part b: non-imaging scintillation detectors unit ii: nuclear medicine measuring devices lectures 7...

Part B: Non-imaging

Scintillation Detectors

Unit II: Nuclear Medicine Measuring Devices

Lectures 7 & 8

ObjectivesObjectives Define scintillationDefine scintillation Describe the structure & purpose of a NaI (Tl) crystal and the crystal’s Describe the structure & purpose of a NaI (Tl) crystal and the crystal’s

proportional response to deposited gamma energyproportional response to deposited gamma energy Describe the components of a photomultiplier tube and their functionDescribe the components of a photomultiplier tube and their function Discuss the purpose of other associated electronics within the Discuss the purpose of other associated electronics within the

scintillation detectorscintillation detector Describe the calibration process for single and multi-channel Describe the calibration process for single and multi-channel

analyzersanalyzers Discuss peak broadening and the determination of a percent energy Discuss peak broadening and the determination of a percent energy

windowwindow Calculate percent energy resolution from FWHM and its importance in Calculate percent energy resolution from FWHM and its importance in

quality controlquality control Describe quality control tests for scintillation detectors and their Describe quality control tests for scintillation detectors and their

required frequencyrequired frequency

http://oldsite.vislab.usyd.edu.au/photonics/devices/semic/images/valcond.gif

Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 13.

Scintillation Process in NaI(Tl)Scintillation Process in NaI(Tl)

Gamma Photon

Excited Electrons



Visible light

(350-500nm λ)

Returning electrons

http://www.webelements.com/webelements/compounds/media/Na/I1Na1-7681825.jpg

Gamma Photon

Excited Electrons


NaI at Room Temperature

Excited Electrons Don’t Fluoresce




Thallium

So we add an impurity – a big ole ugly Thallium Atom



Thallium

Thallium forms a luminescent Center in the gap that catches excited electrons



Thallium

When electrons return to the valence band from the gap, they give off light


Thallium

Therefore, we say our crystal of Sodium Iodide is Thallium Activated

Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) p 53.

Gamma Photon

NaI (Tl) Crystal (hermetically sealed in reflective material)

Visible light

140 keV Gamma Photon

Visible light produced is 325 to 550 nm wavelength

For each keV of gamma photon energy absorbed, 20-30 visible light photons are released by the crystal.

Therefore, a 140 keV photon will cause about 3000 visible light photons to be released from the crystal

Important Magic to RememberImportant Magic to RememberThe higher the gamma photon energy :

The more visible photons created

The visible light emitted from the NaI (Tl) crystal is PROPORTIONAL to the incident energy of the gamma photon.

Timing after a gamma interaction:

Scintillation peak in about 30 nsec and about 2/3 of light emitted after 230 nsec

At lower rates of interaction (low count rate), a scintillation event typically ends before the next

Hence scintillation detectors operate in pulse mode

Photomultiplier TubePhotomultiplier Tube

http://www.kolumbus.fi/michael.fletcher/pmt_1.jpg

http://www.youngin.com/EditData/Editor/211392222620034716115.jpg

Photomultiplier tubes coupled to NaI(Tl) Crystals

Crystal/PMT InterfaceCrystal/PMT Interface

Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 101.

CsSb CsSb PhotocathodePhotocathode

NaI (Tl) NaI (Tl) CrystalCrystal

Gamma PhotonGamma Photon

Visible LightVisible Light

Emitted Emitted electrons from electrons from photocathodephotocathode

Optical Window Optical Window (transparent (transparent

material)material)

Quantum Efficiency: Quantum Efficiency: A measure of A measure of how well a how well a photoemissivephotoemissive material material emits electrons when exposed to emits electrons when exposed to

various wavelengths of lightvarious wavelengths of light

The above graph The above graph shows this shows this

photoemissive photoemissive material is most material is most

productive at 400 nmproductive at 400 nm—about the same —about the same

wavelength created wavelength created by NaI (Tl) by NaI (Tl) scintillationscintillation

Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 102.

