The Impact Of Exceeding Recommended Crucible Volumes In The Technegas Generator

摘要

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. Introduction A Technegas generator is a microprocessor controlled device which produces an ultra fine micro-aerosol of a graphite coated Technetium atom used for ventilation lung scanning (1). Recommended crucible loading activity ranges between 400 and 900 MBq of Sodium Pertechnetate in 0.14 ml (2). High specific concentrations are not always available due to the decay of eluate and/or the generator bound parent. This decay results in significantly reduced available activity as the day and week progress. This is significant, as not only will the amount of 99mTc available from the generator decrease throughout the week, but the eluted 99mTc pertechnetate activity will also decrease throughout the day. Multiple simmer cycles of 6 minutes can be employed to obtain activity suitable for ventilation imaging. Specific concentration has further ramifications on patient management since it is proportional to the efficiency with which an adequate count rate sufficient to complete the ventilation study is achieved. This may be compounded by the symptoms typical of a patient presenting for pulmonary embolism evaluation (i.e. dyspnea and chest pain) (1). Anecdotally, patient compliance tends to decrease as the duration of the ventilation procedure increases, increasing the likelihood that the study will be performed with sub optimal count density and/or room contamination will occur. A high specific concentration should increase the likelihood of obtaining an adequate count rate with minimal inspiration and improved patient compliance.A number of measures are employed to improve available specific concentrations of 99mTc, such as, decreasing the volume of saline used to elute the generator or performing additional elutions throughout the day. Nonetheless, specific concentration of 99mTc eluate is of considerable concern, particularly on days of low activity due to generator decay. Contrary to manufacturer's guidelines (2), anecdotal evidence suggests that it is a common practice to increase the 99mTc pertechnetate volume above the confines of the crucible (convex bubble) to increase activity while limiting the preparation time to a single simmer cycle.Manufacturer's guidelines (2) advise that over filling of the crucible does not con