1. Lecture 2—Associated Electronics and Energy Spectrum
Unit III: Non-imaging Scintillation Detectors.
Lecture 2—Associated Electronics and Energy Spectrum
CLRS 321 Nuclear Medicine Physics and Instrumentation I

2. Objectives
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

3. 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.

4. 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.

5. Pulse Shaping
Sorenson, p. 88

6. Pulse Shaping: RC Circuits and Noise Elimination

7. © 2010 Jones and Bartlett Publishers, LLC
Calibration
Figure 2-7 Calibration of a scintillation detector. Figure 2-7a shows the high-voltage (or calibration) knob with four settings corresponding to the four
energy spectra in Figure 2-7c–2-7f. Figure 2-7b shows the detector count rate at each knob setting, with the letters corresponding to the pulse-height spectra shown in Figures 2-7c–2-7f. The operator-determined LLD and ULD are shown superimposed on the energy spectra. As the high voltage is increased, the sizes of all pulses increase, and the energy spectrum is “stretched out” along the X-axis. The detector is correctly calibrated in Figure 2-7e, when the number of counts registered in the LLD-ULD window reaches a maximum. In Figure 2-7f, the high voltage has increased the pulse size beyond the center of the window; note that the measured count value does not decrease all the way to zero. Adapted from: Christian P. “Radiation Detection.” In Fahey FH, Harkness BA, eds. Basic Science of Nuclear Medicine CD-ROM Reston, VA: Society of Nuclear Medicine. Reprinted with permission.
© 2010 Jones and Bartlett Publishers, LLC

8. Let’s apply our gain control to the real energy spectrum1 2 5 10
GAIN 1 2 5 10
GAIN 1 2 5 10
GAIN 1 2 5 10 40 30 20 10

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

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

11. GAIN 1 2 5 10 40 30 20 10

12. GAIN 1 2 5 10 40 30 20 10

13. Single Channel Analyzer
Pulse Height Analysis
Discriminating between voltage pulse heights in order to get the pulse heights that best represent the source’s level of energy.
After analysis, each accepted pulse is converted to equal value and becomes a “count.”

14. 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

15. 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

16. 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

17. 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

18. 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
Pulses
140 keV 40
180 keV 10
230 keV 2
Volts

19. 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 level
30 keV 6 40 30 20 10
80 keV 10
120 keV 15
Pulses
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?

20. 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?

21. 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

22. 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).

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

24. 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)

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

26. 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

27. 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.

28. 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.

29. 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

30. 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????

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

32. 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

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

34. Anti-coincidence logic
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.

35. 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

36. 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

37. Anti-coincidence logic
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

38. 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

39. 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

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

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

42. 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

43. Fig 2-6 from your Prekeges Text

44. 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

45. 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

46. Full Width at Half Maximum (FWHM)
First…
Find point on scale that correlates to your peak counts. 40 30 20 10
Counts (X 1000)
140 V
Volts

47. Full Width at Half Maximum (FWHM)
42,000 Counts
Next…
Find the maximum counts of the spectrum. 40 30 20 10
140 V

48. Full Width at Half Maximum (FWHM)
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

49. Full Width at Half Maximum (FWHM)
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

50. Full Width at Half Maximum (FWHM)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

51. Full Width at Half Maximum (FWHM)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

52. Full Width at Half Maximum (FWHM)
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

53. Full Width at Half Maximum (FWHM)
A highly resolute photopeak (with a low FWHM) should be skinny. 40 30 20 10

54. 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

55. MultiChannel Analyzer
Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 119.

56. Like SCAs, gamma photons generate a number of pulse sizes along a voltage scale or “channels.”
Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 119.

57. 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.

58. 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.

59. Most scintillation detectors now use MCAs to define and discern gamma emission spectrums
Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 119.

60. 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.

61. The MCA can count from selected multiple channels or can collect a count from all channels.
Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 119.

62. 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.

63. Multi-Channel Analyzer
Fig from Prekeges:

64. Next time: Applications and QC Concepts
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