3 ms·

I am a physicist and you have it pretty close to correct, except reversed. The atom emits a positron (and in the process turns one of its protons into a neutron

I am a physicist and you have it pretty close to correct, except reversed. The atom emits a positron (and in the process turns one of its protons into a neutron), the positron annihilates with an electron and this produces opposite photon pairs which are detected.

heavyset_go 3y ago
Where does the electron come from, the body? Or is it generated in pairs with the positron or something?
- deleted 3y ago
  [deleted]
- eigenket 3y ago
  Yeah, the positron hits an ambient electron from somewhere in the environment, in a PET scan this is usually inside the body.
  - bonzini 3y ago
    ... which is why it's called ionizing radiation, removing an electron (or adding it in the case of beta decay) turns neutral atoms into ions.
    - eigenket 3y ago
      In general yes. In this specific case not entirely, because once the radiating nucleus has undergone positron emission it has turned one of its protons into a neutron, which means it now needs one less electron in order to form a neutral atom. The "spare" electron can wander off and replace (eventually, on average) the electron that gets annihilated by the positron.
platz 3y ago
How does a photon survive going through the body to make it to the detector? Seems like there is a lot of stuff in the way.
- eigenket 3y ago
  A photon from an electron-positron anihilation has frequency around 10^20 Hz, somewhat higher frequency than what we usually call x-rays. Some of them do get absorbed by the body, but enough pass through to be detected usefully.
- magicalhippo 3y ago
  Absorption of light by stuff is frequency dependent. At the right frequency, stuff can be almost entirely transparent. Similar to how by detecting infrared light JWST can peer through the dust in galaxies to reveal detail that the couldn't see in optical light using Hubble[1]. Electron-positron annihilation releases photons with an energy of roughly 0.5MeV[2], and as you can see from this graph[3] the absorption is relatively low at those energy levels. Unless I got my math wrong, about 90% of those photons survive traveling through 1 cm of water or fat (which has roughly the same density as water). [1]: https://jwst.nasa.gov/content/about/comparisonWebbVsHubble.html https://jwst.nasa.gov/content/about/comparisonWebbVsHubble.h... [2]: https://en.wikipedia.org/wiki/Electron%E2%80%93positron_annihilation#Low-energy_case https://en.wikipedia.org/wiki/Electron%E2%80%93positron_anni... [3]: https://physics.nist.gov/PhysRefData/XrayMassCoef/ComTab/water.html https://physics.nist.gov/PhysRefData/XrayMassCoef/ComTab/wat...
- dekhn 3y ago
  https://en.wikipedia.org/wiki/Mean_free_path https://en.wikipedia.org/wiki/Mean_free_path
gpderetta 3y ago
Yes, had to correct myself after checking Wikipedia. In retrospect there was no way a positron would make it out of the body into to the photodetector unscathed (or, better, unscattered).

heavyset_go 3y ago

Where does the electron come from, the body? Or is it generated in pairs with the positron or something?

deleted 3y ago
[deleted]
eigenket 3y ago
Yeah, the positron hits an ambient electron from somewhere in the environment, in a PET scan this is usually inside the body.
- bonzini 3y ago
  ... which is why it's called ionizing radiation, removing an electron (or adding it in the case of beta decay) turns neutral atoms into ions.
  - eigenket 3y ago
    In general yes. In this specific case not entirely, because once the radiating nucleus has undergone positron emission it has turned one of its protons into a neutron, which means it now needs one less electron in order to form a neutral atom. The "spare" electron can wander off and replace (eventually, on average) the electron that gets annihilated by the positron.

platz 3y ago

How does a photon survive going through the body to make it to the detector? Seems like there is a lot of stuff in the way.

eigenket 3y ago
A photon from an electron-positron anihilation has frequency around 10^20 Hz, somewhat higher frequency than what we usually call x-rays. Some of them do get absorbed by the body, but enough pass through to be detected usefully.
magicalhippo 3y ago
Absorption of light by stuff is frequency dependent. At the right frequency, stuff can be almost entirely transparent. Similar to how by detecting infrared light JWST can peer through the dust in galaxies to reveal detail that the couldn't see in optical light using Hubble[1]. Electron-positron annihilation releases photons with an energy of roughly 0.5MeV[2], and as you can see from this graph[3] the absorption is relatively low at those energy levels. Unless I got my math wrong, about 90% of those photons survive traveling through 1 cm of water or fat (which has roughly the same density as water). [1]: https://jwst.nasa.gov/content/about/comparisonWebbVsHubble.html https://jwst.nasa.gov/content/about/comparisonWebbVsHubble.h... [2]: https://en.wikipedia.org/wiki/Electron%E2%80%93positron_annihilation#Low-energy_case https://en.wikipedia.org/wiki/Electron%E2%80%93positron_anni... [3]: https://physics.nist.gov/PhysRefData/XrayMassCoef/ComTab/water.html https://physics.nist.gov/PhysRefData/XrayMassCoef/ComTab/wat...
dekhn 3y ago
https://en.wikipedia.org/wiki/Mean_free_path https://en.wikipedia.org/wiki/Mean_free_path

gpderetta 3y ago

Yes, had to correct myself after checking Wikipedia. In retrospect there was no way a positron would make it out of the body into to the photodetector unscathed (or, better, unscattered).