1. METHODS
A series of measurements were taken from six locations throughout our institution. A standard dose of F-18 was measured in addition to three standards; Co-57 , Ba-133, and Cs The first two locations were PET ionizing chambers, and the following locations, F-18 was measured on a CRC 25W calibrator. A calibration number of 472 was used to measure the dose in the locations. Activity values of the standards were determined by doing a decay correction on the initial activity of the sources.
BACKGROUND
Dose calibrators have been used throughout the history of nuclear medicine to accurately measure the dose for nuclear procedures. Since most nuclear procedures involve low energy isotopes, precise attention has been paid to low energy accuracy test. With newer advancements such as PET, it is now important to evaluate the accuracy of measuring high energy isotopes. A recent publication indicated an issue with dose calibrators, and measuring 511 keV high energy isotopes. This prompted Capintec to publish a new calibration number for measuring F-18. After the change had been made, an experiment was done to determine if the calibrators were consistent for providing the same measurements for a standard dose.
RESULTS
The standard dose of F-18 ranged from 12.0 mCI to 17.2 mCI at the six locations. Five of the six measurements differed by 10% or more when compared to measurement at location #1. The evaluation with Ba-133 also indicates a problem with the calibrator in location #1, although daily constancy checks with Cs-137 were acceptable. The calibrator had also passed the Ba-133 accuracy test that was performed about five months ago. This experiment was done after the calibration number for F-18 was changed.
Co-57
Ba-133
UNITS mCI
LOC #1
LOC #2
LOC #3
LOC #4
LOC #5
LOC #6
Co-57
DECAY CORRECTED
2.35
MEASURED
2.41
2.29
2.31
2.33
2.30
% ERROR
2.6
1.7
0.9
2.1
Ba-133
0.135
0.120
0.142
0.141
0.140
0.134
12.5
5.2
4.4
3.7
0.74
Cs-137
0.201
0.195
0.200
0.208
0.207
0.212
0.209
2.9
0.50
3.4
3.0
4.0
F-18
17.2
15.9
14.8
13.6
13.2
12.0
17.6
16.3
15.1
14.6
%ERROR
10.7
10.1
11.0
10.6
10.0
The measurement of F-18 at location #1 was used as the standard. The measured value was decayed to provide reference values for each of the other locations
CONCLUSION
Cs-137
F -18
AIMS
The experiment was to evaluate six dose calibrators at our institution to determine accuracy in measuring high energy (511 keV) standard doses.
A problem was identified with the dose calibrator in location #1 at our institution, and the calibrator is being repaired. This experiment shows the importance of monitoring accuracy and constancy at high energies, possibly with two different standards.