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GCSE Physics · key term
The process in which a particle and its antiparticle meet and their mass is converted into energy; in PET, a positron and electron produce two gamma-ray photons.
Used by Edexcel
Specification 6.34P · Paper 1
Revise: Medical DiagnosisAfter emission, the positron travels a short distance through tissue and encounters an electron. They undergo annihilation: the positron and electron disappear and their energy is released as two gamma-ray photons. These photons travel away in opposite directions.
For a PET question, describe the complete chain: positron–electron annihilation produces two gamma rays travelling in opposite directions; radiation detectors detect the gamma rays; a computer uses detections from many events to locate the source and construct an image.
Do not say that the PET scanner detects positrons. The positrons travel only a short distance in tissue; the scanner detects the gamma rays produced after annihilation.
Answer the stated question directly. If asked how a tumour is located, lengthy detail about administering the tracer will not replace an explanation of annihilation, detection and location.
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