Unit III Nonimaging Scintillation Detectors Lecture 2Associated Electronics

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

Objectives l l 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 4 X to 5 X Matches impedance to the system’s circuitry Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques , 5 th Ed. (St. Louis: Mosby 2004) pg 60.

Next the signal goes from the preamp to the amp Amplifier Pulse undergoes: 1. Pulse Shaping 2. Linear Amplification (Amplified 1 to 100 X Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques , 5 th Ed. (St. Louis: Mosby 2004) pg 60. by Gain control)

Pulse Shaping Sorenson, p. 88

Pulse Shaping: RC Circuits and Noise Elimination

Calibration Figure 2 -7 Calibration of a scintillation detector. Figure 2 -7 a shows the highvoltage (or calibration) knob with four settings corresponding to the four energy spectra in Figure 2 -7 c– 2 -7 f. Figure 2 -7 b shows the detector count rate at each knob setting, with the letters corresponding to the pulse-height spectra shown in Figures 2 -7 c– 2 -7 f. 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 -7 e, when the number of counts registered in the LLD-ULD window reaches a maximum. In Figure 2 -7 f, 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 GAIN 2 1 40 5 30 10 20 10 0 25 50 75 100 125 150 175 200 225 250 275 300

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

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

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

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

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

225 120 ke. V 30 ke. V 80 ke. V 140 ke. V 180 ke. V We’d get a random mixed bag of photons. 150 125 100 75 50 Pulse Voltage 175 200 230 ke. V Time

225 200 175 150 30 ke. V 80 ke. V 140 ke. V 180 ke. V For the sake of our example, we’ll say we’re detecting only photons of the six above energies. 75 100 125 120 ke. V 50 Pulse Voltage 230 ke. V Time

225 120 ke. V 30 ke. V 80 ke. V 140 ke. V 180 ke. V We’ll let them add up over time. 150 125 100 75 50 Pulse Voltage 175 200 230 ke. V Time

We’ve stopped our detector, and now we’ll tally up each type of photon detected. 30 ke. V 6 80 ke. V 10 120 ke. V 15 140 ke. V 40 180 ke. V 10 230 ke. V 2 Here are our totals. 0 25 50 75 100 125 150 175 200 Volts 225 250 275 300

Next, we’ll stack up our tally count for each photon on our voltage scale according to its calibrated spot on the scale. 6 80 ke. V 10 120 ke. V 140 ke. V 15 40 180 ke. V 10 230 ke. V 2 40 30 Pulses 30 ke. V 20 10 0 25 50 75 100 125 150 175 200 Volts 225 250 275 300

We can see that we have collected mostly 140 ke. V photons— the type of gamma emission associated with Tc-99 m. 6 80 ke. V 10 120 ke. V 140 ke. V 15 40 180 ke. V 10 230 ke. V 2 This is our photopeak because it most repeatedly generated the level of scintillation light that resulted in this voltage pulse level 40 30 Pulses 30 ke. V 20 10 0 25 50 75 100 125 150 175 200 225 250 Volts If our source is indeed Tc-99 m, why are we getting the other photon energy readings? 275 300

Some explanations for these other gammas detected are … 6 80 ke. V 10 120 ke. V 140 ke. V 15 40 180 ke. V 10 230 ke. V 2 40 Counts 30 ke. V Compton Scattered photons Backscattered photons Partially detected photons 30 20 Extra electrons emitted from photocathode 10 0 25 50 75 100 Primary gamma photons 125 150 175 200 225 Two gamma photons detected simultaneously 250 275 Volts The 140 ke. V 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? 300

225 200 175 150 30 ke. V 80 ke. V 140 ke. V 180 ke. V We’ll go back to our collection of pulses over time to see how we can distinguish the 140 ke. V pulses from the other pulses representing detected photons of different energies. 75 100 125 120 ke. V 50 Pulse Voltage 230 ke. V Time

Lower Level Discriminator (LLD) l 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 ke. V), that is, we’re going to electronically tell our system NOT to accept any pulses that do not reach 126 volts in height (126 ke. V).

225 120 ke. V 30 ke. V 80 ke. V 140 ke. V 180 ke. V 200 230 ke. V 75 100 125 150 (at 126 Volts) 50 Pulse Voltage 175 Here’s our LLD line Time

225 120 ke. V 30 ke. V 80 ke. V 140 ke. V 180 ke. V 200 230 ke. V 150 125 100 75 50 Pulse Voltage 175 And here’s its effects Time All pulses less than 126 volts are not seen (counted)

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

225 200 30 ke. V 80 ke. V 140 ke. V 180 ke. V 1 We get nine pulses counted 7 3 2 4 5 6 8 9 75 100 125 150 175 120 ke. V 50 Pulse Voltage 230 ke. V Time But Wait! Some of these are not 140 volt pulses!