For every 3 – 5 visible light photons reaching the photocathode, For every 3 – 5 visible light photons reaching the photocathode, 1 electron is emitted1 electron is emitted

Proportionality MaintainedProportionality Maintained

Note: This is Note: This is the least the least

efficient phase efficient phase of the transferof the transfer

Photomultiplier Tube (PMT) ConstructionPhotomultiplier Tube (PMT) Construction

Focusing Grid – Guides in ElectronsFocusing Grid – Guides in Electrons

Dynodes – Usually positively charged Dynodes – Usually positively charged photoemissive coated electrodes with photoemissive coated electrodes with

increasing voltagesincreasing voltages

Anode – end positively chargedAnode – end positively charged

High voltage power supply – needed to High voltage power supply – needed to increase incrementally the potential increase incrementally the potential

difference between dynodesdifference between dynodes

Increased voltages between dynodes : Increased voltages between dynodes : means increased KE of electrons means increased KE of electrons

between dynodesbetween dynodes

Increased KE of electrons mean that Increased KE of electrons mean that more electrons are knocked off at each more electrons are knocked off at each

dynode (3X to 6X at each)dynode (3X to 6X at each)

At 6X each with 10 dynodes means 6At 6X each with 10 dynodes means 610 10

electrons produced (about 60 million)electrons produced (about 60 million)

(Still small – 1 Amp = 1 C/s; (Still small – 1 Amp = 1 C/s; 1 C = 6.3 X 101 C = 6.3 X 1018 18 electrons)electrons)

Millions of electrons are produced by Millions of electrons are produced by the dynodes the dynodes in direct responsein direct response to the to the

initial few electrons emitted by the initial few electrons emitted by the photocathode photocathode in direct responsein direct response to the to the

visible light photons emitted by the visible light photons emitted by the NaI(Tl) crystal NaI(Tl) crystal in direct responsein direct response to the to the

energy level of the gamma photon energy level of the gamma photon interacting with the crystal interacting with the crystal

End result:End result:

Increased gamma energy : means Increased gamma energy : means increased electrons reaching the anode increased electrons reaching the anode

at the end of the tubeat the end of the tube

This means that the height of the This means that the height of the electric pulse created by the millions of electric pulse created by the millions of

electrons at the anode will be an electrons at the anode will be an indicator of gamma energy level indicator of gamma energy level


Inefficiencies from the transfer Inefficiencies from the transfer of energyof energy

The proportionality of the The proportionality of the system is approximate and system is approximate and contains random statistical errorcontains random statistical errorSee Table 2-1 (p. 21)See Table 2-1 (p. 21)

Note that Tc-99m gamma produces Note that Tc-99m gamma produces less “information carriers” than the less “information carriers” than the higher energy gamma from Cs-137higher energy gamma from Cs-137

Therefore, Cs-137 has better Therefore, Cs-137 has better counting statistics and less variation counting statistics and less variation in the heights of its pulsesin the heights of its pulses

Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) pg 60.

From the PMT the signal goes from the anode to the preampFrom the PMT the signal goes from the anode to the preamp

PreamplifierPreamplifier

Increases pulse Increases pulse 4X to 5X4X to 5X

Matches Matches impedanceimpedance to the to the system’s circuitrysystem’s circuitry



Next the signal goes from the preamp to the ampNext the signal goes from the preamp to the amp

AmplifierAmplifier

Pulse undergoes: Pulse undergoes:

1. 1. Pulse ShapingPulse Shaping

2.2.Linear AmplificationLinear Amplification

(Amplified 1 to 100 X(Amplified 1 to 100 X

by Gain control)by Gain control)

Pulse Shaping (Amplifier)Pulse Shaping (Amplifier)

From Sodee:From Sodee:

““Change to voltage Change to voltage converter that increases converter that increases the signal-to-noise ratio.”the signal-to-noise ratio.”

Makes “splat” of voltage Makes “splat” of voltage pulse into a “pop.”pulse into a “pop.”

Increases count rate Increases count rate capability of the systemcapability of the system

Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 138.

Pulse ShapingPulse Shaping

Sorenson, p. 88Sorenson, p. 88

Pulse Shaping: RC Circuits and Pulse Shaping: RC Circuits and Noise EliminationNoise Elimination

Linear Amplification (Amplifier)Linear Amplification (Amplifier)P

ulse

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Each of these pulses Each of these pulses represents a different gamma represents a different gamma photon energy detected by the photon energy detected by the NaI (Tl) crystal.NaI (Tl) crystal.

We want to preserve this We want to preserve this proportionality in the pulses for proportionality in the pulses for it represents the proportional it represents the proportional differences in the gamma differences in the gamma energies detected.energies detected.

But we need a stronger signal But we need a stronger signal to work with.to work with.


ulse

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The linear amplifier amplifies The linear amplifier amplifies all the pulses proportionally.all the pulses proportionally.


ulse

Vol

tage

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TimeTime

Proportionality Proportionality MaintainedMaintained


From Sodee…From Sodee…

“The pulse height is directly proportional to the energy of the incident gamma photon.”

