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

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

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.

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 Sorenson, p. 88

Pulse Shaping: RC Circuits and Noise Elimination

© 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. 2001. Reston, VA: Society of Nuclear Medicine. Reprinted with permission. © 2010 Jones and Bartlett Publishers, LLC

Let’s apply our gain control to the real energy spectrum 1 2 5 10 GAIN 1 2 5 10 GAIN 1 2 5 10 GAIN 1 2 5 10 40 30 20 10 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 40 30 20 10 0 25 50 75 100 125 150 175 200 225 250 275 300

GAIN 1 2 5 10 GAIN 1 2 5 10 40 30 20 10 0 25 50 75 100 125 150 175 200 225 250 275 300

GAIN 1 2 5 10 40 30 20 10 0 25 50 75 100 125 150 175 200 225 250 275 300

GAIN 1 2 5 10 40 30 20 10 0 25 50 75 100 125 150 175 200 225 250 275 300

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.”

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. 50 75 100 125 150 175 200 225 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. 50 75 100 125 150 175 200 225 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. 50 75 100 125 150 175 200 225 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 0 25 50 75 100 125 150 175 200 225 250 275 300 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 Pulses 140 keV 40 180 keV 10 230 keV 2 0 25 50 75 100 125 150 175 200 225 250 275 300 Volts

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

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. 50 75 100 125 150 175 200 225 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 50 75 100 125 150 175 200 225 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 50 75 100 125 150 175 200 225 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 50 75 100 125 150 175 200 225 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 50 75 100 125 150 175 200 225 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 50 75 100 125 150 175 200 225 Pulse Voltage What the…? Time Is this what we wanted? Are these the counts we need? How can we use this????

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

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.

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.

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

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

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. 50 75 100 125 150 175 200 225 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. 50 75 100 125 150 175 200 225 Pulse Voltage Time

We go from this…. 230 keV 120 keV 30 keV 80 keV 140 keV 180 keV 50 75 100 125 150 175 200 225 Pulse Voltage Time

To this. 230 keV 120 keV 30 keV 80 keV 140 keV 180 keV Pulse Voltage 50 75 100 125 150 175 200 225 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 0 25 50 75 100 125 150 175 200 225 250 275 300 Volts This is a Single Channel Analyzer

Fig 2-6 from your Prekeges Text

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 0 25 50 75 100 125 150 175 200 225 250 275 300 Volts This is a Single Channel Analyzer

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

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

Full Width at Half Maximum (FWHM) 42,000 Counts Next… Find the maximum counts of the spectrum. 40 30 20 10 140 V 0 25 50 75 100 125 150 175 200 225 250 275 300

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

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 0 25 50 75 100 125 150 175 200 225 250 275 300 126 V

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: 0 25 50 75 100 125 150 175 200 225 250 275 300 126 V % Resolution = Upper Scale Reading – Lower Scale Reading X 100 Photopeak scale reading

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 0 25 50 75 100 125 150 175 200 225 250 275 300 126 V % Resolution = 158 V - 126 V X 100 = 23 % 140 V

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 0 25 50 75 100 125 150 175 200 225 250 275 300 126 V % Resolution = 158 V - 126 V X 100 = 23 % 140 V

Full Width at Half Maximum (FWHM) A highly resolute photopeak (with a low FWHM) should be skinny. 40 30 20 10 0 25 50 75 100 125 150 175 200 225 250 275 300

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

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.

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

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

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. 2-10 from Prekeges:

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