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

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

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.

Daily Constancy

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

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

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.

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. --

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 7.078 would pass.

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.

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?)

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)

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.) 1.72116 x 105 cpm X 100 = 15.4% (0.6 μCi)(2.22 x 106 dpm/µCi)(0.838)

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

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.

Next Time… Unit III Test !!!