Prekeges, J.


ulse

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The linear amplifier amplifies The linear amplifier amplifies all the pulses proportionally.all the pulses proportionally.

Pul

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TimeTime

GAIN

1

2 5

10

We can designate (calibrate) the height of our pulses by

Gain Control

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A new set of pulses from photons with the following energies…230

keV80 keV

120 keV140 keV

30 keV

180 keV

10

2

0

30

4

0

5

0

60

Pul

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ulse

Vol

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TimeTime

230 keV80 keV

120 keV140 keV

30 keV

180 keV

10

2

0

30

4

0

5

0

60

(We have magic eyes and know what these energies are before our detecting system does.)

Pul

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We introduce a linear voltage scale…

230 keV80 keV

120 keV140 keV

30 keV

180 keV

For Comparision,

Volts

0 25 50 75 100 125 150 175 200 225 250 275 300

10

2

0

30

4

0

5

0

60

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230 keV80 keV

120 keV140 keV

30 keV

180 keV

Volts

0 25 50 75 100 125 150 175 200 225 250 275 300

GAIN

1

2 5

10

We introduce our gain control voltage of our amplifier to increase the voltage pulse heights associated with photons of the given energies

10

2

0

30

4

0

5

0

60

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230 keV80 keV

120 keV140 keV

30 keV

180 keV

Volts

0 25 50 75 100 125 150 175 200 225 250 275 300

GAIN

1

2 5

10At gain setting “1” we see the pulse voltages below

And they appear on our voltage scale as follows…(Photon energy represented by color points

only)

10

2

0

30

4

0

5

0

60

Pul

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230 keV80 keV

120 keV140 keV

30 keV

180 keV

Volts

0 25 50 75 100 125 150 175 200 225 250 275 300

GAIN

1

2 5

10

Changing the gain to “2” doubles the size of our pulses.

And Shifts our energy points lineup to the right

GAIN

1

2 5

10

10

2

0

30

4

0

5

0

60

Pul

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ulse

Vol

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TimeTime

230 keV80 keV

120 keV140 keV

30 keV

180 keV

Volts

0 25 50 75 100 125 150 175 200 225 250 275 300

GAIN

1

2 5

10

To amplify our pulses even more, we’ll need to change the scale of our pulse voltages so they will fit on our slide.

GAIN

1

2 5

10

10

2

0

30

4

0

5

0

60

Pul

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ulse

Vol

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230 keV80 keV

120 keV140 keV

30 keV

180 keV

Volts

0 25 50 75 100 125 150 175 200 225 250 275 300

GAIN

1

2 5

10

GAIN

1

2 5

10

50

75

1

00

1

25

1

50

1

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2

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22

5

Now that our scale’s adjusted on our graph, we can really start cranking up the gain and see its

effects.

GAIN

1

2 5

10

Let’s see what happens when we crank this puppy up to “5.” (5 X the gain!)

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230 keV80 keV

120 keV140 keV

30 keV

180 keV

Volts

0 25 50 75 100 125 150 175 200 225 250 275 300

GAIN

1

2 5

10

GAIN

1

2 5

10

50

75

1

00

1

25

1

50

1

75

2

00

22

5GAIN

1

2 5

10

(Notice how these photon energy points spread to the right even more.)

GAIN

1

2 5

10

Let’s crank the gain up to “10.” (10 X amplified!!)

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230 keV80 keV

120 keV140 keV

30 keV

180 keV

Volts

0 25 50 75 100 125 150 175 200 225 250 275 300

GAIN

1

2 5

10

GAIN

1

2 5

10

50

75

1

00

1

25

1

50

1

75

2

00

22

5GAIN

1

2 5

10

(-Notice how these points representing the different photon energies now line up with our voltage scale?)

GAIN

1

2 5

10

230 keV80 keV

120 keV140 keV

30 keV

180 keV

Volts

0 25 50 75 100 125 150 175 200 225 250 275 300

GAIN

1

2 5

10

GAIN

1

2 5

10

GAIN

1

2 5

10

GAIN

1

2 5

10With our gain at 10, we’ve made our photon energies register to voltage numbers that equal the keV for each photon.

Because we knew our photon energies before-hand, we can therefore say that we have “calibrated” our voltage scale so that each additional volt means an additional keV of photon energy.

GAIN

1

2 5

10

32

16

8

42

4

2

1

0.5

0.25

Special note: not all gain scales read the same.

Some are “inverse gain” scales and can be considered as representing the denominators of fractions.

For example:

Going from an inverse gain setting of 32 to 16 is like adjusting the voltage control from 1/32 to 1/16.

This still in effect doubles the voltage response of the pulse.

