Topic 11: PET/CT — Principles, Radiopharmaceuticals, Indications and Research
PET/CT stands for Positron Emission Tomography/Computed Tomography.
It is a hybrid imaging technique that combines:
- PET → functional and metabolic information
- CT → anatomical and structural information
The two scans are acquired during the same examination and then fused, allowing the physician to determine:
- Whether abnormal metabolic activity is present
- Where exactly that abnormal activity is located anatomically
PET/CT is widely used in:
- Oncology
- Neurology
- Cardiology
- Infection and inflammation imaging
- Research and development of new molecular tracers
BASIC PRINCIPLE OF PET
PET uses radiopharmaceuticals labeled with positron-emitting radionuclides.
Examples:
- Fluorine-18
- Gallium-68
- Carbon-11
- Nitrogen-13
- Oxygen-15
After administration, the radiopharmaceutical distributes in the body according to its biochemical or receptor-specific properties.
The radionuclide then undergoes positron decay.
POSITRON DECAY
A proton inside the unstable nucleus changes into a neutron.
The positron is a positively charged particle with the same mass as an electron.
ANNIHILATION
After emission, the positron travels a short distance through tissue.
It then meets an electron.
The positron and electron destroy each other in a process called: Annihilation
This produces:
- Two gamma photons
- Each photon has an energy of 511 keV
- They travel in nearly opposite directions, approximately 180° apart
This is the basic physical principle of PET imaging.
COINCIDENCE DETECTION
The PET scanner contains a ring of detectors around the patient.
When two 511 keV photons strike opposite detectors almost simultaneously, the scanner recognizes this as a true annihilation event.
This is called: Coincidence detection
The scanner assumes that the annihilation occurred somewhere along the imaginary line connecting the two detectors.
This line is called the: Line of response
Thousands to millions of coincidence events are recorded.
The computer reconstructs them into tomographic images.
PET DETECTOR SYSTEM
Unlike SPECT, PET does not use a mechanical collimator.
Instead, PET uses: Electronic collimation through coincidence detection
This contributes to PET having:
- Higher sensitivity
- Better spatial resolution
- Better quantitative accuracy than SPECT
The scanner contains multiple detector rings made from scintillation crystals.
Common detector materials include:
- Bismuth germanate
- Lutetium oxyorthosilicate
- Lutetium-yttrium oxyorthosilicate
The exact detector material depends on the scanner.