PET scanning — common mistakes

When discussing PET scanning in exams, always ensure you mention the positron-electron annihilation and the 180-degree emission of two gamma photons due to conservation of momentum. Also, being able to calculate the 0.511 MeV energy of these photons using E=mc² is a key skill.

Cancer cells often have a higher metabolic rate than healthy cells, meaning they consume more glucose. The PET tracer, often a glucose analogue like FDG, accumulates in these 'hungry' cancer cells, making them light up brightly in the scan, helping doctors identify tumours.

Radionuclides used in PET, such as Fluorine-18 (F-18), typically have very short half-lives (e.g., F-18 has a half-life of about 110 minutes). This ensures the patient's exposure to radiation is minimised and the radioactive material decays quickly within the body.

Yes, other imaging techniques exist depending on the diagnostic need. MRI (Magnetic Resonance Imaging) and CT (Computed Tomography) scans provide detailed structural images. SPECT (Single-Photon Emission Computed Tomography) also uses radioactive tracers but detects single gamma rays emitted directly from the decay, not from annihilation, which generally results in lower resolution than PET.

PET provides excellent functional information (metabolic activity) but has poor spatial resolution. CT provides excellent anatomical detail but no functional information. By fusing the images from both scans, doctors get a single image that shows both the precise location (from CT) and the metabolic activity (from PET) of a lesion or tumour.