PET has a very high sensitivity and can register absorption of radiopharmaceutical agents in extremely low concentrations. Since the central atoms in biochemical compounds (carbon, oxygen, nitrogen) all have positron-emitting isotopes that can be produced in small hospital cyclotrons, it is possible to mark a number of central molecules such as oxygen, water, amino acids, various metabolites, hormones, and neurotransmitters.
For clinical PET, dextrose is usually used where a hydroxide group is replaced by 18F (18-flourine), a compound that is called 18F-FDG (flourine-18 labeled deoxyglucose). 18F-FDG has a high affinity for cells with increased metabolism, for example cancer cells. The substance is transported into the cells and phosphorylates glucose to 18F-FDG-phosphate, but no further break-down occurs. Because cell membranes are impermeable to phosphorylated deoxyglucose, an intracellular accumulation of the substance occurs.
- Small tumors ( < 0,5 cm) and tumors with low to moderate absorption can escape detection.
- Inflammatory conditions will produce increased absorption.
- For patients with diabetes (especially those requiring insulin) and non-fasting patients, high muscular absorption will reduce the sensitivity for tumor detection.
- Some tumor types have low FDG absorption (for example, prostate and bronchoalveolar carcinoma).
Sources of error
- Infections and inflammatory conditions (including post-operative changes) will result in increased absorption.
- Normally, the intestine can have a high absorption.
- Myocardium often displays high absorption, also in fasting patients.
- 18 F-FDG is excreted through the kidneys and FDG in the urinary tract can be misinterpreted.
- Absorption in brown fat tissue can be misinterpreted as a tumor if PET is not compared with CT. PET/CT combined in the same apparatus gives better specificity than PET alone.
- Radio-pharmaceutical agent: 18F-FDG is formed by radiating a heavier natural variant of oxygen with protons. This occurs in a cyclotron. Fluorine-18 (18F) is produced at the hospital cyclotron located at Rikshospitalet .