The impact of exceeding recommended crucible volumes in the Technegas generator

Introduction: While Technegas generators have been common place within departments for over a decade, misconceptions still exist concerning appropriate volumes used to fill the Technegas crucible. Anecdotal evidence suggests that it is common clinical practice to over fill the crucible as a means of increasing activity without the need for multiple simmer cycles. Research Question: Is the convex bubble blown off by argon flow? If so, what is the extent of the lost activity? Is over-filling the crucible an effective means of increasing activity without performing multiple simmers? Methodology: Various volumes of 99mTc pertechnetate were assayed. Both volume and activity of 99mTc pertechnetate delivered to the crucible were recorded before the simmer cycle. After completion of the simmer cycle the Technegas crucible was inspected to ensure complete evaporation, and then calibrated to allow calculation of the percentage difference between the expected activity and actual activity. Results: No statistically significant difference was noted between mean loss for volumes within manufacturer specifications (2.0%) and the mean loss for volumes exceeding manufacturer specifications (2.4%) (P = 0.62). Visual inspection of the crucibles immediately following the completion of the simmer cycle demonstrated unevaporated liquid in the crucible 87.5% of assays with volumes greater than or equal to 0.19 ml and no assays within manufacturer specifications. Conclusion: While the argon purge does not blow off the convex meniscus and there is negligible post simmer loss of activity from the over filled crucible, this practice is not an effective means of avoiding the need for multiple simmer cycles to increase crucible activity.