Jerry Allison PhD Department of Radiology Medical College of Georgia Radiation Detectors A note of thanks to Z J Cao PhD Medical College of Georgia And Sameer Tipnis PhD G Donald Frey PhD ID: 653711
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Slide1
Nuclear Medicine Physics
Jerry Allison, Ph.D.Department of RadiologyMedical College of Georgia
Radiation DetectorsSlide2
A note of thanks to Z. J. Cao, Ph.D.Medical College of GeorgiaAnd
Sameer Tipnis, Ph.D.G. Donald Frey, Ph.D.Medical University of South Carolina
forSharing nuclear medicine
presentation contentSlide3
Basic principleRadiation enters a medium, deposits energyEnergy deposition produces ionizations, scintillations Signal converted to electrical current/pulses
Current/pulses amplified (original current/pulses generally small)Amplified current is measured or amplified pulses are counted/sorted by their energies, and recordedDisplay of radiation level or energy spectrum2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABRSlide4
Radiation Detectors in NMSurvey meters (gas-filled detector)
Ionization chambers (IC)Geiger Müeller (GM)Dose calibrator (gas-filled detector)
Well counter (scintillation detector) Thyroid probe (scintillation detector) Miniature g
-probe (scintillation) Slide5
Survey meters (IC)
Gas-filled detectors
GM chamber “pancake” (GM)
Dose calibrators (IC)
2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABRSlide6
Gas-filled DetectorsBasic gas-filled detector consists ofgas mediumpositive and negative charged electrodesShapesVersatile: cylindrical, flat, well-type
2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABRSlide7
Radiation ionizes gas molecules to produce +/- ion pairs Electric field draws e- to anode, generates signal Signal characteristics depend on the applied voltage
Gas-filled Detectors
2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABRSlide8
Gas-filled detectorsHow it works?
.Slide9
IC regionCurrent pulse (signal) produced by radiationSignal strength is proportional to energy depositedUsed for measuring “amount” of radiation (i.e., exposure, air kerma)
Ionization Chamber Region
S
1
S
2
2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABRSlide10
Ionization ChambersGas used Survey meter: airDose calibrator: Argon (10 – 20 atmospheres, less in PET)
Low efficiency (gas has low density)Air chambers are temperature/pressure sensitiveFairly rugged, not easily saturated with radiation
2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABRSlide11
Ionization Survey MetersCan be used to accurately measure:Exposure (measure of ionization in air, C/kg)C/kg SI units
Roentgen (R) traditional units1R = 2.58 x 10-4 C/kg33 ev deposited per ion pair createdAir Kerma (absorbed dose in air)
Kerma: kinetic energy released in mediaGray SI units1 Gy = 33.7 C/kg1 R of exposure = 0.00869
Gy of absorbed dose [AIR]
2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABRSlide12
Dose calibratorMeasure activity only
Select correct isotope buttonDrop a sample to the bottom to avoid position effectQuality control is regulated by NRC or Agreement StateEvery patient dose must be assayed before administration
Slide13
Dose calibratorRadionuclide selection
Ion chamber well
Shielded syringe transportSlide14
Dose calibrator quality control Constancy: daily, using Cs-137
(660 keV, 30 y) and Co-57 (122 keV, 9 mo) for all nuclide settings, error <
10% Linearity: quarterly, using 300mCi Tc-99m, down to 10
Ci or lineators, error < 10%
Accuracy:
yearly
, using Cs-137 and Co-57
,
error
< 5%
Geometry:
upon installation
, using 1
mCi
Tc-99m
with different volumes, error < 10%
Syringes (1ml, 3ml, 5ml, 10ml)Vial (10ml)Slide15
Dose calibrator - daily constancy test
When
performing the constancy test, one must check every setting that might be used that day starting officially at 12:01
AM.
A QC was performed
on
the
dose calibrator at 7:00 AM
today.
If the
technologist
is called in
for a lung V/Q study
at 12:15
AM tomorrow
morning,
she/he must check the constancy of Tc-99m and Xe-133 settings even though the elapsed time is less than 24 hours. Slide16
16
Linearity test using lineators Lineators: a set of lead sleeves with summed thickness to mimic physical decaySlide17
Linearity test using lineators The whole test must be done within 5 min.The initial ratios of decay among the sleeves
are verified and calibrated by the physical decay method. The initial ratios are then used to compare with the ratios obtained from
later tests.17Slide18
What if a test fails?If deviation is out of the limit, obligation is to record value, note repair and recalibration, retest, and record new values.
Until repair and recalibration are accomplished, every measured dose must be corrected mathematically. Slide19
GM regionHigh voltage applied to anodeIniitial ionizations produced by radiation and secondary ionizations produced by accelerating electronsSignal strength is independent of energy depositedUsed for measuring “presence” of radiation
Geiger-Müller Region
S
2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABRSlide20
GM CountersSealed pressurized chambers for maximum detection efficiencyNot possible to identify energyExtremely sensitive to radiationCan be saturated (“zero” reading if radiation flux too high)Typical applications is detection of trace radiation (contamination)
2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABRSlide21
21Quality control of survey meterChecking battery: before each use
Checking the reference source: before each useCalibration: before initial use and every yearSlide22
Scintillation Detectors2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR
(Main detectors in
NM imaging,
including gamma
cameras
)Slide23
Scintillation Detectors Two main components -Scintillator Radiation deposits energy in scintillator causing light flashes (fluorescence)Photomultiplier tube (PMT)Used to detect fluorescence from scintillator and amplify the signalNM – Inorganic solid scintillator (e.g.
NaI(Tl)) and PMT2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABRSlide24
OperationRadiation + scintillator produce fluorescence proportional to energyLight strikes PMT photocathode, ejecting e- e-
successively accelerated towards 8 – 12 dynodesSignal amplified ~ 106 -107 Signal read out and processed
2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABRSlide25
Energy resolution
∆E
E
2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABRSlide26
Energy resolution
Energy resolution plays an important role in scatter rejection / image quality
High energy resolution
Image quality
∆E
E
2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABRSlide27
Well-counters (NaI(Tl))Daily wipe tests
Scintillation Detectors
2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABRSlide28
Well counter NaI(Tl)Measure small amount of radioactivity (< 1 m
Ci for daily wipe tests)Main components: single NaI(Tl) crystal (4.5×
5 cm or 1.6×3.8 cm) with a hole for samplePMTpreamplifier
amplifierPHAreadout deviceSlide29
Thyroid probe (NaI(Tl))
Scintillation Detectors
2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABRSlide30
Thyroid probe Measure thyroid uptake of I-131 in-vivo
5×5 cm NaI(Tl) with 15 cm long conical collimator
pointing to neck and thigh (bkg)calibration phantom with known activity for calculating
uptake1 – 2 cm difference in depth 10 – 40% difference in count rateSlide31
Thyroid probe Thyroid uptake neck phantom
Slide32
32QC of well counter and thyroid probe
Constancy: before each use, using a Cs-137 source Chi-square: quarterly, using a Cs-137 source Do a series of counts have a Poisson distribution?Energy resolution
: quarterly, using a Cs-137 source Slide33
Miniature g probe99mTc-colloid injected before surgery
99mTc-colloid is concentrated in the sentinel lymph nodes.Detecting sentinel lymph nodes using the g probe in surgery
Probe: 5 × 10 mm, high directional sensitivity, light, easy to operate
Also detecting other isotopes, e.g. I-131