1.1.5 PET/SPECT: Metabolism & Function
PET and SPECT are functional imaging modalities: beyond anatomy, they visualize metabolism, perfusion, and receptor distribution using radioactive tracers.
PET: coincidence detection and LOR
PET relies on positron annihilation:
- Inject a positron-emitting tracer (e.g., ¹⁸F-FDG)
- Positron annihilates with an electron
- Two 511 keV photons are emitted ~180° apart
- Coincidence detection forms a Line of Response (LOR)
Common tracers include ¹⁸F-FDG and ⁶⁸Ga-PSMA.
PET-CT / PET-MRI
Hybrid imaging combines:
- anatomical localization (CT/MRI)
- attenuation correction and improved quantification
A modern PET-CT scanner
SPECT: single photons + collimator
SPECT detects single gamma photons and uses a collimator to infer direction, trading sensitivity for directionality. It is widely used due to accessibility and mature clinical workflows (e.g., bone imaging, perfusion studies).