Positron emission tomography (PET) is a modern non-invasive imaging technique for quantification of radioactivity in vivo. It involves the intravenous injection of a positron-emitting radiopharmaceutical, waiting to allow for systemic distribution and then scanning for detection and quantification of patterns of radiopharmaceutical accumulation in the body.

As with SPECT imaging, PET scan data can be reconstructed and displayed as a three dimensional image. This is in contrast to scintigraphy, which yields planar data which can only be used to create a two dimensional image.

Although the physiologic information afforded by PET and SPECT imaging is invaluable, the quality of obtained data is poor and limits imaging spatial resolution.  For this reason, PET and SPECT scans are often combined with CT imaging, allowing correlation between functional and anatomical imaging (“hybrid imaging”). More recently PET-MRI scanners have become available although their use remains limited and they are generally only found in the larger academic centres often in a research setting. 

Physics

A radiolabelled biological compound such as F-18 Fluorodeoxyglucose (FDG) is injected intravenously.

Uptake of this compound is followed by further breakdown occurring in the cells. Tumour cells have a high metabolic rate and hence this compound is also metabolised by tumour cells.

FDG is metabolised to FDG-6-phosphate which cannot be further metabolised by tumour cells and hence it accumulates and concentrates in tumour cells. This accumulation is detected and quantified.

Radiopharmaceutical detection

The positron emitting isotope administered to the patient undergoes β+ decay in the body, with a proton being converted to a neutron, a positron (the antiparticle of the electron, sometimes referred to as a β+ particle) and a neutrino. The positron travels a short distance and annihilates with an electron. The annihilation reaction results in the formation of two high energy photons which travel in diametrically opposite directions.

Each photon has an energy of 511 keV. Two detectors at opposite ends facing each other detect these two photons travelling in opposite directions and the radioactivity is localised somewhere along a line between the two detectors. This is referred to as the line of response.

Procedure

  • Patient should be fasting for 4-6 hours
  • blood glucose level <150 mg/dL
  • avoid strenuous activity 24 hours prior to imaging
  • avoid speech 20 minutes prior to imaging
  • the scan is carried out 60 minutes post-injection of FDG

In cases of fusion imaging such as PET-CT, the whole body CT scan is conducted first, followed by the whole-body PET scan and subsequently the two sets of images are co-registered.

A standardised uptake value (SUV) is calculated at the end of the study i.e. ratio of activity per unit mass tissue to injected dose per unit body mass.

Limitations

Motion artifacts result in an inaccurate anatomical coregistration of the CT and PET studies. The distance (2-3 mm) the positron travels before annihilation and the detector element size both contribute to relatively poor spatial resolution.

Physiological muscle uptake usually appears symmetrically and diffusely on PET imaging.

Applications