Positron emission tomography

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 to detect 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/noisy 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").

Physics

A radiolabelled bio compound such as 2-fluoro-2-deoxy-D-glucose (FDG) is injected intravenously.

Uptake of this compound followed by further breakdown occurs 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

Radionuclides decay in the body with the release of a positron, i.e. a beta particle. The beta particle travels a short distance and annihilates with an anti-particle (electron). The annihilation reaction results in the formation of two high energy photons which travel in 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 refered to as the line of response.

Procedure

• 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 standard 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 artefacts result in an inaccurate co-registration of the CT and PET studies.

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

Attenuation correction

When combined with CT, the CT imaging can be used to generate an attenuation map which is used to correct the PET imaging for attenuation. This attenuation correction can add a number of further artefacts.

• breathing
  • artefacts related to respiratory motion causes the 'mushroom effect' where an artefact is sometimes seen in the lung bases because of the different phases of respiratory motion
• implants and prostheses
  • metallic implants such as joint prostheses can create significant artefact on PET images as the attenuation correction cannot deal with/correct for markedly high densities
• truncation 
  • CT field of view is limited whereas PET field of view is usually larger; if patients are scanned with arms by their side this can lead to abnormal reconstruction of the images

Normal physiological uptake

• brain tissue
• skeletal muscle, especially after strenuous activity and laryngeal muscles following speech
• myocardium
• gastrointestinal tract, e.g. intestinal wall
• genitourinary tract: FDG is excreted via the renal system and passes into the collecting systems
• brown fat
• thymus ⁴
• bone marrow ⁵

False-positive FDG uptake

This may occur due to the following conditions:

• granulomatous disease
• abscess
• surgical changes
• foreign body reaction
• inflammation
• excessive bowel uptake with metformin therapy

Applications

• oncologic
  • detection, staging, drug response
  • differentiation between radiation necrosis and recurrence
• neurologic
  • early diagnosis of Alzheimer disease
  • localisation of seizure focus in interictal phase
  • localising eloquent areas (e.g. speech, motor function)
• cardiac
  • identification of hibernating myocardium