Talk:Technetium-99m/Archive 1

Archive 1

Dose

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)

"Synthetic substance"

An editor recently removed the word "synthetic" from the phrase "Technetium-99m is produced from the synthetic substance molybdenum-99" with the explanation "Molybdenum-99 is not a "synthetic substance".

We don't need an 'it isn't' 'it is' exchange in edit summaries. A quick Google search for the term 'synthetic nuclide' shows numerous reputable uses of the term. (It means a nuclide produced by particle bombardment.)

• http://www.google.com/search?&q=synthetic+nuclide

If you disagree, please discuss here, otherwise lets restore the word 'synthetic'.

--Hroðulf (or Hrothulf) (Talk) 07:15, 1 March 2010 (UTC)

Mo-99 is indeed an example of a synthetic substance. Somebody else took the word out, and I restored it. Then, instead of realizing it had been fixed, you reverted me and thus YOU removed it again! [1]. It's still gone, due to YOUR edit. I think you are confused, and arguing with the wrong person. Fix your own mistake if you want the word back, since you took it out the second time. SBHarris 07:49, 1 March 2010 (UTC)

SBHarris, I see why I appear confused. I reverted your edit on purpose, because you did not state any reason in your edit summary - you just accused the previous editor of being wrong. Although, in hindsight, I agree with you, I am no expert in the jargon and I don't know of an authoritative dictionary, so it is better discussed here before restoring it. I understand your urgency, but prefer caution. --Hroðulf (or Hrothulf) (Talk) 14:46, 1 March 2010 (UTC)

Sadly, I resorted to Google again, and found just one radiological glossary.

• http://www.remm.nlm.gov/dictionary.htm

It doesn't use the word synthetic, and seems to prefer the word "artificial" without defining it. Since we write for a general audience, I will insert "artificial" into our article (pending better information or further discussion.)

--Hroðulf (or Hrothulf) (Talk) 15:08, 1 March 2010 (UTC)

Either one is acceptable, but I actually liked "synthetic" better, since it's clearly synthesized in a manmade nuclear reaction. Artificial is correct for the same reason, but artificial carries more of a connotation of "ONLY artificial", with the idea that the only source of it is industry, like Nylon. Of course all nuclides are produced in the universe somewhere, in stars. A few short-lived ones are even found on Earth, as decay products, like technetium itself, which as a name that means "artificial," but was later found in nature. SBHarris 17:55, 1 March 2010 (UTC)

Merge with generator article?

I propose that we merge the brief article Technetium-99m generator into the more general article Technetium-99m. Technetium-99m is quite a short article anyway, and already has a section on production of the species that is longer than the generator article. --Hroðulf (or Hrothulf) (Talk) 15:16, 1 March 2010 (UTC)

Technetium generators are complex things and an article on them has the potential to eventually be long enough to need is own article for length, per WP:SS. Let us not be in such a hurry to combine things on WP. If we did that, most stubs would disappear into any article that referenced them. But stubs and short articles are buds from which WP grows. Patience! Of course, you're welcome to put on a {proposed merge} tag and get comment from others. SBHarris 17:59, 1 March 2010 (UTC)

I agree with Sbharris. --mav (Urgent FACs/FARs/PRs) 18:57, 1 March 2010 (UTC)

Okay, as that's the only comment we've gotten in the month, I'm going to remove the merge suggestion flag. SBHarris 22:03, 28 March 2010 (UTC)

History

The history section only contains information on the shortages in 2009 and 2010 (I suggest that the actual years be specified because, if I am not mistaken, the years in question are not correctly described as "the late 2000s"78.48.121.170 (talk) 15:43, 31 May 2010 (UTC)). It would be interesting to include information on the discovery of the species, early production methods, and the introduction of imaging and therapeutic uses, if any volunteers can provide them. --Hroðulf (or Hrothulf) (Talk) 15:04, 25 February 2010 (UTC)

Additional references for history section: http://www.bnl.gov/bnlweb/pubaf/bulletin/1998/bb112098.pdf http://www.bnl.gov/bnlweb/history/tc-99m.asp 130.199.3.130 (talk) 18:17, 29 April 2010 (UTC)

Great. I will be interested to download them when I have more time, but do you think you can use the information yourself to expand the article? ---Hroðulf (or Hrothulf) (Talk) 20:34, 29 April 2010 (UTC)

Is low energy "safer"?

