
From: The Essential Physics of Medical Imaging (Bushberg, et al)
Fall 2011, Copyright UW Imaging Research Laboratory
Tuesday, September 27, 11
In a pho tomu l t i p l i e r t ube , t he photocathode is at a positive voltage with respect to the first dynode.
From: The Essential Physics of Medical Imaging (Bushberg, et al)
False
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.
Question (True or False)
Fall 2011, Copyright UW Imaging Research Laboratory
Tuesday, September 27, 11
A 1 MeV beta particle produces a pulse of the same amplitude in a G-M detector as a 200 keV beta particle.
From: The Essential Physics of Medical Imaging (Bushberg, et al)
True
Question (True or False)
Fall 2011, Copyright UW Imaging Research Laboratory
Tuesday, September 27, 11
Which detector system is most appropriate and accurate for the measurement of a pure beta source:
From: Raphex
a. Ionization chamberb. Geiger Muller tubec. NaI(Tl) well scintillation counterd. Thermoluminescent dosimetere. Liquid scintillation counter
Question
Fall 2011, Copyright UW Imaging Research Laboratory
Tuesday, September 27, 11
a. Identify the energy of a radionuclideb. Reject Compton scattered photonsc. Separate a mixture of radionuclidesd. Alter the sensitivity or resolution of the
systeme. All of the above
From: Raphex
A pulse height analyzer (PHA) window can be used to:
Question
Fall 2011, Copyright UW Imaging Research Laboratory