Inverse gain scales

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230 keV80 keV

120 keV140 keV

30 keV

180 keV

Volts

0 25 50 75 100 125 150 175 200 225 250 275 300

50

75

1

00

1

25

1

50

1

75

2

00

22

5

We have collected these six photons of various energies over this given time period…

…and have appointed them each a place on our voltage scale (which now represents 1 keV per volt).

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230 keV80 keV

120 keV140 keV

30 keV

180 keV

50

75

1

00

1

25

1

50

1

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2

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22

5

Now,

What if we let the clock run and keep on detecting more photons?

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230 keV80 keV

120 keV140 keV

30 keV

180 keV

50

75

1

00

1

25

1

50

1

75

2

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22

5

We’d get a random mixed bag of photons.

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230 keV80 keV

120 keV140 keV

30 keV

180 keV

50

75

1

00

1

25

1

50

1

75

2

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22

5

For the sake of our example, we’ll say we’re detecting only photons of the six above energies.

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230 keV80 keV

120 keV140 keV

30 keV

180 keV

50

75

1

00

1

25

1

50

1

75

2

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22

5

We’ll let them add up over time.

230 keV

80 keV

120 keV140 keV

30 keV

180 keV

Volts

0 25 50 75 100 125 150 175 200 225 250 275 300

6

10

15

40

10

2

We’ve stopped our detector, and now we’ll tally up each type of photon detected.

Here are our totals.

230 keV

80 keV

120 keV140 keV

30 keV

180 keV

Volts

0 25 50 75 100 125 150 175 200 225 250 275 300

6

10

15

40

10

2

Next, we’ll stack up our tally count for each photon on our voltage scale according to its calibrated spot on the scale. 40

30

20

10

Count

s

230 keV

80 keV

120 keV140 keV

30 keV

180 keV

Volts

0 25 50 75 100 125 150 175 200 225 250 275 300

6

10

15

40

10

2

The colored bars do not represent pulse heights here…

40

30

20

10

Count

s

But rather the total amount of each type of energy detected.

230 keV

80 keV

120 keV140 keV

30 keV

180 keV

Volts

0 25 50 75 100 125 150 175 200 225 250 275 300

6

10

15

40

10

2

We can see that we have collected mostly 140 keV photons—the type of gamma emission associated with Tc-99m.

40

30

20

10

Count

s

If our source is indeed Tc-99m, why are we getting the other photon energy readings?

This is our This is our photopeakphotopeak because because

it most repeatedly it most repeatedly generated the level of generated the level of scintillation light that scintillation light that

resulted in this resulted in this voltage pulsevoltage pulse

230 keV

80 keV

120 keV140 keV

30 keV

180 keV

Volts

0 25 50 75 100 125 150 175 200 225 250 275 300

6

10

15

40

10

2

Some explanations for these other gammas detected are …

40

30

20

10

Count

s

Partially detected photons

Back-scattered photons

Compton Scattered photons Primary

gamma photons Extra

electrons emitted from photo-cathode

Two gamma photons detected simulta-neously

The 140 keV primary gamma photons are coming directly from the source. How do we extract them from the others so they can give us some reliable information?

If we lived in a perfect world with no If we lived in a perfect world with no scatter and perfect detectors, we would scatter and perfect detectors, we would get a gamma “pulse-height” spectrum that get a gamma “pulse-height” spectrum that looked like…looked like…

0 25 50 75 100 125 150 175 200 225 250 275 300

40

30

20

10

Count

s

Volts

Figure 03: Peak broadening as seen with scintillation detectorsFigure 03: Peak broadening as seen with scintillation detectors

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How do we extract How do we extract the pulses that the pulses that represent the true represent the true gamma energy of gamma energy of a radionuclide?a radionuclide?

Answer: Pulse Answer: Pulse Height AnalysisHeight Analysis

Pulse Height AnalyzerPulse Height Analyzer

According to Sodee text:According to Sodee text: “ “The pulse height analyzer is an electronic The pulse height analyzer is an electronic

device that enables the operator to select device that enables the operator to select pulses of a certain height and to reject all pulses of a certain height and to reject all pulses of a different height.”pulses of a different height.”

Paul Early, D. Bruce Sodee, Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear MedicinePrinciples and Practice of Nuclear Medicine, 2nd Ed., , 2nd Ed., (St. Louis: Mosby 1995), pg. 141.(St. Louis: Mosby 1995), pg. 141.

Single Channel Analyzer (SCA)Single Channel Analyzer (SCA)

A pulse height analyzer that detects only A pulse height analyzer that detects only one set of pulses.one set of pulses.