The article contains this sentence: "The isotope is also of a very low energy level for a gamma emitter. Its ~140 keV of energy make it safer for use because of the substantially reduced ionization compared with other gamma emitters." Though I've read that before, and I'm sure it could be supported by a cite, this explanation seems too simple to me. Sure, each photon carries less energy, but how many photons do you have to use? Does a lower energy photon cause less biological damage per photon? Is a lower energy photon just as detectable by the camera as a higher energy one? The absorption spectrum of human tissue and the sensitivity spectrum of the gamma camera come into play, and then you might have to look at different possible camera designs. From the bits and pieces that I know, it seems that the final conclusion is probably not wrong, it's just the explanation that's naive. Ultraviolet has much lower energies than x-ray, but using a tanning bed every day is probably more dangerous than getting a chest x-ray.--Yannick (talk) 12:45, 9 May 2012 (UTC)

If you had as much energy-deposited in your tissues from a chest X-ray as you do with any single UV tanning session, you wouldn't survive even one. Of course there are many issues here, and some of them you understand better than I do, apparently, since you've been explaining them to me over on the RBE article. But insofar as I can tell, this breaks down to several issues:

• Fractional energy deposited per beam energy. X-rays over about 100 keV go mostly "all the way through" (which is why they are used for imaging), as do therapeutic photons in the MeV range. By "all the way through," I mean that the differential absorption per volume of tissue is such a small fraction that there isn't much difference in dose/volume close to the source (or the skin where external beam enters) than at the skin where the radiation leaves. There is indeed a "gamma window" where a larger fraction of energy deposited makes beams of a given energy a little more dangerous in the 2-10 MeV range (this applies all the way through), but in practice all this is taken care of at the first step of calculating rads absorbed. This MeV window is due to pair-production which gives an extra absorption mechanism (the upper end is where cross section decreases again, at VERY high energies), but this doesn't affect RBE, since (as noted) it's all factored in at the rad calculation stage, before you ever consider RBE. For photons, the RBEs are all the same. So this by itself is not a factor in using Tc-99m, vs. isotopes like I-131 or F-18 that emit (directly or indirectly) hotter gammas.

• However, it is an issue in how much isotope (in Bq) to image. On the whole, gamma cameras are quantum detection devices, which detect single photons with about the same efficiency at all frequencies in the imaging range. Thus, you get the same image-quality from an isotope that emits N photons/sec of 364 keV (like I-131) as you do with one that emits the same N photons at 144 keV, since the detector sees them both equally well. But the beam of N photons at 364 keV has roughly 2.5 times the energy, so even at roughly the same photon RBE or Wr (which it should have) it's going to give you 2.5 times the dose in both rads and (therefore) sieverts, to get the same image. You gain in this manner by using radionuclides with lower energy gammas, so long as they are not so low energy that your cammera quantum detection begins to suffer.

• For any nuclide already diffusely inside the body (as for imaging), there is the matter of what fraction of the energy goes into photons, which only do a tiny fraction of the damage per energy that ingested high energy electrons do. Here RBE and local Wr are important, since you're looking at gamma vs high energy electron (in practice only beta or IC emission, as Auger electrons are so low energy that they seem to do damage only for nuclides directly attached to DNA). Decay channels determine dose for each type of radiation. Tc-99m is nice in having < 12% of emissions come out as internal conversion electrons, which have a high RBE and Wr and are completely absorbed, but they are only 12%-- the gamma emissions don't make an IC electron. However, an isotope like I-131, used often in imaging, decays >80% of the time to both 364 KeV gammas AND 600 KeV betas (mean) and those betas (one beta comes out for every gamma) push the equivalent dose way up per unit of imaging pixels. So Tc-99m gains there, not for the softness of its gamma, but for the relative purity of its gamma.

• Finally, Tc-99m gains inasmuch as its 6 h half life is one of the shortest in the business (only F-18 is shorter among commonly used imaging isotopes). This usually gives you a very small dose, since you'd like the half life for dose-only reasons to be as close as possible to the time it takes to do the scan, yet long enough that you can milk the isotope from the generator (or make it and transport it). Tc-99m is nearly perfect there (it would be even nicer if it had a 1 hour half-life, but then you wouldn't have time to use it except as the straight pertechnetate, maybe not enough time to do any radiotracer ligand chemistry like DOTAC-combining). This isn't discussed in the article, and probably should be. As you note, the article talks only about the softness of the Tc-99m gamma, and although that does cut the equivalent dose substantially over higher energy gamma-emitters, it's far from the whole story about why this isotope delivers less dose per image than any other. SBHarris 18:36, 9 May 2012 (UTC)

Thanks, that gives me quite a bit to chew on. I think I understand most of that, but I'm struggling with the fractional energy deposited per beam energy. I understand that it's not an RBE issue, but I'm not clear on what "differential absorption" is. Here's what I think you're saying: so long as the body is basically transparent to the beam energy, doubling the number of photons absorbed would just mean more contrast which means you could use half the photons for the same quality image. So any absorption variation with beam energy would have no net effect on safety, so long as it is not so great as to start obscuring the inner features. Did I get that right? The rest (gamma purity, quantum detection, half life) is quite clear. The benefit of short half-life is mentioned in the second paragraph of "Radiation exposure," though maybe it could be clearer.