PET (Positron Emission Tomography)
Medical Imaging - OCR A-Level Physics
Key Definition
PET (Positron Emission Tomography)
An imaging technique that detects pairs of gamma photons produced by positron-electron annihilation to create 3D images of metabolic activity in the body.
An imaging technique that detects pairs of gamma photons produced by positron-electron annihilation to create 3D images of metabolic activity in the body.
- The patient is injected with a radioactive tracerA short-lived radioactive substance injected into the body; its gamma emissions are detected to image metabolic activity. that emits positrons (β⁺ decay). A common tracer is fluorodeoxyglucose (FDG) labelled with fluorine-18.
- The emitted positron travels a short distance (a few mm) before it annihilatesWhen a particle meets its antiparticle and both are destroyed, producing gamma photons (e.g. e⁻ + e⁺ → 2γ). with an electron, producing two gamma photons.
- The two gamma photons travel in opposite directions (180° apart), each with energyThe capacity to do work. Measured in joules (J). 511 keV (= rest mass energyThe capacity to do work. Measured in joules (J). of an electron, $E = m_{e} c^2)$.
- A ring of gamma detectors around the patient detects the two photons simultaneously (coincidence detection). The line connecting the two detectors identifies the line along which the annihilationThe process in which a particle and its corresponding antiparticle collide and convert their combined rest mass into energyThe capacity to do work. Measured in joules (J)., typically producing two gamma photons. occurred.
- Many such lines are combined by computer to produce a 3D image of tracer concentration.
- PET shows functional/metabolic activityThe number of nuclear decays per unit time. Measured in becquerels (Bq), where 1 Bq = 1 decay per second. (e.g. glucose uptake in tumours), not just anatomy. Often combined with CT to overlay function on structure (PET-CT).
$$\begin{aligned}
E &= 2m_e \(c^{2}\) \\
&= 2 \times 0.511 \\
&= 1.022 \text{ MeV}
\end{aligned}$$
Common Mistake
MEDIUM
Wrong: PET scanning detects positrons directly.
Right: PET detects the gamma photons produced by positron-electron annihilationThe process in which a particle and its corresponding antiparticle collide and convert their combined rest mass into energy, typically producing two gamma photons., not the positrons themselves. The positrons annihilate within mm of emission.
Right: PET detects the gamma photons produced by positron-electron annihilationThe process in which a particle and its corresponding antiparticle collide and convert their combined rest mass into energy, typically producing two gamma photons., not the positrons themselves. The positrons annihilate within mm of emission.