We will use a Single Channel Analyzer example to We will use a Single Channel Analyzer example to demonstrate how we can separate our 140 keV photons demonstrate how we can separate our 140 keV photons from the photons of other energies.from the photons of other energies.

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230 230 keVkeV

80 keV80 keV

120 120 keVkeV140 140 keVkeV

30 keV30 keV

180 180 keVkeV

50

75

1

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1

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1

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1

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2

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50

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1

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1

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1

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2

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25

We’ll go back to our collection of We’ll go back to our collection of pulses over time to see how we can pulses over time to see how we can distinguish the 140 keV pulses from distinguish the 140 keV pulses from

the other pulses representing the other pulses representing detected photons of different detected photons of different

energies.energies.

Lower Level Discriminator (LLD)Lower Level Discriminator (LLD)

The electronically and arbitrarily The electronically and arbitrarily established threshold that a pulse much established threshold that a pulse much reach in order to be counted as detected.reach in order to be counted as detected.

For our example, we’ll establish a threshold of For our example, we’ll establish a threshold of 10% below the 140 volt pulse (140 keV),10% below the 140 volt pulse (140 keV),

that is, we’re going to electronically tell our that is, we’re going to electronically tell our system NOT to accept any pulses that do not system NOT to accept any pulses that do not

reach 126 volts in height (126 keV).reach 126 volts in height (126 keV).

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230 230 keVkeV

80 keV80 keV


30 keV30 keV

180 180 keVkeV

50

75

1

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1

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1

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1

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2

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50

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1

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1

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1

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Here’s our LLD Here’s our LLD lineline(at 126 Volts)(at 126 Volts)

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230 230 keVkeV

80 keV80 keV


30 keV30 keV

180 180 keVkeV

50

75

1

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1

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1

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1

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2

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50

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1

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And here’s its effectsAnd here’s its effects

All pulses less than 126 volts are All pulses less than 126 volts are not seen (counted)not seen (counted)

Let’s count our pulses Let’s count our pulses and see what we got.and see what we got.

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230 230 keVkeV

80 keV80 keV


30 keV30 keV

180 180 keVkeV

50

75

1

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1

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1

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1

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2

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50

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1

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But Wait! Some of But Wait! Some of these are not 140 volt these are not 140 volt

pulses!pulses!

11

22

3344 55 66

77

88

99

We get nine We get nine pulses countedpulses counted

Upper Level Discriminator Upper Level Discriminator (ULD)(ULD)

An Upper Level Discriminator is just a second An Upper Level Discriminator is just a second Lower Level Discriminator.Lower Level Discriminator.

It also has an electronic threshold that will only It also has an electronic threshold that will only recognize pulses of an arbitrarily selected recognize pulses of an arbitrarily selected voltage height.voltage height.

The ULD threshold is set above the LLD The ULD threshold is set above the LLD threshold.threshold.

Let’s set our ULD for 10% above our Let’s set our ULD for 10% above our desired 140 volt (140 keV) pulse height.desired 140 volt (140 keV) pulse height.

This would come to 154 volts (154 keV).This would come to 154 volts (154 keV).

This means all pulses BELOW 154 volts This means all pulses BELOW 154 volts would be NOT be counted.would be NOT be counted.

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230 230 keVkeV

80 keV80 keV


30 keV30 keV

180 180 keVkeV

50

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1

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1

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2

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1

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Here’s the ULD Here’s the ULD thresholdthreshold

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230 230 keVkeV

80 keV80 keV


30 keV30 keV

180 180 keVkeV

50

75

1

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2

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50

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1

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And here’s its effectsAnd here’s its effects

Is this what we wanted? Are these the Is this what we wanted? Are these the counts we need? How can we use counts we need? How can we use

this????this????

What the…?What the…?

Anticoincidence Logic CircuitAnticoincidence Logic Circuit

Simon Cherry, James Sorenson, & Michael Phelps, Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear MedicinePhysics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 116., 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 116.

Pulses Pulses from from

AmplifieAmplifierr

LLDLLD

ULDULD

Anti-Anti-coincidenccoincidenc

e logice logic

OutpuOutputt

All of our All of our pulses come pulses come in from the in from the amplifier at amplifier at

their their proportional proportional

voltage voltage heightsheights


AmplifieAmplifierr

LLDLLD

ULDULD


e logice logic

OutpuOutputt

One copy of One copy of pulses goes to pulses goes to

the LLD.the LLD.

One copy of One copy of pulses goes to pulses goes to

the ULD.the ULD.


