Answer to Question #10802 Submitted to "Ask the Experts"

Category: Nuclear Medicine Patient Issues — Diagnostic Nuclear Medicine

The following question was answered by an expert in the appropriate field:

Q

I read Question #8060 and Question #7251 regarding cardiac stress tests but still have a few questions. First, they say that one receives about 7 mSv of radiation from the exercise part and 3 mSv from the other part of the stress test. This is compared to the 3 mSv one gets from background radiation over one year. But that is 7 mSv in the course of a couple hours, while the 3 mSv of background is spread out over the year. Doesn't the intensity of the radiation, not just the total dose, have to be considered as well? Same for a chest x ray: I've heard it's less than what you get from flying in a plane, but a plane flight could be six hours or more, while the x ray takes place in a millisecond.

My mother recently had a cardiac stress test using Cardiolite, where 99mTc is administered. I read online in a cardiac journal that there is greater risk to family members when 99mTc is used rather than 201Tl. I didn't realize this until a day after the test, but I had already spent lots of time with her after the test, in close proximity driving her home, being in the kitchen, and sharing a bathroom. Is there any risk to me and other family members? Should we have stayed farther away? She is also concerned because she has had several computed tomography (CT) scans and x rays in the past two years, so cumulative exposure is an issue.

Lastly, I read online that 99mTc decays to a more stable, but still radioactive 99Tc with a half-life of 211,000 years. So does that mean that residues from sweat, fluids, etc., could still affect us long after the 99mTc is decayed?

A

It is true that both dose and dose rate make a difference in the biological response to radiation. This question has been answered before. Please see Question 9503 and Question 10788.

Regarding whether 99mTc cardiac stress tests with Cardiolite or Myoview (another 99mTc cardiac imaging agent) are of greater risk to family members than 201Tl, I cannot comment without seeing the article to which you refer. The amount of 99mTc administered is greater than the amount of 201Tl and the radiation energy is also greater (meaning that more of the radiation will leave the body). This would tend to support 99mTc being of higher risk. But the half-life of 99mTc is much shorter, so after a couple of days, there is insignificant 99mTc in the body, but a much higher percentage of 201Tl would be present. (I had a staff member set off an alarming area detector every time he passed by it even four weeks after he had a 201Tl stress test.) Regardless of which radionuclide is administered, however, the radiation doses to family members are very low and the risk is minimal to nonexistent. It is noteworthy that the patient dose is significantly higher for 201Tl than 99mTc cardiac imaging agents even considering that more 99mTc is administered.

99mTc does indeed decay to 99Tc which is radioactive, but the 99Tc is of no consequence. 99mTc has a six-hour half-life and 99Tc has a 211,000 year half-life. This means that the rate at which 99Tc atoms give off radiation is much, much slower than 99mTc. For example, if we start with a pure 99mTc sample, in six hours there will be half as many atoms of 99mTc as were there initially. These atoms would now be 99Tc. In other words, after one half-life there would be an equal number of atoms of 99mTc and 99Tc. But we do not measure radioactivity in terms of the number of atoms. Instead we measure amounts of radioactivity in terms of how frequently it emits radiation, e.g., 1 radioactive decay per second. The reason that 99Tc lasts so much longer than 99mTc is that the average time between each decay is much longer.

Think about it this way. You have two tubs, each with a drain. The drain valves are spring loaded so that the higher the water pressure, the larger the opening and the more water is released. Tub 1 has a large opening (or a loose spring) and Tub 2 has a small opening (or a tight spring). Tub 1 drains into Tub 2. Since water pressure decreases as the water level goes down, the less water in Tub 1 the slower the water flow into Tub 2. At some point essentially all the water in Tub 1 has drained into Tub 2. But since the Tub 2 valve is much more restrictive, the water flow out of Tub 2 is much less than the initial flow rate out of Tub 1, even after Tub 2 has filled. The water level in each tub represents the number of radioactive atoms. The water flow represents the amount of radioactivity. 

Mathematically it looks like this: 

A = amount of radioactive material in number of decays per time, e.g., decays per second
T = half-life in units of time
N = number of atoms
k = constant (in this case, the natural logarithm of 2, which is ~0.693).

A = kN/T.

For 99mTc let’s use:

N = 1,000,000 atoms
T = 6 hours = 21,600 seconds.

The activity is:

A = (0.693 × 1,000,000)/21,600 = 32 decays per second.

For 99Tc, T = 211,000 years = 6,658,653,600,000 seconds.

A = (0.693 × 1,000,000)/6,658,653,600,000 = 0.000000104 decays per second = 3.3 decays per year.

It would take about 10 years for 99Tc to emit the same amount of radiation as is emitted from 99mTc in one second.

If your mother received 1.2 GBq of 99mTc, then she would have ~4 Bq (assuming no biological elimination) of 99Tc after all the 99mTc decayed. The amount of naturally occurring radioactive materials, such as 40K, in the body greatly exceed this quantity.

Kent Lambert, CHP

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
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