PET scanning
Imaging with antimatter
- A PET scan lights up the body's most active tissue — like a tumour.
- Remarkably, it works using antimatter: positrons.
- It maps activity, not just structure.
The tracer
- 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.
Practice
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 — mass becomes energy.
- Two photons fly off in opposite directions, each $511\ \text{keV}$ ($= m_e c^{2}$), conserving momentum.
Practice
When the emitted positron meets an electron, they:
Matter and antimatter annihilate: their mass turns into two photons.
Practice
Each photon from electron–positron annihilation has an energy of (in keV):
Each carries $m_e c^{2} \approx 0.511\ \text{MeV} = 511\ \text{keV}$.
Practice
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 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.
Practice
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
Key idea
- 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