AmplifieAmplifierr

LLDLLD

ULDULD


e logice logic

OutpuOutputt

Only the 140, 180, & 230 V Only the 140, 180, & 230 V pulse copies cross the LLD pulse copies cross the LLD

threshold and are accepted.threshold and are accepted.

Only the 180 & Only the 180 & 230 V pulse copies 230 V pulse copies

cross the ULD cross the ULD threshold and are threshold and are

acceptedaccepted


AmplifierAmplifier

LLDLLD

ULDULD

OutpuOutputt

In the anticoincidence In the anticoincidence logic circuit the copies logic circuit the copies

of the 180 & 230 V of the 180 & 230 V pulses arrive at the pulses arrive at the same time (for they same time (for they

were generated at the were generated at the same time.)same time.)

The copy of the 140 V pulse The copy of the 140 V pulse arrives by itself because its arrives by itself because its

copy broke the LLD copy broke the LLD threshold but not the ULD threshold but not the ULD

thresholdthreshold..


AmplifieAmplifierr

LLDLLD

ULDULD

OutpuOutputt

Because the 180 and Because the 180 and 230 V pulse copies 230 V pulse copies arrived at the same arrived at the same

time (they were time (they were generated at the same generated at the same time) the coincidence time) the coincidence

logic cancels them out.logic cancels them out.

The single 140 V (140 keV) The single 140 V (140 keV) pulse has no copy and pulse has no copy and

survivessurvives


AmplifierAmplifier

LLDLLD

ULDULD


e logice logic

OutpuOutputt

From all the From all the pulses we pulses we collect one collect one “count” of a “count” of a 140 V pulse 140 V pulse

(140 kev (140 kev photon).photon).

Pul

se V

olta

geP

ulse

Vol

tage

TimeTime

230 230 keVkeV

80 keV80 keV


30 keV30 keV

180 180 keVkeV

50

75

1

00

1

25

1

50

1

75

2

00

50

75

1

00

1

25

1

50

1

75

2

00

22

52

25

In Effect, our Coincidence Circuit In Effect, our Coincidence Circuit enables us to cancel out our enables us to cancel out our unwanted oversized pulses.unwanted oversized pulses.

Pul

se V

olta

geP

ulse

Vol

tage

TimeTime

230 230 keVkeV

80 keV80 keV


30 keV30 keV

180 180 keVkeV

50

75

1

00

1

25

1

50

1

75

2

00

50

75

1

00

1

25

1

50

1

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2

00

22

52

25

And get only the desired And get only the desired pulses.pulses.

Pul

se V

olta

geP

ulse

Vol

tage

TimeTime

230 230 keVkeV

80 keV80 keV


30 keV30 keV

180 180 keVkeV

50

75

1

00

1

25

1

50

1

75

2

00

50

75

1

00

1

25

1

50

1

75

2

00

22

52

25

We go from this….We go from this….

Pul

se V

olta

geP

ulse

Vol

tage

TimeTime

230 230 keVkeV

80 keV80 keV


30 keV30 keV

180 180 keVkeV

50

75

1

00

1

25

1

50

1

75

2

00

50

75

1

00

1

25

1

50

1

75

2

00

22

52

25

To this.To this.

230 230 keVkeV

80 keV80 keV

120 120 keVkeV

140 140 keVkeV

30 keV30 keV

180 180 keVkeV

VoltsVolts

0 25 50 75 100 125 150 175 200 225 250 0 25 50 75 100 125 150 175 200 225 250 275 300 275 300

66

1010

1515

4040

1010

22

4040

3030

2020

1010

Cou

nts

Cou

nts

We end up with an energy “window” that We end up with an energy “window” that discriminates against photon energies that are from discriminates against photon energies that are from

indirect sources.indirect sources.

This is a Single Channel AnalyzerThis is a Single Channel Analyzer

(LLD)(LLD) (ULD)(ULD)

Fig 2-6 from your Prekeges TextFig 2-6 from your Prekeges Text

230 230 keVkeV

80 keV80 keV

120 120 keVkeV

140 140 keVkeV

30 keV30 keV

180 180 keVkeV

VoltsVolts

0 25 50 75 100 125 150 175 200 225 250 0 25 50 75 100 125 150 175 200 225 250 275 300 275 300

66

1010

1515

4040

1010

22

4040

3030

2020

1010

Cou

nts

Cou

nts



In reality, we add up the counts from the different In reality, we add up the counts from the different photon energies and get something like this…photon energies and get something like this…

230 230 keVkeV

80 keV80 keV

120 120 keVkeV

140 140 keVkeV

30 keV30 keV

180 180 keVkeV

VoltsVolts

0 25 50 75 100 125 150 175 200 225 250 0 25 50 75 100 125 150 175 200 225 250 275 300 275 300

66

1010

1515

4040

1010

22

4040

3030

2020

1010

Cou

nts

Cou

nts



This shows a 20% energy (window) around the 140 keV This shows a 20% energy (window) around the 140 keV photopeak.photopeak.

