Explain that, in PET scanning, positrons emitted by the decay of the tracer annihilate when they interact with electrons in the tissue, producing a pair of gamma‑ray photons travelling in opposite directions.
A Positron Emission Tomography (PET) scan is a medical imaging technique that shows how tissues and organs are functioning. It uses a radioactive tracer that emits positrons.
When the positron meets an electron, they annihilate each other. Think of it like two dancers (positron and electron) performing a perfect pirouette and then releasing two fireworks that shoot straight out in opposite directions.
Mathematically, the annihilation can be written as:
\$e^+ + e^- \rightarrow \gamma + \gamma\$
Each gamma photon has an energy of 511 keV.
In a PET scanner, detectors are arranged in a ring around the patient. When a pair of opposite photons hits the detectors simultaneously, a coincidence event is recorded.
Using the line of response (the straight line connecting the two detectors), the scanner reconstructs the location of the annihilation event. By collecting many such events, a 3‑D image of tracer concentration is produced.
Remember:
• Key terms: positron, electron, annihilation, gamma photon, coincidence detection.
• Formula to cite: \$e^+ + e^- \rightarrow \gamma + \gamma\$
• Why opposite directions matter: Conservation of momentum & energy.
• Visual aid: Draw a simple diagram with two arrows pointing opposite each other to illustrate the back‑to‑back photons.
• Practice question: “Explain how the conservation laws lead to the detection principle in PET.”
| Step | Process | Key Point |
|---|---|---|
| 1 | Tracer decays (β⁺) | Produces positron (\$e^+\$) |
| 2 | Positron meets electron (\$e^-\$) | Annihilation → two γ photons |
| 3 | Photons detected in coincidence | Reconstruct annihilation location |