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Nuclear Medicine Physics - PowerPoint Presentation

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Nuclear Medicine Physics - PPT Presentation

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

medicine nuclear tipnis sameer nuclear medicine sameer tipnis physics phd residents radiology dabr 2015 dose radiation energy gas detectors

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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 300mCi 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