PET scan

Positron emission tomography (PET) is a nuclear medicine modality that uses positron emitters such as fluorine-18, oxygen-15, nitrogen-13, or carbon-11, and biologically active molecules. The fact that these nuclides are components of common biologic molecules makes PET particularly suitable for visually capturing various biologic pathways. Thus, PET images reflect the biochemical function of tissues and organs rather than their structure, producing information complementary to that obtained from CT and MR. It may have a useful role in helping to define the edge of ill-defined, metabolically highly active tumours such as gliomas of the brain.

Both CT and MR imaging may not yield much information regarding the involvement of regional lymph nodes, and often involvement can only be inferred when the node is significantly enlarged. In contrast, PET may help to determine whether a node of borderline size is likely to be involved, and it can even detect a tumour in nodes that appeared normal using CT or MR.

PET imaging relies on the simultaneous detection (a coincident event) of both photons which were created from annihilation of a positron. These two photons travel at the speed of light in exactly opposite directions (i.e., 180 degrees apart). Coincident detection of these two photons by two oppositely positioned detectors in the PET scanner results in images with a much higher resolution compared with the conventional, single-photon nuclear medicine studies and produces the possibility of quantitative measurement of the tracer uptake in a volume of interest. Because PET images are rather low in resolution compared to CT and MR, PET machines are frequently coupled with CT to provide a straightforward hardware registration of the two image sets.