INFLUENCE OF HIGH-ENERGY PHOTONS FROM CO-57 FLOOD SOURCES ON SCINTILLATION CAMERA UNIFORMITY IMAGES

Cobalt-57 flood sources are often used for system flood-field uniformi ty checks of scintillation camera performance. Such sources are now av ailable in large sizes and with high activities. Uniformity images usi ng new Co-57 sources have shown artefacts which are not present in uni formity images obtained from technetium-99m flood sources of the same activity. The high-energy photons emitted by Co-57 and cobalt isotope impurities appear to influence the images obtained. This was investiga ted in three cameras using three Co-57 sources of different age. The f lood-field images obtained with the Co-57 sources showed non-uniformit y patterns that were dependent on the age of the cobalt source, the di stance of the source to the collimator, and the specific camera type. Quantification of the uniformity images reflected these findings. Ener gy spectra of a new Co-57 source, obtained with an external 1024-chann el analyser connected to the camera, showed a broad tail of high-energ y photons above the 122-keV photopeak, due to Compton scatter and coll imator penetration. This tail diminished with older sources and with i ncreased source to collimator distance, indicating that in both situat ions fewer high-energy photons were being measured by the camera syste m. This tail of high-energy photons contributes to the total count rat e, but because this is not obvious from the counts observed in the pho topeak window, the camera can unsuspectingly be operated at too high a total countrate where pile-up effects become significant. Caution is therefore advised when using Co-57 sources. Problems may be minimized by purchasing low-activity sources (certainly no more than 370 MBq), b y placing the source at a distance from the collimator, e.g. 50 cm, by allowing a new source to decay, and by manufacturers producing Co-57 sources without detectable radioactive contaminants.