Scintillation Detectors

Scintillation Detectors
Part B: Non-imaging Scintillation Detectors Unit II: Nuclear Medicine Measuring Devices Lectures 7 & 8

Objectives Define scintillation
Describe the structure & purpose of a NaI (Tl) crystal and the crystal’s proportional response to deposited gamma energy Describe the components of a photomultiplier tube and their function Discuss the purpose of other associated electronics within the scintillation detector Describe the calibration process for single and multi-channel analyzers Discuss peak broadening and the determination of a percent energy window Calculate percent energy resolution from FWHM and its importance in quality control Describe quality control tests for scintillation detectors and their required frequency

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

Excited Electrons Gamma Photon

Visible light (350-500nm λ) Returning electrons

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NaI at Room Temperature
Excited Electrons Gamma Photon

Excited Electrons Don’t Fluoresce

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

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

NaI (Tl) Crystal (hermetically sealed in reflective material)
Visible light Gamma Photon Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) p 53.

For each keV of gamma photon energy absorbed, 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 Visible light produced is 325 to 550 nm wavelength 140 keV Gamma Photon

Important Magic to Remember
The 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 Tube

Photomultiplier tubes coupled to NaI(Tl) Crystals

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

CsSb Photocathode NaI (Tl) Crystal Gamma Photon Visible Light
Emitted electrons from photocathode Optical Window (transparent material)

Quantum Efficiency: A measure of how well a photoemissive material emits electrons when exposed to various wavelengths of light The above graph shows this photoemissive material is most productive at 400 nm—about the same wavelength created by NaI (Tl) scintillation Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 102.

Proportionality Maintained
Note: This is the least efficient phase of the transfer For every 3 – 5 visible light photons reaching the photocathode, 1 electron is emitted

Photomultiplier Tube (PMT) Construction
Focusing Grid – Guides in Electrons Dynodes – Usually positively charged photoemissive coated electrodes with increasing voltages Anode – end positively charged High voltage power supply – needed to increase incrementally the potential difference between dynodes

(Still small – 1 Amp = 1 C/s; 1 C = 6.3 X 1018 electrons)
Increased voltages between dynodes : means increased KE of electrons between dynodes Increased KE of electrons mean that more electrons are knocked off at each dynode (3X to 6X at each) At 6X each with 10 dynodes means 610 electrons produced (about 60 million) (Still small – 1 Amp = 1 C/s; C = 6.3 X 1018 electrons)

Millions of electrons are produced by the dynodes in direct response to the initial few electrons emitted by the photocathode in direct response to the visible light photons emitted by the NaI(Tl) crystal in direct response to the energy level of the gamma photon interacting with the crystal End result: Increased gamma energy : means increased electrons reaching the anode at the end of the tube This means that the height of the electric pulse created by the millions of electrons at the anode will be an indicator of gamma energy level

Inefficiencies from the transfer of energy
The proportionality of the system is approximate and contains random statistical error See Table 2-1 (p. 21) Note that Tc-99m gamma produces less “information carriers” than the higher energy gamma from Cs-137 Therefore, Cs-137 has better counting statistics and less variation in the heights of its pulses

From the PMT the signal goes from the anode to the preamp Preamplifier Increases pulse 4X to 5X Matches impedance to the system’s circuitry Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) pg 60. Proportionality Maintained

Next the signal goes from the preamp to the amp
Amplifier Pulse undergoes: 1. Pulse Shaping Linear Amplification (Amplified 1 to 100 X by Gain control) Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) pg 60.

Pulse Shaping (Amplifier)
From Sodee: “Change to voltage converter that increases the signal-to-noise ratio.” Makes “splat” of voltage pulse into a “pop.” Increases count rate capability of the system Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 138.

Pulse Shaping Sorenson, p. 88

Pulse Shaping: RC Circuits and Noise Elimination

Linear Amplification (Amplifier)
Each of these pulses represents a different gamma photon energy detected by the NaI (Tl) crystal. We want to preserve this proportionality in the pulses for it represents the proportional differences in the gamma energies detected. But we need a stronger signal to work with. Pulse Voltage Time

The linear amplifier amplifies all the pulses proportionally. Pulse Voltage Time

Proportionality Maintained Pulse Voltage Time

From Sodee… “The pulse height is directly proportional to the energy of the incident gamma photon.” Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) pg 60. Prekeges, J.

