Answer to Question #12552 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 know that one meter away from an x-ray beam scatter radiation measures 0.1% of the primary beam. My question is with regards to nuclear medicine. Is the scatter at one meter the same for nuclear medicine? Also, I know some tests such as heart stress tests result in a dose that can be as much as 40 mSv. Would a distance of two meters be sufficient so as not to receive a scatter dose of radiation from such a high dose?

A

With respect to your questions about the patient dose and dose to others, 40 mSv would not be considered a large dose to an individual who received a cardiac stress test. It is less than the annual limit of radiation exposure allowed for radiation workers.  

Due to self-absorption within the patient and external distance factors, the dose to a family member or member of the general public would be very small and a distance factor of one meter would be more than adequate.  

Also, since 99mTc has a half-life of six hours and is excreted rapidly, external dose rates would be reduced by approximately 95% within 24 hours after administration and barely detectable after 48 hours.

When we perform a diagnostic nuclear medicine study, like a cardiac stress test, radiopharmaceuticals administered to the patient give off radiation, some of which is absorbed in the tissues of the body and some is transmitted through the tissues, so we can capture the emissions and create an image from it.  

At low energies, most of the radiation is absorbed internally, but at medium to high energies, more of the radiation is emitted and can be captured for imaging purposes.  

With regard to your comment about scatter, I believe that you might be confusing concepts. Generally, for x-ray devices, the Food and Drug Administration manufacturing standard is that the x-ray tube housing must shield the leakage radiation to at least as low as 1/1,000th (0.1%) of the primary beam dose rate at 1 m. See Table B-2 of the National Council on Radiation Protection and Measurements Report No. 49 (NRCP 1976). The patient does not emit any radiation after they have received an x ray.

Michael J. Bohan
Radiation Safety Officer

Reference  

National Council on Radiation Protection and Measurements. Structural shielding design and evaluation for medical use of x-rays and gamma rays of energies up to 10 MeV. Bethesda, MD: NCRP; NCRP Report No. 49; 1976.

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