Non-imaging Non-imaging devices devices in Nuclear Medicinein Nuclear Medicine

Pawitra Masa-at4937092 SIRS/MMarch 23, 2007

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Type of non-imaging Type of non-imaging devicedevice Gas-filled detectors

Radionuclide dose calibrators

Scintillation detectorScintillation detector Gamma well counter The thyroid uptake probe Liquid scintillation detector

Radionuclide dose Radionuclide dose calibratorscalibrators

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Gas-Filled DetectorsGas-Filled Detectors

I ; Ionization chamber regionP ; Proportional regionGM ; Geigur-Mueller region

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In the ionization chamber region The number of ion pairs collected by the

electrodes is equal to the number of ion pair produced by the radiation in the detector.

There is no change in the number of ion pairs collected as the voltage increase.

Ionization Chamber

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Ionization Chamber

HV

+

-

Negative ion

Positive ion

1234

Electrometer

The response is proportional toionization rate (activity, exposure rate)

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Dose calibrators (Activity meter) A radionuclide calibrator is in essence a well-type gas

ionization chamber into the well of which a radioactive material is introduced for measurement.

The activity of the material is measured in terms of the ionization current produced by the emitted radiations which interact in the gas.

The chamber is sealed, usually under pressure, and has two co-axial cylindrical electrodes maintained at a voltage difference derived from a suitable supply, the axial space constituting the well.

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Dose calibrators

In the associated electrometer, the ionization current is converted to a voltage signal, which is amplified, processed and finally displayed, commonly in digital form in units of activity - becquerels (Bq) or curies (Ci).

This is possible since for a given radionuclide, assuming a fixed geometry and a linear response, ionization current is directly proportional to activity.

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Dose calibrators

SC97

Proportionality between the number of photons emitted and the ionization current

Well-shaped ionization chamberfilled with a gas of high atomicnumber (e.g. Xenon) and keptunder pressure

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Dose calibrators

The response of the detector will depend on:

• Radionuclide (energy and abundance of photons).• Geometry of the detector.• Geometry of the source.• The condition of the instrument (QC).

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ACTIVITY MEASUREMENT

Setting Measured activityTc-99m 1.00Co-57 1.19In-111 2.35Tl-201 1.76Ga-67 1.12I-123 2.19I-131 1.43

Measured activity/True activity of Tc.99m if the indicated settings are used

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Geometric efficiencyThe quotient: number of photons reaching the detector overthe number of photons emitted from the sample

Increasing geometric efficiency

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SAMPLE HOLDER(reproducible geometry)

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Quality control of the dose calibrator.should include:

Test of precision and accuracy Test of linearity of activity response Test of reproducibility (Constancy test) Check of background

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Sealed sources for calibration of activity meters

• Long half-life• Range of photon energies• Range of activities• Calibrated within 5%

Co-57, Ba-133, Cs-137, Co-60

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Sealed sources for calibration of activity meters

Radionuclide Photon energy (keV)

Half-life Activity

(MBq)

Co-57 122 271 d 185

Ba-133 81, 356 10.7 y 9.3

Cs-137 662 30 y 7.4

Co-60 1173, 1332 5.27 y 1.9

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Source (sealed): Cs-137 or Co-57

Procedure: Select settings for the radionuclideand adjust background. Insert source in holderand make 10 measurements.

Data analysis: To assess precision, calculate foreach source (i) the percentage difference between themeasured activity Ai and their mean Amv. (+/-5%)To assess accuracy, calculate the percentage differencebetween the mean activity and the certified activity.(+/- 10%).

Measurement of precision and accuracy

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Mv: 9.834SD: 0.125

0

5

10

15

20

0 20 40 60

Measurement no

Act

ivit

y

Measure the activity of a sealed referencesource e.g. every morning. Use Tc-99m settings.

Measurement of reproducibility

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Measurement of reproducibility

•Check of Reproducibility. Part of control chart. The Cs-137 Source used had amean measured activity of 4.55 MBq (123 uCi) on 1 April.

•The limits of accepability indicated correspond to +/- 5% of the expected activity.

•The initial discrepant result on 17 May arose from failure to allow sufficient time for the reading to stabilize

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Use a radionuclide with short half-life e.g. Tc-99mMake repeated measurements during several half-lives.

