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Designing For Airborne Radioisotopes
By: Tobias Gilk
For PET imaging, Fludeoxyglucose (FDG) is far-and-away the most commonly used radiopharmaceutical, relying on fluourine-18 as the empowering isotope. Most often FDG is injected intravenously, but what if you wanted to do an inhalation (ventilation / perfusion) study? Well it turns out that there are some very effective radiopharms for doing this, and your existing PET scanner (with the right accessories) can handle these studies just fine. But if you’re using any gaseous or aerosolized isotopes, you need to have some specific provisions in the scanner room.
Unlike injected materials, which tend to hang around inside the body for a while, whatever you inhale will have a very short residency inside your lungs. Because of this, these studies require special respirators which deliver the radio-tagged media with breathable air, then collect it again on exhalation. But as effective as these respirator systems are, some of the material will escape into the exam room. But depending on what the material is, the appropriate design responses will be quite different.
Aerosol: If you take the cup of coffee from your desk – with its one good swig remaining – and turn it over, you’d better run and grab a stack of paper towels. But if you take that same splash of Starbucks and put it into a spray bottle that atomizes the liquid into small enough droplets, you can spritz the bottle and watch the mist just hang in the air. So it is with aerosolized isotopes, such as Technetium DTPA. The material is heavier-than-air, but the aerosolized droplets are so fine that they will be carried by the air until they eventually settle out. Aerosolized radioisotopes will flow in whatever direction (and to whatever destinations) the air in the room would normally flow… typically exhaust air ducts.
Lighter-than-air: What’s a party without balloons? Some kid always unties the balloon, inhales the helium, and does his best Alvin the chipmunk impression, right? When they exhale, the unrestrained helium does precisely what it was trying, unsuccessfully, to do while trapped in the balloon (or the kid’s lungs)… get up above what it experiences as the thick, oppressive soup of nitrogen and oxygen that we call air. So it is with lighter-than-air isotopes, like helium, they rise up to the ceiling… except that, while it remains there, the radioactive isotope continues to decay, adding to the cumulative dose of anyone in the immediate vicinity.
Heavier-than-air: Xenon-based radiopharmaceuticals are significantly heavier-than-air (about 4.5 times) which, in addition to making them laborious to breathe, means that they will sink -- quickly -- to the floor like some imaginary liquid when they escape containment. The extreme (comparative) weight of Xenon gasses means that it would take gale-force velocities in a ceiling exhaust register to suck up Xenon isotopes off of the floor.
What these different options for aerosolized or gaseous isotopes mean is that the locations and capacities of air vents need to be planned, prior to equipment installation, to accommodate whichever may be used.
The US Department of Veterans Affairs is in the midst of updating their Design Manual for HVAC systems (the soon-to-be-superseded 2008 edition is, at the time of this article, available for download HERE ). This document is a very ‘nuts and bolts’ description of how heating, cooling, and ventilation should be provided for VA facilities, including nuclear medicine suites.
The new Design Manual is expected to state that the minimum required exhaust airflow from PET scanner rooms (or any nuclear imaging modality) must be provided in full duplicate for the use of gaseous / aerosolized radiopharmaceuticals. If the minimum room exhaust requirement was calculated to be 200 cubic-feet per minute (cfm), then, in order to accommodate the potential for either lighter-than-air or heavier-than-air radiopharmaceuticals, the room must be provided with the capacity to exhaust 200 cfm both near the ceiling and 200 cfm near the floor.
Retrofitting duplicate exhaust in an already-operating suite would be both complicated and expensive. While inhalation studies are not nearly as common as injection studies, at least today, it is important to protect the clinical capabilities of the scanner for all of it’s 10-15 year expected life. The modest incremental cost of incorporating dual exhausts for gaseous / aerosolized radiopharms in suite design represents pennies on the dollar, compared to the costs of retrofit.
As with all radiology, radiation therapy and nuclear medicine suite designs, it isn’t enough to plan a facility based on historical uses. Clinical demands, reimbursement structures, and imaging technology shift annually, and the designer’s goal should be to preserve the healthcare provider’s flexibility to respond to these changes.
If you would like to learn about how RAD-Planning may help you find opportunities to make the most of your next Imaging installation, please contact us at TheRadiant@RAD-Planning.com.
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802 Broadway 5th Floor
Kansas City, MO 64105