PET scanning
| English | Chinese | Pinyin |
|---|---|---|
| antimatter | 反物质 | fǎn wù zhì |
| PET scan | PET扫描 | PET sǎo miáo |
| positron | 正电子 | zhèng diàn zi |
| tracer | 示踪剂 | shì zōng jì |
| annihilate | 湮灭 | yān miè |
| coincidence | 符合 | fú hé |
Imaging with antimatter 反物质
- A PET scan PET扫描 lights up the body's most active tissue — like a tumour.
- Remarkably, it works using antimatter: positrons 正电子.
- It maps activity, not just structure.
PET scan route
Follow positron emission to a ring of detected photons.
The tracer 示踪剂
- A tracer is a $\beta^{+}$ (positron) emitter, attached to a molecule the body takes up (e.g. glucose).
- Active tissue absorbs more of it, so it decays more there.
A PET scan of the brain: warm colours (red, yellow) show where the tracer collected -- the most active tissue
A PET tracer is:
It is a β$^{+}$ emitter on a molecule (like glucose) that active tissue absorbs more of.
Annihilation
- The emitted positron meets an electron and they annihilate 湮灭 — mass becomes energy.
- Two photons fly off in opposite directions, each $511\ \text{keV}$ ($= m_e c^{2}$), conserving momentum.
When the emitted positron meets an electron, they:
Matter and antimatter annihilate: their mass turns into two photons.
Each photon from electron–positron annihilation has an energy of (in keV):
Each carries $m_e c^{2} \approx 0.511\ \text{MeV} = 511\ \text{keV}$.
The two annihilation photons travel in opposite directions.
The pair was almost at rest, so to conserve momentum the photons go back-to-back.
Coincidence 符合 detection
- A ring of detectors records the two simultaneous photons (a coincidence).
- That fixes the line the annihilation happened on; many lines build a 3-D map of activity.
Applying a p.d. deforms the crystal; squeezing it makes a p.d.
Two photons detected at the same instant — a ____ — fix the line the annihilation happened on.
Many such coincidence lines, from many angles, let a computer build a 3-D map of the tracer.
You've got it
- a PET tracer is a $\beta^{+}$ emitter taken up by active tissue
- positron + electron annihilate → two $511\ \text{keV}$ photons in opposite directions
- a coincidence of two photons fixes the line; many give a 3-D activity map