Year 9, Number 36, April 2007

 

PET/CT; State of the art and future prospects

 

 

PET radiotracers


Unstable nuclides which emit positron are used in PET imaging.  The positron has the same mass as an orbital electron but is positively charged.  A unique characteristic of the positron is that it can not exist at rest in nature. Once it loses its kinetic energy, the positron immediately combines with a negatively charged electron and undergoes annihilation reaction in which the masses of the two particles are completely converted into energy in the form of two 0.511-Mev annihilation photons, which leave their production site at 180 degrees from each other. The coincidence (simultaneous) detection of the two 511-keV gamma rays forms the basis for imaging with PET 67.

The biologic ubiquity of the elements available as positron emitters gives PET an unprecedented power to image the distribution and kinetics of natural and analog biologic tracers 8. The most commonly used radiotracer in clinical practice and the study of cancers is FDG labeled with 18F. The FDG actively fixes at the cellular level thanks to glucose transporters, is phosphorylated and no longer metabolized, and therefore remains trapped within the cell. Positron-emitting radionuclides with a very short half-life, such as oxygen-15, nitrogen-13 and carbon-11, can only be used at the production sites equipped with a cyclotron. 18F with a relatively  longer half-life of about 110 min can instead be distributed from the production site to other centers equipped with only a scanner and which are within two hours traveling distance. Other radiotracers contribute to the study of protein  metabolism (11C-methionine, 11C-tyrosine), the extent of cellular proliferation (18F-fluorothymidine) , the metabolism of membrane phospholipids (11C-choline), hypoxia (18F-fluo-romisonidazole), apoptosis (18F-annexin V) and angiogenesis (18F-rginine-glycine aspartic acid). Tumors take up PET radiotracers more intensely than healthy tissue and are therefore recognizable as areas of increased radioactive concentration 2.

 


Abstract | Introduction | PET radiotracers | PET and PET/CT scanners | Challenges with PET/CT | Main indications for PET/CT | Development prospects for PET/CT | Conclusions | References | Print

 

 

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