The linear amplifier amplifies all the pulses proportionally. Pulse Voltage Time

We can designate (calibrate) the height of our pulses by
Gain Control Pulse Voltage GAIN 1 2 5 10 Time

A new set of pulses from photons with the following energies…
230 keV 120 keV 30 keV 80 keV 140 keV 180 keV Pulse Voltage Time

230 keV 120 keV 30 keV 80 keV 140 keV 180 keV (We have magic eyes and know what these energies are before our detecting system does.) Pulse Voltage Time

We introduce a linear voltage scale…
230 keV 120 keV 30 keV 80 keV 140 keV 180 keV For Comparision, We introduce a linear voltage scale… Pulse Voltage Volts Time

GAIN 1 2 5 10 230 keV 120 keV 30 keV 80 keV 140 keV 180 keV We introduce our gain control voltage of our amplifier to increase the voltage pulse heights associated with photons of the given energies Pulse Voltage Volts Time

At gain setting “1” we see the pulse voltages below
2 5 10 230 keV 120 keV 30 keV 80 keV 140 keV 180 keV At gain setting “1” we see the pulse voltages below Pulse Voltage And they appear on our voltage scale as follows… (Photon energy represented by color points only) Volts Time

Changing the gain to “2” doubles the size of our pulses.
1 2 5 10 GAIN 1 2 5 10 230 keV 120 keV 30 keV 80 keV 140 keV 180 keV Changing the gain to “2” doubles the size of our pulses. Pulse Voltage And Shifts our energy points lineup to the right Volts Time

GAIN 1 2 5 10 GAIN 1 2 5 10 230 keV 120 keV 30 keV 80 keV 140 keV 180 keV To amplify our pulses even more, we’ll need to change the scale of our pulse voltages so they will fit on our slide. Pulse Voltage Volts Time

GAIN 1 2 5 10 GAIN 1 2 5 10 230 keV 120 keV 30 keV 80 keV 140 keV 180 keV Now that our scale’s adjusted on our graph, we can really start cranking up the gain and see its effects. Pulse Voltage Volts Time

Let’s see what happens when we crank this puppy up to “5
Let’s see what happens when we crank this puppy up to “5.” (5 X the gain!) GAIN 1 2 5 10

GAIN 1 2 5 10 GAIN 1 2 5 10 GAIN 1 2 5 10 230 keV 120 keV 30 keV 80 keV 140 keV 180 keV Pulse Voltage (Notice how these photon energy points spread to the right even more.) Volts Time

Let’s crank the gain up to “10.” (10 X amplified!!)
2 5 10

GAIN 1 2 5 10 GAIN 1 2 5 10 GAIN 1 2 5 10 GAIN 1 2 5 10 230 keV 120 keV 30 keV 80 keV 140 keV 180 keV Pulse Voltage (-Notice how these points representing the different photon energies now line up with our voltage scale?) Volts Time

GAIN 1 2 5 10 GAIN 1 2 5 10 GAIN 1 2 5 10 GAIN 1 2 5 10 230 keV 120 keV 30 keV 80 keV 140 keV 180 keV With our gain at 10, we’ve made our photon energies register to voltage numbers that equal the keV for each photon. Volts 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.

Special note: not all gain scales read the same.
Some are “inverse gain” scales and can be considered as representing the denominators of fractions. GAIN 1 2 5 10 32 16 8 4 0.5 0.25 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

230 keV 120 keV 30 keV 80 keV 140 keV 180 keV 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). Pulse Voltage Volts Time

What if we let the clock run and keep on detecting more photons?
230 keV 120 keV 30 keV 80 keV 140 keV 180 keV Now, What if we let the clock run and keep on detecting more photons? Pulse Voltage Time

We’d get a random mixed bag of photons.
230 keV 120 keV 30 keV 80 keV 140 keV 180 keV We’d get a random mixed bag of photons. Pulse Voltage Time

230 keV 120 keV 30 keV 80 keV 140 keV 180 keV For the sake of our example, we’ll say we’re detecting only photons of the six above energies. Pulse Voltage Time

We’ll let them add up over time.
230 keV 120 keV 30 keV 80 keV 140 keV 180 keV We’ll let them add up over time. Pulse Voltage Time

We’ve stopped our detector, and now we’ll tally up each type of photon detected.
30 keV 6 80 keV 10 Here are our totals. 120 keV 15 140 keV 40 180 keV 10 230 keV 2 Volts

Next, we’ll stack up our tally count for each photon on our voltage scale according to its calibrated spot on the scale. 30 keV 6 40 30 20 10 80 keV 10 120 keV 15 Counts 140 keV 40 180 keV 10 230 keV 2 Volts

The colored bars do not represent pulse heights here…
30 keV 6 40 30 20 10 80 keV 10 120 keV 15 Counts 140 keV 40 180 keV 10 230 keV 2 Volts But rather the total amount of each type of energy detected.

We can see that we have collected mostly 140 keV photons—the type of gamma emission associated with Tc-99m. This is our photopeak because it most repeatedly generated the level of scintillation light that resulted in this voltage pulse 30 keV 6 40 30 20 10 80 keV 10 120 keV 15 Counts 140 keV 40 180 keV 10 230 keV 2 Volts If our source is indeed Tc-99m, why are we getting the other photon energy readings?

