¹³³Xe Contamination Found in Internal Bacteria Filter of Xenon Ventilation System

MATERIALS AND METHODS

Each internal bacteria filter (n = 6) (Fig. 1) was evaluated after approximately 1 mo of normal use. The xenon ventilation system (Ventil-Con 3; RADX) was evacuated twice to eliminate ¹³³Xe in the system before removal of the bacteria filter. Upon removal, the filter was immediately placed in a resealable plastic bag and monitored using a survey meter equipped with an energy-compensated probe. The maximum surface radiation level was recorded. The filter was then imaged for 5 min using a scintillation camera equipped with a low-energy all-purpose collimator and 20% energy window centered around 81 keV. The bacteria filter was removed from its resealable plastic bag and stored in a fume hood for several days. It was periodically monitored and imaged during that time. A used ¹³³Xe vial was assayed in a dose calibrator and imaged on the scintillation camera to estimate the ¹³³Xe activity in the bacteria filter. Background images were obtained for all sets of images.

• Download figure
• Open in new tab
• Download powerpoint

FIGURE 1.

Bacteria filter (arrow) (15 cm from filter inlet to filter outlet, 9-cm maximum diameter) inside xenon ventilation system. CO₂ absorber is located in canister directly below filter. Filter and CO₂ absorber are in constant contact with ¹³³Xe in ventilation system. Moisture absorber is located in canister directly across from CO₂ absorber canister. Moisture absorber comes into brief contact with ¹³³Xe only when system is evacuated or during washout phase of ventilation study. During that time, ¹³³Xe is trapped in internal shielded xenon filter (i.e., activated charcoal).

Based on imaging, the polyurethane adhesive used by the manufacturer to attach the fiberglass filter medium to the filter housing was thought to be the primary cause of the ¹³³Xe retention. To confirm this suspicion, a small sample (∼4 × 5 × 15 mm) of the adhesive from a filter was placed in a sealed vial with ¹³³Xe for several days. Upon removal from the vial, the adhesive sample was imaged over several days.

We also evaluated if ¹³³Xe activity was found in the CO₂ and moisture absorbers (i.e., soda lime granules and silica gel, respectively, n = 2 for both) on their removal from the ventilation system. After the bacteria filter was changed, each absorber was removed and placed in separate resealable plastic bags. The bags were monitored using a survey meter, and then, like the filter, they were imaged on a scintillation camera. The absorbers were stored in the fume hood with the plastic bag opened for approximately 7 h if ¹³³Xe activity was detected. At that time, they were monitored and imaged.

RESULTS

Estimated ¹³³Xe activity in each bacteria filter immediately after removal ranged from 132 to 2,035 kBq (3.6–55.0 μCi), based on imaging. The higher the estimated ¹³³Xe concentration (MBq/L) of the ventilation system was before evacuation, the higher was the initial filter activity. Initial maximum surface-radiation levels ranged from 0.4 to 4.5 μSv/h (0.04–0.45 mrem/h), which correlated well with the estimated activity (Fig. 2). Background levels were 0.1 μSv/h (0.01 mrem/h). The ¹³³Xe activity did readily leave the filter over time (i.e., time to reach half the counts of the initial decay-corrected image ranged from <6 to >72 h) (Fig. 3).

• Download figure
• Open in new tab
• Download powerpoint

FIGURE 2.

Linear regression analysis between maximum surface-radiation level and estimated ¹³³Xe activity in each bacteria filter immediately after removal from xenon ventilation system.

• Download figure
• Open in new tab
• Download powerpoint

FIGURE 3.

Percentage of activity remaining in bacteria filter based on initial decay-corrected scintillation camera counts versus time after filter removal.

The majority of the image counts (∼70%) were seen in 2 distinctive areas in the bacteria filter (Fig. 4). They corresponded to sites where the manufacturer used polyurethane adhesive to attach the fiberglass filter medium to the polypropylene filter housing (Fig. 5). To confirm that the adhesive was the primary cause of the ¹³³Xe retention, a sample of the adhesive from a filter was placed in a sealed vial with ¹³³Xe for several days. Imaging of the adhesive on removal from the vial demonstrated that the adhesive retained ¹³³Xe activity, which was released slowly over time, as had been the case with the internal bacteria filters.

• Download figure
• Open in new tab
• Download powerpoint

FIGURE 4.

Images of typical bacteria filter after removal from xenon ventilation system. Top left is immediately after removal. Top right is 0.3 d after removal, with 75% of ¹³³Xe activity remaining based on decay correction. Respective left and right bottom images are 3 and 7 d after removal, with 39% and 24% of ¹³³Xe activity remaining based on decay correction. Two hottest bands of activity correspond to 2 polyurethane adhesive areas in filter.

• Download figure
• Open in new tab
• Download powerpoint

FIGURE 5.

Cross-section of bacteria filter. Polyurethane adhesive area (open arrow) toward filter inlet is circular, with 6-cm diameter and 3- to 5-mm thickness. Other polyurethane adhesive area (solid arrows), toward filter outlet, is doughnut shaped with 4-cm inner diameter, 7.5-cm outer diameter, and 4- to 5-mm thickness. Fiberglass filter medium is between and attached to 2 adhesive areas so as to secure it to polypropylene filter housing.

One bacteria filter had increased initial ¹³³Xe activity throughout. The increase was probably due to incomplete evacuation of the ¹³³Xe activity from the xenon ventilation system before removal. This would explain its quicker release of ¹³³Xe activity (○ line in Fig. 3) when compared with the other filters.

The moisture absorber showed no detectable ¹³³Xe activity on removal, whereas the CO₂ absorber did. The 2 CO₂ absorbers had estimated ¹³³Xe activities of 157.7 and 308.8 kBq (4.3 and 8.3 μCi), with low maximum surface-radiation levels of 0.3 and 0.4 μSv/h (0.03 and 0.04 mrem/h), respectively. No detectable ¹³³Xe activity in the CO₂ absorber could be found after storage in the fume hood for 7 h.

DISCUSSION

Normally, we think of radioxenon contamination as resulting from a xenon spill (e.g., vial breaks or patient removes mask during ventilation study). We have shown that ¹³³Xe contamination is present in the reusable internal bacteria filter and CO₂ absorber of our ventilation system. One would not expect to see this type of contamination—of an inert radioactive gas—in a bacteria filter or CO₂ absorber. Because the filter is internal, there is a constant presence of ¹³³Xe in the filter (and the CO₂ absorber) if the ventilation system has activity loaded (i.e., does not include activity in the xenon trap), thus allowing time for the ¹³³Xe to adsorb to the plastic contents of the bacteria filter (i.e., primarily to the polyurethane adhesive) and to the CO₂ absorber. Radioxenon adsorption to plastic syringes (1,2) and to rubber and plastic stoppers and components (1–5) has been reported. Radioxenon contamination of polystyrene packing materials from ¹³³Xe shipments (5–7), plastic packaging jackets that house shielded ¹³³Xe vials (6), and polyurethane packing materials (8) and activated carbon absorbent packets (9) from ¹³¹I capsule shipments (i.e., ^131mXe contamination) has been reported.

CONCLUSION

¹³³Xe contamination within the reusable internal bacteria filter and CO₂ absorber of our ventilation system was easily detected by survey meters and imaging. Although initial activities and surface radiation levels were low, radiation safety practices would dictate that a ¹³³Xe-contaminated bacteria filter and CO₂ absorber be stored preferably in a fume hood until it cannot be distinguished from background before autoclaving or disposal.