Why is Positron Emission Tomography not deadly?

Positrons are anti-electrons–right?

As in…antimatter?

As in–if it touches ordinary matter, everything for a mile around disintegrates?

Yet we have Positron Emission Tomography–PET scans. In hospitals. Used on innocent children! Innocent AMERICAN children, goddammit!

C’mon, who’s pulling whose leg here! Are we supposed to believe you can generate positrons with something less than a multi-billion-dollar synchrotron? Yeah, right! Hey, I WATCH STAR TREK, you guys.

Or does the reputation preceding the humble positron over-estimate its abilities?

Actually, the fact that positrons annihilate with electrons is the basis for how PET works. The annihilations just aren’t energetic enough to be noticable, other than with the fancy detectors the hospital uses.

The particular radioactive atoms involved that emit the positrons when they decay aren’t readily found in nature. They thus have to be produced and this typically does involve bombarding other atoms, though on a somewhat smaller scale than in a typical high energy physics lab. More details.

A postiron has a mass of 0.511 MeV/c[sup]2[/sup] (the same as an electron obviously). In a low-enrgy collison the two particles mass are turned into energy (photons), which would be a total of a little over 1.022 MeV or 1.637*10[sup]-13[/sup] joules which isn’t very much energy at all, though the photons produce are Gamma rays (there are 2 photons produced in each anihilation)

In PET the positrons are a decay product of a certain nuclide

0.511 Mev is 1.95E-17 kilocalories per photon, or 11,744,850 kcal/mole, which is a good deal larger than the typical carbon-carbon bond strength of about 85 kcal/mole.

0.511 MeV would correspond to a frequency of about 10[sup]20[/sup] Hz which puts the photons well into the gamma range, but gamma radiation isn’t that harmful due to the fact that it isn’t easily absorbed.

I thought gamma radiation was the most deadly of all…

Okay, hopefully some more radiation savvy dopers will come to clarify, but here’s the gist as I understand it: The dangerousness of radiation is based on two things; the “punch” of the particles and their penetration. Alpha particles have a lot of punch, but can be stopped by your clothes or even the layer of dead skin surrounding your body. This makes them harmless unless you get an Alpha emitter inside your body, such as by swallowing or inhalation. Gamma radiation is fairly weak, but penetrates VERY well, hence the traditional thick lead shielding. Now, this penetrating ability can also reduce its danger to you, because it’s quite likely it’ll fly right through you and smack into the wall. So, to summarize the deal with Gamma radiation; very little of it will actually be interacting with you, and whatever does isn’t going to be doing a terribly large amount of damage.

Basically a dose of alpha or beta radiation is going to be a lot more harmful, but they are both easily (extremely easily in the case of alpha) protetced against. Gamma radfiation certainly can ionize your molecules causing potentially harmful effects, but it can also go straight through you.

PET is harmful, which is why there are a limited number of times medical professionals can use it before the cummulative risk of radiation becomes too dangerous. After about five or six sessions, it’s no longer permitted. PET also can’t be used with very young children whose brains are still growing rapidly.

Is this over the patient’s lifetime? I’m a little surprised at this, since there are private hospitals here that advertise PET for cancer screening as part of a full physical exam. I’d be interested in reading more about this rule (guideline?) if you have a cite.

There are private medical establishments that advertize yearly full-body CT scans, too. That doesn’t mean it’s healthy.

I don’t actually know precisely how full-body PET differs from brain scans using PET. As I recall, there’s a limited number of times PET brain scans can be done before the risk grows too great.

Hmmm… briefly scanning the web, I can find only a few references to limits on the number of times PET can be performed.

Here’s one: unfortunately, it’s not clear. I can’t tell whether this refers to the number of times scanning can be done in a session, or the limit on the number of times it can be done ever.

Alereon is basically correct, which is why there are multiple measures for radiation exposure. Some measure total energy, while others measure the net effect of the radiation on organisms.

For example, REMs (roentgen equivalent mammal) is a dose of radiation that produces the biological side effects of one roentgen’s worth of X-rays or gamma (both of which are extremely penetrating but relatively non-interacting). How the dose is received matters a great deal: alpha particles are essentially harmless outside of the body (they interact very strongly and so are stopped by the outer layer of skin, but are deadly if they’re emitted inside the body), so an equivalent amount of radiation might have a different REM rating depending on how it was administered.


Now y’see, folks, this just shows you can’t believe everything you read in comic books or see on STAR TREK.

I always thought:

(a) that anti-particles could ONLY be produced through high-energy collisions a la a cyclo-, synchro-, beva-, or other such “tron.” Never knew “ordinary matter” of certain types actually gave off positrons.

(b) that even a single collision of anti=particle with the particle it’s “anti” generated so much energy that everything and its grandmother blew up in a massive chain reaction. But it sounds like the antiproton is the real bugger, eh?


a) In beta decay an anti-lepton is always produced. In the above form of beta decay a positron and neutrino are produced and in the more familiar form an electron and anti-neutrino (though I’m sure I’m going to be reminded that the neutrino is possibly a self-conjugate particle, i.e. it might be it’s own antiparticle), thus the lepton quantum number is conserved (though this is not always the case as lepton number is almost certainly violated in the process of Hawking radiation).

b) It depends on the decay product: obviously the fact that the deacy product of the anilhation is not easily absorbed is one reason for there being no chain reaction (though the decay product of an electron-positron collision is not always two gamma protons, this is just the low-energy case). A proton-antiproton collision has roughly 2000 times more mass than an electron-positron collision and therefore 2000 times more energy, the products are mesons, mostly pions.

The main thing is the very small quantity of antimatter, if you were to meet an anti-person and anihilate with him, thewn the energy released would be huge.

I wouldn’t call it “ordinary matter.” PET uses radioactive isotopes which decay and emit positrons. While there are some radioactive isotopes found in nature (uranium and plutonium, for example), you need isotopes with short half-lives for medical use. Those can only be produced using nuclear reactors or accelerators. Isotopes used for PET scans are created using cyclotrons.