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PET/CT Gamma Ray Protection with Lead Apron

  1. Aug 3, 2016 #1
    I was informed I might get a better response by posting to this forum, so sorry for the double post.

    Hello all,
    I am currently attending a service course for a PET/CT system for my employer. We have been covering the physics behind the function of detectors and my instructor said something that I totally disagreed with and the rest of the class looked at me like I was just dumb. It had to do with why PET/CT techs do not wear lead aprons.

    You probably already know the process of PET, but it helps me learn if I can explain in detail.

    In positron emission tomography a radioactive isotope is combined with a substance that would interact with some biologic process. During the decay of the isotope a positron is emitted and interacts with an electron, resulting in annihilation and the perpendicular emission of gamma rays eventually interacting with the crystals mounted on the pick up tubes, translating into visible light and being measured.

    The instructor said that PET techs do not wear lead aprons because the lead apron would slow down the gamma rays and allow the photons to be absorbed by cells of the human body. Essentially the instructor was saying that wearing a lead apron in a PET room is more dangerous than not.

    I totally disagreed and he began to try and explain I was wrong.

    I disagreed because I was under the impression that photons of any wavelength or frequency never change speed, they can change direction through interactions with atoms or be completely absorbed and expelled, but photons can surely not be slowed down, it may appear they slow down, but it is merely absorption and expulsion of the photon in several directions that make the photon appear to slow down, but they physically are not able to slow down.

    Once I conveyed my message clear enough, the instructor said, well the photons impart energy into the lead apron, transferring energy to the atoms, causing the gamma rays to be of less strength, thus being able to be easier absorbed by the human body. This explanation made more sense to me, but I am still skeptical of this explanation, as I believe some lead is better than no lead, but really this explanation only made sense if we were discussing a charged particle rather than an electromagnetic wave. This happens in the generation of x-rays when electrons are attracted to an anode of an xray tube, the electrons interact in the proximity of a nucleus, altering its path and expelling a softer photon that results in the xray generation profile, but I don't see how an electromagnetic wave can be directed without actually exciting an electron or nucleus.

    The argument soon became an argument about whether you can slow a photon down, and I ended up arguing against my co-workers and lab partners. I said lead aprons are negligible protection from the powerful gamma rays produced but they didn't cause the radiation to somehow become more dangerous. One student went so far to say that photon speed has been shown scientifically to slow down, but all the explanations I have read state the appearance of a slower photon, but really the path is being altered not the photon speed. The student then said "when you are driving a car and you turn on your headlights, now that light is traveling faster that it normally would". I immediately thought to ask if he was a creationist, because I know Einsteins Theory of Relativity states that light will always travel at C from any relative observation, it's time dilation that occurs at increasing speeds, but that's beside the point of the previous argument but I thought I would share that because it really got my goat after he said Einstein's theory of relativity has been proven wrong!!

    Are my physics wrong? Can a lead apron cause gamma rays to become more dangerous to a tech wearing it?
  2. jcsd
  3. Aug 3, 2016 #2

    Vanadium 50

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    It certainly won't help. A thin lead apron won't absorb much radiation - it will take a gamma ray and it will Compton scatter giving you a lower energy gamma ray and an electron, such that the total energy is more or less unchanged.

    Will it hurt? Maybe a tiny bit in theory - it's possible that a high energy gamma would go through you, and a lower energy gamma and electron would be absorbed, so you in fact get more dosage. It's also possible that if you were turned away from the source, the apron could produce a backscattered electron, increasing your dose. I don't think these are large effects, and they may not even be measurable.

    For dental x-rays, the biggest benefit of the lead apron is that it minimizes patient movement, and thereby reduces the probability than an additional x-ray needs to be taken.
  4. Aug 3, 2016 #3
    Thank you so much for the response, it makes sense to me now. I got caught up in the photon slowing down argument and lost focus on whether the photon would have a greater probability of interacting with cells. Good to know
  5. Aug 4, 2016 #4


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    An effective shield against gamma rays would be too heavy to carry around. The gamma ray to electron conversion can also happen in the human body, I'm not sure if light shielding would be harmful directly, but it would not help either. It would make everything take more time which increases the radiation exposure.

    Google finds this document suggesting that technicians receive a dose of up to a few microsievert per patient.
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