Adsorption of TcO4- by zeolites and other crystalline minerals for testing small-animal imaging-system performance

When evaluating small-animal imagers, small pointlike sources (1mm diam.) are useful for testing system performance. Slightly larger sources (1-3 mm) can also be used to mimic tumors. Ideal sources should have high uptake of TcO4-, and should not be damaged during preparation or handling. Various crystalline minerals are able to adsorb a variety of chemical species, and may satisfy these criteria. We evaluated the utility of a number of such materials for SPECT and PET imaging. Initial tests of 12 different types of crystalline minerals convinced us that the following five materials were promising with regard to size, uptake, and physical robustness: (1)PSA diatomaceous earth (2))Isolite calcined diatomaceous earth (3)Profile calcined clay (4)Bear River zeolite (5)ZeoPro lite. Three samples of each material were soaked in a solution of 1,850 MBq/ml TcO4- for 10 min, while three other pieces of each type were soaked for 60 min. Samples were air-dried, scanned for 30 min in the Harvard microSPECT system, and later scanned on a GE Locus microCT to determine the volume and density variation of each piece. Each sample's specific activity was measured by drawing regions on the microSPECT images, which had been reconstructed for 11 iterations of OSEM. After 10 min. of soaking, the uptake varied from a low of 265/-32 MBq/cc with material 4, up to a high of 850/81 MBq/cc with material 3. After 60 min of soaking there was little change in specific activity. Other properties of the 5 zeolites varied considerably. Material 1 is fragile, and easily fragments into smaller pieces. Material 3 was robust to handling, and showed the highest and most uniform uptake, as well as uniform density. Material 2 exhibited variable density, while the the zeolite samples (4 and 5) were more uniform in density, with moderate uptake. Uptake comparison with cation and anion exchange beads was also performed. Finally, we conducted preliminary evaluations of the adsorption of 18F-FDG on selected materials, and found similar relative uptake for different materials, except that the average adsorption efficiency of FDG was about an order-of-magnitude less than that of TcO4-, presumably because of differences in molecular size and charge effects.