1. CLRS 321 Nuclear Medicine Physics & Instrumentation I
Non-imaging Scintillation Detectors
Unit III: Lectures 3 &4
Applications and QC Concepts
CLRS 321 Nuclear Medicine Physics & Instrumentation I

2. Objectives Discuss the uses of non-imaging scintillation detectors.
Describe collimation techniques for non-imaging scintillation detectors.
Describe quality control tests for scintillation detectors and their required frequency

3. Figure 09A: Scintillation detector probe geometry

4. Figure 09B: Scintillation detector well geometry

5. 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.

6. Daily Constancy

7. Energy Resolution (quarterly)
Measures FWHM (percent energy resolution)
Increasing value means problems…
Electronic noise
Decoupled crystal (from PMT)
Yellow or cracked crystal
Often part of daily calibration (as with our machine)
Cs-137 should be less than 10% FWHM

8. Chi-Square Test (quarterly)
Does your detector produce results that agree with the random statistical distribution associated with radioactive decay (Poisson distribution)?
10 measurements of a radioactive source (usually Cs-137) are made for the same amount of time
Ideally the time for each measurement should produce approximately 10,000 counts for each measurement
Is there the predicted statistical variation?
Too much variation—inconsistent results
Too little variation—a systematic problem exists in the system

9. Chi-Square
A test to see if your system is compliant with the laws of nuclear radiation statistics.
A nuclear counting system should detect radiation events within an expected range of variance consistent with a Poisson Distribution
Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pp. 185&186.

10. Chi Square Example
X2 results are then compared to a probability chart based on the degrees of freedom
(Which is N-1)
We will do this in the last instrumentation lab. --

11. Chi Square Example Use Probability Table (From Sodee)
p < 0.1 = too much variability
p > 0.9 = not enough variability
Use Probability Table (From Sodee)
Our degrees of Freedom are 10-1 or 9.
Our results need to fall between the probabilities of 0.90 and 0.10 (remember, our result cannot be significant [0.05] on either end).
Our Chi-square of would pass.

13. Energy Linearity Testing (quarterly)
A calibrated NaI (Tl) detector may have problems accurately representing lower (<200 keV) energies
Testing a radionuclide with a wide range of energies (such as Eu-152) can see if energies are inaccurately being represented at any particular level.

14. Efficiency Testing
To determine the efficiency factor—how the count rate actually represents disintegrations per unit of time.
(how much of the activity present are we actually detecting?)

15. Calculating Efficiency
Counts/unit time
X 100
Disintegrations/unit time x Mean number/disintegration
Things to consider…
The raw definition of activity is disintegrations per unit of time, so usually you need to convert your activity to disintegrations
Often, your source has decayed, so you will need to calculate the activity when you conduct your efficiency calculation
The mean number/disintegration is derived from the percent abundance, which usually can be found in tables with information about the radionuclide
This is the percent of the disintegrations that are giving off the gamma photons you are measuring (and therefore, you are not measuring ALL of the disintegrations going on)
The percent is converted to decimal amount (e.g. 75% = 0.75)

16. Defininition: 1 μCi = 2.22 x 106 dpm
Efficiency Example
1.2 μCi I-131 Assayed 8 days ago
Counted at 172,116 cpm (1.72 X 105)
364 keV abundance in I-131 = 83.8 %
Defininition: 1 μCi = 2.22 x 106 dpm
(1st you’d have to decay calculate the 1.1 µCi. Conveniently the time elapsed is 1 half-life, so in this case, you have half the dose, or 0.6 µCi.)
x 105 cpm
X 100
= 15.4%
(0.6 μCi)(2.22 x 106 dpm/µCi)(0.838)

17. Finding Actual Counts using Efficiency as a Factor
From the previous efficiency example:
Efficiency = 15.4%
(Efficiency factor would be 0.154)
Measured amount was 172,116 cpm
What was the actual count rate considering the counting efficiency of our detector?
dpm = net cpm / efficiency factor
dpm = 172,116 cpm / 0.154
dpm = 1,117,636 dpm

18. Determining Acceptability
There is really no set numbers by the NRC for scintillation detector operational acceptability.
Best to go by manufacturer’s acceptance limits.
Chi-square acceptance based on statistical laws
Most manufacturers recognize that it is desirable to have constancies less than ± 10%.
Most manufacturers recognize that a <10% FWHM for Cs-137 is desirable.

19. Next Time…
Unit III Test !!!