Talk:Technetium-99m

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The dose calculations presented on this page are misleading. In order for the emitted 140KeV gamma to deposit any dose (other than from the insignificant recoil of the emitting Technetium atom), the gamma in question would have to be absorbed in the body. So, essentially no dose results from the gammas detected in the SPECT camera. Rather, those gammas that may interact in the patient's body (or for that matter in the body of another person standing nearby for example) are responsible for the dose. Thus, to calculate the dose one needs to assess the likelihood that a gamma passing through a particular path in the patient's body will interact, which is a function of the material along that path and is different for tissue like bone possessing atoms with higher electron densities such as calcium. This is further complicated because the Technetium is typically distributed widely throughout the body depending on the type of carrier molecule in which it has been embedded, and the direction if gamma emission from each decay is essentially random. Finally, the amount of energy deposited in each gamma interaction typically is not the entire energy of the gamma, but for example in some cases some fraction of the energy is transferred when the gamma scatters off of an electron losing some of its energy to that electron, proceeding off in another direction with a lower energy.

So, the calculation mentioned in the article is the dose that would be absorbed if all of the energy of a single technetium gamma were entirely absorbed and somehow spread over the entire body of the patient. While the conclusion that this dose is negligible is certainly correct, it does not follow that the total absorbed dose locally in all of the tissues of the body due to such a scan is so negligible as to be totally disregarded. In fact, it is because this dose is actually measurable on normal industrial personal dosimeters such as those worn by workers at our National Laboratories, that such workers are not allowed to wear their badges for some period immediately after having had such medical tests.

Lpinsky 03:11, 7 August 2007 (UTC)