PET scanning

Step 1 — Tracer. Short-half-life $\beta^+$ emitter (F-18 in fluorodeoxyglucose, $t_{1/2} \approx 110$ min) injected. Accumulates where metabolism is high (e.g. tumour, active brain).

Step 2 — Decay. Nucleus emits POSITRON. Positron travels few mm, slows, annihilates with nearby ELECTRON.

Step 3 — Annihilation. $e^+ + e^- \to 2\gamma$. Each photon has $E = m_e c^2 = 0.511$ MeV. By momentum conservation (initial system $\approx$ at rest), photons emitted at 180° (back-to-back).

Step 4 — Detection. Ring of detectors around patient. Pairs of detectors in COINCIDENCE — both photons arriving within a few ns → annihilation happened on the line connecting them.

Step 5 — Reconstruction. Many lines of response → 3D image of tracer concentration.

Functional imaging. Unlike CAT (structure), PET shows METABOLIC ACTIVITY → tumours, brain activity.

Cambridge tip. Always include 'coincidence' AND '0.511 MeV' AND '180°' in your description.