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The letters PET stand for Positron Emission Tomography. It works by you ingesting a material that emits positrons. Positrons are antimatter electrons, they anni

The letters PET stand for Positron Emission Tomography. It works by you ingesting a material that emits positrons. Positrons are antimatter electrons, they annihilate with electrons in your body and produce EM radiation which is detected by the scanner machine.

gpderetta 3y ago
More specifically, the material emits high energy photons, that almost immediately "decay" into positron/electron pairs that travel at almost exactly opposite directions. Then detectors at opposite ends of the machine detect the electron and the positron, match the event pairs and can then reconstruct the line of travel and the emission depth (from the detection timestamp delta). IANAP, but did a bit of work on tomographic image reconstruction. edit: never mind, I got that wrong (it has been a while...): the material does emit a positron that decays into opposite photon pairs.
- eigenket 3y ago
  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).

gpderetta 3y ago

More specifically, the material emits high energy photons, that almost immediately "decay" into positron/electron pairs that travel at almost exactly opposite directions. Then detectors at opposite ends of the machine detect the electron and the positron, match the event pairs and can then reconstruct the line of travel and the emission depth (from the detection timestamp delta). IANAP, but did a bit of work on tomographic image reconstruction. edit: never mind, I got that wrong (it has been a while...): the material does emit a positron that decays into opposite photon pairs.

eigenket 3y ago
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).