0 25 50 75 100 125 150 175 200 225 250 0 25 50 75 100 125 150 175 200 225 250 275 300 275 300

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Let’s apply our gain control to the real energy spectrumLet’s apply our gain control to the real energy spectrum

GAINGAIN

11

22 55

1010

0 25 50 75 100 125 150 175 200 225 250 0 25 50 75 100 125 150 175 200 225 250 275 300 275 300

4040

3030

2020

1010

GAINGAIN

11

22 55

1010

0 25 50 75 100 125 150 175 200 225 250 0 25 50 75 100 125 150 175 200 225 250 275 300 275 300

4040

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1010

GAINGAIN

11

22 55

1010

GAINGAIN

11

22 55

1010

0 25 50 75 100 125 150 175 200 225 250 0 25 50 75 100 125 150 175 200 225 250 275 300 275 300

4040

3030

2020

1010

GAINGAIN

11

22 55

1010

GAINGAIN

11

22 55

1010

GAINGAIN

11

22 55

1010

0 25 50 75 100 125 150 175 200 225 250 0 25 50 75 100 125 150 175 200 225 250 275 300 275 300

4040

3030

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1010

GAINGAIN

11

22 55

1010

GAINGAIN

11

22 55

1010

GAINGAIN

11

22 55

1010

GAINGAIN

11

22 55

1010

0 25 50 75 100 125 150 175 200 225 250 0 25 50 75 100 125 150 175 200 225 250 275 300 275 300

4040

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1010

Full Width at Half Maximum Full Width at Half Maximum (FWHM)(FWHM)

A A measurement measurement

of energy of energy resolution—a resolution—a

means of means of showing how showing how

well your well your detector can detector can discriminate discriminate

energy energy differences.differences.

VoltsVolts

0 25 50 75 100 125 150 175 200 225 250 0 25 50 75 100 125 150 175 200 225 250 275 300 275 300

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Cou

nts

(X

C

ou

nts

(X

1

00

0)

10

00

)


First…First…

Find point on Find point on scale that scale that

correlates to correlates to your peak your peak

counts.counts.140 V140 V

0 25 50 75 100 125 150 175 200 225 250 0 25 50 75 100 125 150 175 200 225 250 275 300 275 300

4040

3030

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1010


Next…Next…

Find the Find the maximum maximum

counts of the counts of the spectrum.spectrum. 140 V140 V

42,000 Counts42,000 Counts

0 25 50 75 100 125 150 175 200 225 250 0 25 50 75 100 125 150 175 200 225 250 275 300 275 300

4040

3030

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1010


Then…Then…

Figure out Figure out where ½ the where ½ the

maximum maximum counts counts

intersects the intersects the peak of the peak of the spectrumspectrum

140 V140 V

21,000 Counts 21,000 Counts (1/2 Maximum)(1/2 Maximum)

0 25 50 75 100 125 150 175 200 225 250 0 25 50 75 100 125 150 175 200 225 250 275 300 275 300

4040

3030

2020

1010


Now…Now…

Determine Determine how the full how the full width of the width of the

photopeak at photopeak at ½ maximum ½ maximum

counts counts translates to translates to

the scale the scale belowbelow

126 V126 V


158 V158 V

0 25 50 75 100 125 150 175 200 225 250 0 25 50 75 100 125 150 175 200 225 250 275 300 275 300

4040

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The FWHM is The FWHM is based on the based on the

following:following: 126 V126 V


158 V158 V

% Resolution = % Resolution = Upper Scale Reading – Lower Scale ReadingUpper Scale Reading – Lower Scale Reading X 100 X 100 Photopeak scale readingPhotopeak scale reading

0 25 50 75 100 125 150 175 200 225 250 0 25 50 75 100 125 150 175 200 225 250 275 300 275 300

4040

3030

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1010


For our system, For our system, our calculations our calculations

would be as would be as follows:follows:

126 V126 V


158 V158 V

% Resolution = % Resolution = 158 V - 126 V158 V - 126 V X 100 = 23 % X 100 = 23 %

140 V140 V

0 25 50 75 100 125 150 175 200 225 250 0 25 50 75 100 125 150 175 200 225 250 275 300 275 300

4040

3030

2020

1010


126 V126 V


158 V158 V

% Resolution = % Resolution = 158 V - 126 V158 V - 126 V X 100 = 23 % X 100 = 23 %

140 V140 V

A FWHM of 23 % A FWHM of 23 % actually stinks.actually stinks.