Slope = -0.11471, R2=0.9999T=6.04 h

1

10

100

1000

10000

100000

0 20 40 60

Time (h)

Acti

vit

y (

MB

q)

Measurement of linearity

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Type of non-imaging Type of non-imaging devicedevice Gas-filled detectors

Radionuclide dose calibrators

Scintillation detectorScintillation detector Gamma well counter The thyroid uptake probe Liquid scintillation detector

Gamma well counterGamma well counter The thyroid uptake The thyroid uptake probeprobe

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Scintillation detectors can be used as a part of both non-imaging and imaging devices.

From the non-imaging devices, scintillation well counters and thyroid probes are used.

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The gamma well counter The gamma well counter

The gamma well counter consists of a scintillation detector with a hole in the center, for a sample to be placed inside for increasing the geometric efficiency and hence the counting efficiency of the counter, and other associated electronics.

Well counters are used namely for in vitro measurements of different samples. They are usually available with automatic sample changers and are mostly programmable with computers.

Their major advantage is high detection efficiency which is from 50% to 70%  for 140 keV gamma photons.

 

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RIA 125I

Kidney clearance 51Cr

Vitamin B12 deficiency 57Co,58Co

Ferrokinetic studies 59Fe

Total body water 3H

Blood volume 125I, 51Cr, 99mTc

Biomedical research 3H, 14C

Examples of use of sample counters

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HV

Ampl. PHA

Timer

Scaler

Rate-meterGain Base Window

Voltage

Detector Sample

Lead shield PM-tube

Gamma counter

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Scintillation detector

Detector

PhotocathodecathoddDynodes

Anode

Amplifier

PHA

ScalerProportionality between thesignal and the energy absor-bed in the detector

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Pulse height analyzer

UL

LL

Time

Pulse height (V)

The pulse height analyzer allows only pulses of a certain height(energy) to be counted.

counted not counted

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Pulse-height distributionNaI(Tl)

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The The TT hyroid hyroid PProberobe

The thyroid probe is a scintillation counter used for measuring radioacitivity above the thyroid gland to assess the uptake of 131I after its oral administration.

In contrast to well counter the thyroid probe must be equipped with collimator, which limits the field of view.

This is a cylindrical barrel made of lead and it covers all the detector including PM tube. It prevents the gamma radiations from other organs to reach the detector.

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HV

Ampl. PHA

Timer

Scaler

Rate-meterGain Base Window

Voltage

Recorder

Collimator

PM

D

Probe system

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Quality control of any scintillation detector systemshould include tests such as: Energy calibration Energy resolution and energy response Sensitivity Counting precision Background count rate Linearity of activity response

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Energy window setting depends on the energy resolution of the detector and the photon energies

Window settingWindow setting

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Gamma counterGamma counterDifferent design of the detector

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COUNT LOSSES(LINEARITY OF ACTIVITY RESPONSE)

•Decaying source method•Graded source method

Liquid scintillation Liquid scintillation countercounter

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Liquid scintillation Liquid scintillation countercounter In the beta-counter or liquid scintillation counter, the sample is

dissolved in an organic scintillation solution. Due to the resulting 100% counting geometry and the absence of any detector window,this means that the counter has excellent properties in detecting radionuclides of low activity emitting low energy beta-particles, such as H3 and C14.

The light photons from the sample are collected by two photomultipliers in coincidence.

This arrangement will reduce the background due to thermal noise and only true scintillation events will be analysed and counted.

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Liquid scintillation Liquid scintillation countercounter The main problem in liquid scintillation counting is the varying

counting efficiency due to quenching of scintillation events. This process is caused by chemical contamination of the sample

and/or a coloured sample. This means that the counting efficiency has to be determined for every sample.

Therefore a quality control of the instrument must include a control of the correction methods. Otherwise the QC methods will be the same as for any scintillation counter.

The sources needed for QC of a liquid scintillation counter include calibrated sources of H3 and C14 with different counting efficiencies as well as a background sample.

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PM PM

Coinc

Ampl PHA ScalerTimer

No window100% geometricefficiency

Liquid scintillation counter

Sample mixed with scintillation solution

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•Counting efficiency•Quenching•Sample preparation•Window setting•Reproducibility•Background

•Counting efficiency•Quenching•Sample preparation•Window setting•Reproducibility•Background

Operational considerations

Liquid scintillation counter

Thank you Thank you Pawitra Masa-atPawitra Masa-at 4937092 SIRS/M4937092 SIRS/MMarch 23, 2007March 23, 2007