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

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

225 120 ke. V 30 ke. V 80 ke. V 140 ke. V 180 ke. V 200 230 ke. V 150 125 100 75 50 Pulse Voltage 175 Here’s the ULD threshold Time

225 120 ke. V 30 ke. V 80 ke. V 140 ke. V 180 ke. V 200 230 ke. V 150 125 75 100 What the…? 50 Pulse Voltage 175 And here’s its effects 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 , 3 d Ed. , (Philadelphia: Saunders (Elsevier) 2003), pg. 116.

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

One copy of pulses goes to the ULD Anticoincidence logic Pulses from Amplifier LLD One copy of pulses goes to the LLD. Output

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

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

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 ke. V) pulse has no copy and survives LLD

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

120 ke. V 30 ke. V 80 ke. V 140 ke. V 180 ke. V In Effect, our Coincidence Circuit enables us to cancel out our unwanted oversized pulses. 150 125 100 75 50 Pulse Voltage 175 200 225 230 ke. V Time

225 120 ke. V 30 ke. V 80 ke. V 140 ke. V 180 ke. V 75 100 125 150 And get only the desired pulses. 50 Pulse Voltage 175 200 230 ke. V Time

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

225 120 ke. V 30 ke. V 80 ke. V 140 ke. V 180 ke. V 200 230 ke. V 150 125 100 75 50 Pulse Voltage 175 To this. Time

We end up with an energy “window” that discriminates against photon energies that are from indirect sources. 6 80 ke. V 10 120 ke. V 140 ke. V 15 40 Counts 30 ke. V 30 (LLD) (ULD) 20 40 180 ke. V 10 230 ke. V 2 10 0 25 50 75 100 125 150 175 200 225 Volts This is a Single Channel Analyzer 250 275 300

l Fig 2 -6 from your Prekeges Text

This shows a 20% energy (window) around the 140 ke. V photopeak. 6 80 ke. V 10 120 ke. V 140 ke. V 15 40 Counts 30 ke. V 30 (LLD) (ULD) 20 40 180 ke. V 10 230 ke. V 2 10 0 25 50 75 100 125 150 175 200 225 Volts This is a Single Channel Analyzer 250 275 300

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

First… Find point on scale that correlates to your peak counts. Counts (X 1000) Full Width at Half Maximum (FWHM) 40 30 20 140 V 10 0 25 50 75 100 125 150 175 200 Volts 225 250 275 300

Full Width at Half Maximum (FWHM) Next… Find the maximum counts of the spectrum. 42, 000 Counts 40 30 20 140 V 10 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 21, 000 Counts (1/2 Maximum) 140 V 20 10 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 21, 000 Counts (1/2 Maximum) 20 158 V 10 0 25 50 75 100 125 126 V 150 175 200 225 250 275 300

Full Width at Half Maximum (FWHM) 40 30 21, 000 Counts (1/2 Maximum) 20 The FWHM is based on the following: 158 V 10 0 25 50 75 100 125 150 175 200 126 V % Resolution = Upper Scale Reading – Lower Scale Reading X 100 Photopeak scale reading 225 250 275 300

Full Width at Half Maximum (FWHM) 40 30 For our system, our calculations would be as follows: 21, 000 Counts (1/2 Maximum) 20 158 V 10 0 25 50 75 100 125 150 175 200 126 V % Resolution = 158 V - 126 V X 100 = 23 % 140 V 225 250 275 300

Full Width at Half Maximum (FWHM) A FWHM of 23 % actually stinks. 7 or 8 % would be a more desirable value. 40 30 21, 000 Counts (1/2 Maximum) 20 158 V 10 This means our photopeak should be much slimmer. 0 25 50 75 100 125 150 175 200 126 V % Resolution = 158 V - 126 V X 100 = 23 % Our system likely needs repair. 140 V 225 250 275 300

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

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

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

Multi Channel Analyzer l l Calibration—HV should be set so that the same energy level (662 ke. V 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. l l HV supply Optic coupling Hermetic seal Correction factors are applied to channels to relate to other energy levels.

Multi-Channel Analyzer l Fig. 2 -10 from Prekeges:

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