New proposed methods to prepare Dose Calibrator for Sc44 activity measurements

Objectives: Sc-44 is an attractive positron emitter radioisotope for imaging biomarkers using PET. Some of the advantages are its 3.97 hours half-life, relatively low positron range (2.5 mm), high percent of positron emission (94.3%), and similar radiolabeling properties as for Ga-68. In the commonly used dose calibrators the Sc-44 isotope cannot be selected for measurement, and also there is no available reference source to perform the calibration either. Our aim was to develop alternative methods to calibrate the activity meter for Sc-44, which can be easily performed in a radiochemistry or pre-clinical imaging laboratory.

Methods: Sc-44 isotope was obtained with the elution of Ti-44/Sc-44 generator, providing activity typically in the range of 1-4 MBq. The sensitivity of a dual head gamma camera was measured with F-18 in the 450-650keV energy window taking into account the detector-sample geometry, dead time and background correction. Based on the known sensitivity factor the activity of Sc-44 samples were measured and calculated. Alternatively, an energy channel of the dose calibrator was set for the Na-22 positron emitter (branching ratio 90.36%) reference source as an appropriate substitution of the Sc-44. This is based on the similar energy spectra of the Na-22 and Sc-44, especially to have the very same second gamma line, 1.247 MeV (branching ratio 99.94%) and 1.157MeV (branching ratio 99.88%), respectively. For all measurements a high resolution gamma-ray spectroscopy provided the true reference activity of Sc-44 samples.

Results: The gamma camera measurement provided calibration factor with an accuracy less than 10% error. The channel calibrated for Na-22 overestimate Sc-44 by ~6%.

Conclusion: The initial results suggest that calibrating the dose calibrator based on the measured activity of Sc-44 by gamma camera is feasible. The statistical uncertainties can be decreased by further experiments, as well as to control the stability of the entire procedure, improving the accuracy. The systematic bias caused by the calibration with Na-22 for Sc-44 can be eliminated by a multiplication, since all calibration factors are simple multiplication constants. Thus with the help of Na-22 reference source the proper calibration factor for Sc-44 can be estimated. Research Support: No any research support