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Lead Alternatives For CT Room Shielding
By: Tobias Gilk
They banished smokers to the outdoor ‘butt huts’, that was before they banned smoking on hospital campuses altogether. Soda machines are disappearing in favor of fruit juice and bottled water machines. As paragons of health, hospitals have been making a concerted effort at improving the healthfulness of the hospital environment.
“BEEP! BEEP! BEEP! BEEP!”
What’s that sound, you ask? That’s the truck delivering a few thousand pounds of toxic heavy metal that we’re going to build into the walls of your new CT suite.
For all of our focus on environmental health does it seem incongruous to you that we still use lead as the predominant shield material for X-ray technologies?
Lead interferes with a variety of body processes and is toxic to many organs and tissues including the heart, bones, intestines, kidneys, and reproductive and nervous systems. It interferes with the development of the nervous system and is therefore particularly toxic to children, causing potentially permanent learning and behavior disorders. Symptoms include abdominal pain, confusion, headache, anemia, irritability, and in severe cases seizures, coma, and death.
So, if lead is so nasty, why do we continue to use it? Two reasons, density and cost:
Density: X-rays are high-energy electromagnetic energies that rocket outward from the emitter. They sail between the sparse atomic structures of gasses (e.g. air) and liquids (e.g. soft human tissues), and are only effectively stopped when they wind up in a comparatively dense cluster of protons found in solids (which is why bones contrast soft tissues so nicely on X-ray images, denser bones absorb the X-rays much more effectively than the surrounding softer tissues).
You could shield X-rays with a gas, but until you got up in the periodic table to Xenon, you’d be looking at hundreds of times the thickness of lead needed to achieve the same degree of attenuation with a gas. Most liquids would require tens of times the thickness of lead. (Liquid mercury, however, would be similar in attenuation capabilities to lead, but would be even more toxic and much harder to keep contained.) You could get away with significantly less material if you opted for uranium, or – even better – plutonium, but those present an entirely different set of problems.
So are there non-toxic, non-radioactive elements that would be effective alternatives for lead? Yes, there are two metals with much lower toxicity that would be similar to lead in shielding capabilities…
Cost: Those two alternative metals are gold and platinum, and if you thought your last CT construction project was expensive, imagine what it would cost if you lined the walls with platinum!
As lead has been banished from materials used for dishes, paints, silverware, gasoline, Happy Meal toys, etc…, the reduced demand has helped to keep the cost depressed. There are few elemental materials cheaper than lead. And for shielding, there are very few (that don’t also irradiate us) that are equally effective.
There are new high specific gravity (HSG) composite materials which use tungsten (only a few hops down the periodic table from lead), encapsulating the metal in a polymer that can be molded or formed. Effectively ‘diluting’ the metal with the polymer means that you’ll increase the overall thickness of shield material needed to provide the same degree of attenuation. HSG polymers are currently being used for shielding, though presently their use is limited to shielding built into the X-ray device itself. With increase scrutiny of the use of lead, particularly in healthcare, it’s likely only a matter of time before sheets of HSG plastics (or similar materials) begin replacing sheets of lead for X-ray shielding in our walls, floors and ceilings.
A couple of elements are happening in the CT industry that will help make transition away from lead to HSG polymers less costly: dose reduction and beam containment.
As we reduce the dose associated with an X-ray exam, we also decrease the requirements for shielding. Lower-energy scans mean that less material (or a less efficient shielding material) can be used in architectural shielding. And with each successive generation of CT scanner is released, the scatter radiation decreases, meaning ‘off-axis’ shielding requirements are greatly diminished. In some situations today, it may be possible to do away with control room wall shielding, almost entirely, because the emitters and gantries are better designed to reduce X-ray radiation scatter.
In the near term, lead will remain the most economical X-ray shield material for buildings. Fortunately, lead is only toxic upon ingestion, so it’s not as if our existing shielding assemblies are somehow poisoning us by their presence. But at some time, those shield materials will need to be removed and disposed of, and that long view shows us where the problem with lead ultimately lies.
As we continue to make our hospitals healthier environments and responsive to a broader mission of sustainable design, it will become harder to justify the use of lead as a shielding material, when less-toxic options exist. As the safe handling and disposal of lead becomes increasingly burdensome, expect to see shifts to newer, innovative architectural shielding materials.
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802 Broadway 5th Floor
Kansas City, MO 64105