On the Imaging of Very Weak Sources in an LSO Pet Scanner

When PET studies such as radiolabeled cell trafficking or gene expression imaging are being performed the amount of activity in the scanner can be very low. For an LSO scanner, such as the microPET R4, there are detected true events originating from the beta decay of ¹⁷⁶Lu in the LSO followed by prompt gamma emission. This contributes a background to the final image, similar to that due to un-corrected random counts, and reduces the contrast of any sources being imaged. This is very prominent when a lower level discriminator (LLD) of 250 keV is used. Measurements of the microPET R4 background count rate with no source in the scanner showed that the trues rate drops from over 1000 cps with a window of 250-750 keV to 2 cps for 400-750 keV. However, the system sensitivity as measured with a 68Ge line source decreased by 50% for the same narrow window. Noise equivalent count rates (NECR) were estimated vs. LLD for low activity levels and showed a strongly peaked shape with NECR being highest for LLD levels of 400 keV or more for activities < 500 nCi. A weak source phantom containing three 1 cm^{3 68}Ge sources with activity 1.5, 6, and 25.5 nCi was scanned using windows of 250-750 keV, 350-750 keV and 400-750 keV. The resulting images showed that the background dominated the image for the 250-750 keV window, making the sources difficult to resolve. The 400-750 keV image clearly resolved all 3 sources and contained very little background. These results indicate that in order to successfully image sources of < 25 nCi in an LSO based scanner system sensitivity to true emission events must be sacrificed by raising the LLD sufficiently high to exclude most of the background.