7 or 8 % would 7 or 8 % would be a more be a more

desirable value.desirable value.

This means our This means our photopeak photopeak

should be much should be much slimmer.slimmer.

Our system Our system likely needs likely needs

repair.repair.

0 25 50 75 100 125 150 175 200 225 250 0 25 50 75 100 125 150 175 200 225 250 275 300 275 300

4040

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A highly A highly resolute resolute

photopeaphotopeak (with a k (with a

low low FWHM) FWHM)

should be should be skinny.skinny.

MultiChannel Analyzer (MCA)MultiChannel Analyzer (MCA)

A “digital” means to collect and record A “digital” means to collect and record counts along a set of voltage channelscounts along a set of voltage channels

Uses Analogue to Digital Conversion Uses Analogue to Digital Conversion (ADC) to discern pulse sizes and assign (ADC) to discern pulse sizes and assign them to memory locationsthem to memory locations

Greatly increases the flexibility of selecting Greatly increases the flexibility of selecting and measuring counts from various energy and measuring counts from various energy sourcessources

MultiChannel AnalyzerMultiChannel Analyzer



Like SCAs, gamma photons generate a number Like SCAs, gamma photons generate a number of pulse sizes along a voltage scale or of pulse sizes along a voltage scale or

“channels.”“channels.”


These pulse sizes are converted to a discrete These pulse sizes are converted to a discrete value based on the channel in which they fall. value based on the channel in which they fall. This is called Analogue to Digital Conversion.This is called Analogue to Digital Conversion.


In other words, there is a rounding off of pulse In other words, there is a rounding off of pulse sizes so that they equal a digitized amount, sizes so that they equal a digitized amount, such as 2.8 and 3.2 are assigned to digital such as 2.8 and 3.2 are assigned to digital

value “3.”value “3.”


Most scintillation detectors now use MCAs Most scintillation detectors now use MCAs to define and discern gamma emission to define and discern gamma emission

spectrumsspectrums


The MCA can select digital channels for The MCA can select digital channels for analysis of digitized counts that represent analysis of digitized counts that represent

incident photons energies upon the incident photons energies upon the scintillation detectorscintillation detector


The MCA can count from selected multiple The MCA can count from selected multiple channels or can collect a count from all channels or can collect a count from all

channels.channels.

Multi Channel AnalyzerMulti Channel Analyzer

Calibration—HV should be set so that the same Calibration—HV should be set so that the same energy level (662 keV for Cs-137) is assigned to energy level (662 keV for Cs-137) is assigned to an acceptable range of channels or data bins.an acceptable range of channels or data bins.

Frequent changes to HV to adjust the energy level Frequent changes to HV to adjust the energy level to the channels means that something is amiss.to the channels means that something is amiss. HV supplyHV supply Optic couplingOptic coupling Hermetic sealHermetic seal

Correction factors are applied to channels to Correction factors are applied to channels to relate to other energy levels.relate to other energy levels.

Multi-Channel AnalyzerMulti-Channel Analyzer

Fig. 2-10 from Prekeges:Fig. 2-10 from Prekeges:

Figure 09A: Scintillation detector probe geometryFigure 09A: Scintillation detector probe geometry

Figure 09B: Scintillation detector well geometryFigure 09B: Scintillation detector well geometry

Thyroid Probe and Well CounterThyroid Probe and Well CounterQuality ControlQuality Control

Daily:Daily:

ConstancyConstancy

Calibration of photopeakCalibration of photopeak

Quarterly:Quarterly:

Chi-SquareChi-Square

Energy Resolution (Sodee)Energy Resolution (Sodee)

Linearity (Sodee)Linearity (Sodee)

Confirm Windows (Sodee)Confirm Windows (Sodee)

Annually:Annually:

Efficiency (Prekeges)Efficiency (Prekeges)Paul Early, D. Bruce Sodee, Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear MedicinePrinciples and Practice of Nuclear Medicine, 2nd Ed., (St. , 2nd Ed., (St.

Louis: Mosby 1995), pg. 149.Louis: Mosby 1995), pg. 149.

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part b: non-imaging scintillation detectors unit ii: nuclear medicine measuring devices lectures 7...

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