Positron and Single photon Emission Tomography

The tracer usually chosen is 18F-fluorodeoxyglucose (FDG). It is a short half-life glucose analogue. It becomes concentrated in metabolically active tissues. PET can be combined with CT or MRI to provide high-quality images combining anatomy with physiology

About

• A test that identified structures by their rate of metabolism and complements other imaging methods
• Useful in the key investigations in malignancy, but also has a wide range of other uses.

Aetiology

• Positron Emission Tomography measures glucose uptake through the use of radiolabelled glucose
• Flourodeoxyglucose which is given to the patient.
• The glucose is taken up by areas of high metabolic activity
• FDG decays rapidly to produce a positron which encounters tissue
• It annihilates with an electron to produce a pair of high-energy photons which can be detected
• Areas of high metabolic uptake e.g. heart and brain are identified.
• Useful especially in identifying lung tumours and areas of acute inflammation.
• Can complement other imaging modalities such as CT or MRI.

Detects

• Lung tumours and extrathoracic metastases in bone and adrenals
• Useful for lung, melanoma, oesophageal as well as lymphoma
• Hot areas seen with TB and sarcoid
• Not useful for brain metastases as background metabolic activity too high
• Slow growing tumours e.g. bronchoalveolar carcinomas may be negative
• PET can also be used to image occult sources of infection eg. PUO
• New areas
  • 11C-labelled metomidate to detect tumors of adrenocortical origin
  • Somatostatin tracers in neuroendocrine tumours
  • Amyloid tracers in Alzheimer’s disease

High Uptake areas normally

• Brain, liver, kidney, bladder,
• Larynx, and lymphoid tissue of pharynx

Single photon emission computed tomography (SPECT)

• This is closely similar to PET but uses a radioisotope-labelled molecule detected by gamma cameras for detection.
• The images produced are of lower resolution than PET but the isotopes used are longer lived and more easily available.
• Examples include myocardial perfusion scanning