The 18F atome emits a positron which collides with a nearby electron and disintegrates, thus creating two diametrically opposed photons.
PetScan is a functional imaging technique which is more correlated to the function of an organ than to its morphology. As for scintigraphy, PetScan imaging is performed through the infusion of a radioactive tracer, the most frequently used to date being 18Fluoro-Desoxy-Glucose or 18FDG, whose distribution in the body is detected through a specific scanner: a coincidence camera.
The frequently used isotopes have a short half-life (from a few minutes to a few hours). During their isotopic disintegration, they emit one positron which is the anti-particle of an electron. This positron has a short penetration through matter (1 to 3 mm), before meeting its anti-particle, one electron. From this collision, a destruction reaction produces two photons which are emitted in diametrically opposite directions.
The following diagram, borrowed from the petscanonline.com website, explains for the production of two opposite photons by 18F.
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The 18F atome emits a positron which collides with a nearby electron and disintegrates, thus creating two diametrically opposed photons. |
The camera detects the two photons by simultaneous detection (coincidence detection). The camera detectors are located as a crown around the patient.
The 180° emitted photons are detected by camera crystals situated in opposite directions, and the precise point of atomic destruction (of emitted positron and electron) is then located on a line joining these two crystal detectors. By recording all of these lines in different directions, the PetScan computer system can then calculate the precise position of 18F uptake.
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18FDG (2-18F-2-desoxy-D-Glucose) is nowadays the most commonly used istope for clinical studies. It is a glucose molecule, a hydroxyl (OH) group in position 2 has been exchanged by a radioactive fluor atom (18F). The cancer cells, which have a more active metabolism than normal cells, need more glucose than normal cells. When 18FDG has penetrated the cell, it is not metabolised like glucose but stays longer in the cell. When undifferentiated cancer cells are very aggressive and multiply, they will uptake great quantities of 18F-Glucose.
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| This diagram shows the difference between a benign tumour (left image) and a malignant tumour (centre image) and with metastatic lesion (right image). Diagram courtesy of thePetScanOnline website |
This glucose increased metabolism can also be observed during infectious or inflammatory diseases or for very active cells like brain or muscle cells. On the contrary, certain malignant tumours may have a low glucose consumption, particularly low grade tumours or necrotic tumours. Therefore the specificity of PetScan is not absolute.
For French speaking readers may wish to consult a very interesting teaching site at petscan.fr which includes numerous illustrations.
Next pages are about:
Producing images with PetScan