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Nuclear Medical Imaging. How Radioactive Am I?

Posted by Jenna Mast  
Jenna Mast September 12, 2018 02:05AM
I thought some here might be interested in this.

I recently had some medical nuclear imaging done in which an isomer called Technetium-99m was used as a tracer. After the injection I was given a card to identify myself as someone who has recently had nuclear imaging done, as apparently the tracer is radioactive enough to set off government radiation detectors. This, of course, piqued my curiosity, as I'm sure it would that of others here, so eventually I decided to see just how radioactive I was and unpacked my GM counter.

Unpleasantly enough, roughly 7 hours after the tracer injection, I was saturating the most sensitive scale and clocking in at around 3 mR per hour...far more than any of my radioactive specimens register. The total dose is estimated to be around 5 mSv.
Dean Allum September 12, 2018 03:32AM
I recall that radiation industry employees have a limit of 100 mR per year. Understand that these people are exposed to radiation for many years of their life so this limit is kept conservative.

A web search showed that Technetium-99m has a half life of 6 hours, so you were probably at the 6 mR/hour level at the beginning, but should be below 0.4 mR/hr a day from now. Seems like you will stay below the 100mR total, but I am not familiar with the 'locational' aspect of this dose.

I also noticed that the gamma rays have 140KeV energy. Quite low compared to most radioactive isotopes.

Keep drinking lots of water. Let us know if you glow in the dark.
Alysson Rowan September 12, 2018 08:51AM
99mTc Is a short-lived pure gamma emitter which decays to 99Tc which is a long-lived pure beta emitter.

Yes, this sounds alarming.

When the metastable nuclide has reached background levels, the remaining radionuclide will barely increase your body's radioactivity at all - considering that we all include naturally occurring Potassium-40, Carbon-14 and Tritium (radioactive hydrogen).

When nuclear systemic imaging is performed, the amount of radionuclide is calculated on the basis of being sufficient to produce the required imaging, and also to not increase your annual dose (what you get during the imaging cycle) or your lifetime dose commitment (what is left in your body for months/years afterward) above pre-set limits.

As a rule of thumb, the radiation dose you receive from the technitium will be about the same as you would expect from a standard X-ray series.
Matt Courville September 12, 2018 12:17PM
Hi Jenna,

I had looked into similar questions for my worried mother a little while back. This Canadian government site (posted at bottom) has some good info, but I specifically appreciated the 'Radiation Dose Examples' diagram which shows user-friendly comparisons for those without a scientific/nuclear properties education.(almost at the end of the page)

It surprised me to realize that a chest CT scan emits 70 times radiation than a chest X-Ray, and yet popular culture would have you fear the x-ray.

It would seem that you would require 10 of these doses you had mentioned, before reaching the 'annual dose limit for nuclear energy workers' based on the Canadian limits. Modern medicine sees the pros outweighing the cons in such treatments.

Hope this helps - Matt


Edited 2 time(s). Last edit at 09/12/2018 12:19PM by Matt Courville.
Reiner Mielke September 13, 2018 05:59PM
I wonder if you would be allowed on an airplane in the US? I can't send a U or Th bearing sample to the US by air even if is below background, which is approximately 0.01 MR/HR. in my area. I guess maybe I wouldn't have the same problem if it were Technetium-99?
Doug Daniels September 13, 2018 06:56PM
As far as airplanes in the US (and elsewhere, I think), passengers have to go through a metal detector, but not a radiation detector. Then again, I haven't flown in about 20 years. And, if they did screen you and found you to be "radioactive", I'm sure the Feds wouldn't care what isotope was causing the detection. On a side note, aren't bananas slightly radioactive due to their potassium content (albeit it low)?
Becky Coulson September 13, 2018 07:42PM
Jenna mentioned that she was given a card identifying her as a patient.
Jeff Weissman September 13, 2018 09:08PM
Radiation detectors can be quite sensitive, and may be in place in airports without any obvious detector visible - in fact I recently learned about what is essentially a "banana detector," a machine that can detect the relatively weak K40 decay from bananas, enabling border control to determine if a shipment of bananas needs to be further inspected or let through, while stopping and inspecting other radioactive cargo.
Colin Robinson September 13, 2018 09:12PM
My wife has recently had a couple of radionuclide scans, one using Tc99. She was advised to avoid being close to small children or pregnant women for 24 hours after which time there would be so little radioactivity as to be barely noticeable above background. I can't imagine there would be enough to set off detectors, certainly after a day or two, although if you tried to board a plane immediately after the procedure you might set off the alarms. in my student days one of my classmates was so paranoid about radioactivity that he hid behind a lab bench when the lecturer introduced the group to some simple alpha, beta and gamma emitters. The same student dropped from the course shortly after that. Presumably science was too dangerous for him.
Alfredo Petrov September 14, 2018 08:46AM
My favorite nuts, of which I have been eating several per day for about 20 years, turn out to be the most radioactive of all foods, certainly more than bananas, because brazil nuts absorb barium from the soil, and along with the barium it inevitably sucks up whatever minute traces of (the chemically very similar) radium are in the soil too.
Jenna Mast September 20, 2018 07:20PM
I was still somewhat more radioactive than usual the next morning. I wasn't told to stay away from small children but I had checked I didn't have any and knowing how radioactive I was with respect to background levels,, I would have not wanted to be near small children.

I made an audio recording. I will try to find a way to upload it.
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