Some explanations for these other gammas detected are …
Compton Scattered photons Primary gamma photons 30 keV 6 40 30 20 10 Back-scattered photons 80 keV 10 Extra electrons emitted from photo-cathode Partially detected photons 120 keV 15 Two gamma photons detected simulta-neously Counts 140 keV 40 180 keV 10 230 keV 2 Volts 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 scatter and perfect detectors, we would get a gamma “pulse-height” spectrum that looked like… 40 30 20 10 Counts Volts

Figure 03: Peak broadening as seen with scintillation detectors

Answer: Pulse Height Analysis
How do we extract the pulses that represent the true gamma energy of a radionuclide? Answer: Pulse Height Analysis Pulse Voltage Time

Pulse Height Analyzer According to Sodee text:
“The pulse height analyzer is an electronic device that enables the operator to select pulses of a certain height and to reject all pulses of a different height.” Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 141.

Single Channel Analyzer (SCA)
A pulse height analyzer that detects only one set of pulses. We will use a Single Channel Analyzer example to demonstrate how we can separate our 140 keV photons from the photons of other energies.

230 keV 120 keV 30 keV 80 keV 140 keV 180 keV We’ll go back to our collection of pulses over time to see how we can distinguish the 140 keV pulses from the other pulses representing detected photons of different energies. Pulse Voltage Time

Lower Level Discriminator (LLD)
The electronically and arbitrarily established threshold that a pulse much reach in order to be counted as detected. For our example, we’ll establish a threshold of 10% below the 140 volt pulse (140 keV), that is, we’re going to electronically tell our system NOT to accept any pulses that do not reach 126 volts in height (126 keV).

Here’s our LLD line (at 126 Volts) 230 keV 120 keV 30 keV 80 keV
Pulse Voltage Time

All pulses less than 126 volts are not seen (counted)
230 keV 120 keV 30 keV 80 keV 140 keV 180 keV And here’s its effects Pulse Voltage Time All pulses less than 126 volts are not seen (counted)

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

We get nine pulses counted
230 keV 120 keV 30 keV 80 keV 140 keV 180 keV 7 1 3 8 4 5 6 2 9 Pulse Voltage But Wait! Some of these are not 140 volt pulses! Time

Upper Level Discriminator (ULD)
An Upper Level Discriminator is just a second Lower Level Discriminator. It also has an electronic threshold that will only recognize pulses of an arbitrarily selected voltage height. The ULD threshold is set above the LLD threshold.

Let’s set our ULD for 10% above our desired 140 volt (140 keV) pulse height.
This would come to 154 volts (154 keV). This means all pulses BELOW 154 volts would be NOT be counted.

Here’s the ULD threshold
230 keV 120 keV 30 keV 80 keV 140 keV 180 keV Here’s the ULD threshold Pulse Voltage Time

What the…? And here’s its effects
230 keV 120 keV 30 keV 80 keV 140 keV 180 keV And here’s its effects Pulse Voltage What the…? Time Is this what we wanted? Are these the counts we need? How can we use this????

Anticoincidence Logic Circuit

Anti-coincidence logic
All of our pulses come in from the amplifier at their proportional voltage heights ULD Anti-coincidence logic Output Pulses from Amplifier LLD

One copy of pulses goes to the ULD.
Anti-coincidence logic Output Pulses from Amplifier LLD One copy of pulses goes to the LLD.

Only the 180 & 230 V pulse copies cross the ULD threshold and are accepted ULD Anti-coincidence logic Output Pulses from Amplifier LLD Only the 140, 180, & 230 V pulse copies cross the LLD threshold and are accepted.

In the anticoincidence logic circuit the copies of the 180 & 230 V pulses arrive at the same time (for they were generated at the same time.) ULD Output Pulses from Amplifier The copy of the 140 V pulse arrives by itself because its copy broke the LLD threshold but not the ULD threshold. LLD

The single 140 V (140 keV) pulse has no copy and survives
Because the 180 and 230 V pulse copies arrived at the same time (they were generated at the same time) the coincidence logic cancels them out. ULD Output Pulses from Amplifier The single 140 V (140 keV) pulse has no copy and survives LLD

ULD Output Anti-coincidence logic Pulses from Amplifier From all the pulses we collect one “count” of a 140 V pulse (140 kev photon). LLD

230 keV 120 keV 30 keV 80 keV 140 keV 180 keV In Effect, our Coincidence Circuit enables us to cancel out our unwanted oversized pulses. Pulse Voltage Time

