MODIFIED TC-99M PHYTATE COLLOID FOR LIVER-SPLEEN IMAGING

In contrast to the excellent images of liver and spleen obtained with Tc-99m-sulfur colloid, the poor splenic visualization following intravenous administration of Tc-99m-stannous phytate to patients is considered a serious drawback. Attempts to measure the biologic distribution and target to nontarget ratios in mice and rats for sulfur colloid and phytate reveal a discrepancy between the animal data and human scan findings. The rodent data indicate a nearly identical organ distribution for both radiocolloids 30 or 60 min after injection with 80 plus or minus 5% of the Tc-99m activity in the liver and 3 plus or minus 1% in the spleen suggesting both agents to be equally effective in imaging the spleen. In all primates tested the spleen/liver ratio for Tc-99m-sulfur colloid is 3-3. 5 times greater than that obtained for Tc-99m-stannous phytate (0. 70 vs 0. 22). The spleen/liver ratios were calculated with the aid of a computer using an interactive light pen and a region of interest program calculating the average number of counts/cell for each organ. On a total organ basis these values are equivalent to a spleen uptake of 3-4% for the sulfur colloid and 1% for the stannous phytate. Modificiation of the radiopharmaceutical is accomplished by adding various amounts of calcium ion and stabilizer (albumin or gelatin) to the reconstituted solution of Tc-99m-stannous phytate. One to two mg of Ca and 50 mg of albumin are found to give an in vitro colloid with biologic properties nearly identical to sulfur colloid.