And get only the desired pulses.
230 keV 120 keV 30 keV 80 keV 140 keV 180 keV And get only the desired pulses. Pulse Voltage Time

We go from this…. 230 keV 120 keV 30 keV 80 keV 140 keV 180 keV
Pulse Voltage Time

To this. 230 keV 120 keV 30 keV 80 keV 140 keV 180 keV Pulse Voltage
Pulse Voltage Time

This is a Single Channel Analyzer
We end up with an energy “window” that discriminates against photon energies that are from indirect sources. 30 keV 6 40 30 20 10 80 keV 10 (LLD) (ULD) 120 keV 15 Counts 140 keV 40 180 keV 10 230 keV 2 Volts This is a Single Channel Analyzer

Fig 2-6 from your Prekeges Text

In reality, we add up the counts from the different photon energies and get something like this… 30 keV 6 40 30 20 10 80 keV 10 (LLD) (ULD) 120 keV 15 Counts 140 keV 40 180 keV 10 230 keV 2 Volts This is a Single Channel Analyzer

This shows a 20% energy (window) around the 140 keV photopeak. 30 keV 6 40 30 20 10 80 keV 10 (LLD) (ULD) 120 keV 15 Counts 140 keV 40 180 keV 10 230 keV 2 Volts This is a Single Channel Analyzer

Let’s apply our gain control to the real energy spectrum
1 2 5 10 40 30 20 10

GAIN 1 2 5 10 40 30 20 10

GAIN 1 2 5 10 GAIN 1 2 5 10 40 30 20 10

GAIN 1 2 5 10 GAIN 1 2 5 10 GAIN 1 2 5 10 40 30 20 10

GAIN 1 2 5 10 GAIN 1 2 5 10 GAIN 1 2 5 10 GAIN 1 2 5 10 40 30 20 10

Full Width at Half Maximum (FWHM)
A measurement of energy resolution—a means of showing how well your detector can discriminate energy differences. 40 30 20 10

First… Find point on scale that correlates to your peak counts. 40 30 20 10 Counts (X 1000) 140 V Volts

42,000 Counts Next… Find the maximum counts of the spectrum. 40 30 20 10 140 V

Then… Figure out where ½ the maximum counts intersects the peak of the spectrum 40 30 20 10 21,000 Counts (1/2 Maximum) 140 V

Now… Determine how the full width of the photopeak at ½ maximum counts translates to the scale below 40 30 20 10 21,000 Counts (1/2 Maximum) 158 V 126 V

40 30 20 10 21,000 Counts (1/2 Maximum) 158 V The FWHM is based on the following: 126 V % Resolution = Upper Scale Reading – Lower Scale Reading X Photopeak scale reading

40 30 20 10 21,000 Counts (1/2 Maximum) For our system, our calculations would be as follows: 158 V 126 V % Resolution = 158 V V X 100 = 23 % 140 V

A FWHM of 23 % actually stinks. 7 or 8 % would be a more desirable value. This means our photopeak should be much slimmer. Our system likely needs repair. 40 30 20 10 21,000 Counts (1/2 Maximum) 158 V 126 V % Resolution = 158 V V X 100 = 23 % 140 V

A highly resolute photopeak (with a low FWHM) should be skinny. 40 30 20 10

MultiChannel Analyzer (MCA)
A “digital” means to collect and record counts along a set of voltage channels Uses Analogue to Digital Conversion (ADC) to discern pulse sizes and assign them to memory locations Greatly increases the flexibility of selecting and measuring counts from various energy sources

MultiChannel Analyzer

Like SCAs, gamma photons generate a number of pulse sizes along a voltage scale or “channels.”

These pulse sizes are converted to a discrete value based on the channel in which they fall. This is called Analogue to Digital Conversion. Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 119.

In other words, there is a rounding off of pulse sizes so that they equal a digitized amount, such as 2.8 and 3.2 are assigned to digital value “3.” Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 119.

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

The MCA can select digital channels for analysis of digitized counts that represent incident photons energies upon the scintillation detector Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 119.

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

Multi Channel Analyzer
Calibration—HV should be set so that the same energy level (662 keV for Cs-137) is assigned to an acceptable range of channels or data bins. Frequent changes to HV to adjust the energy level to the channels means that something is amiss. HV supply Optic coupling Hermetic seal Correction factors are applied to channels to relate to other energy levels.

Multi-Channel Analyzer
Fig from Prekeges:

Figure 09A: Scintillation detector probe geometry

Figure 09B: Scintillation detector well geometry

Thyroid Probe and Well Counter
Quality Control Daily: Constancy Calibration of photopeak Quarterly: Chi-Square Energy Resolution (Sodee) Linearity (Sodee) Confirm Windows (Sodee) Annually: Efficiency (Prekeges) Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 149.

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