Positron emission tomography PET

This new technique utilises positron-emittting isotopes (radionuclides) bound to compounds of biological interest to study specific physiological processes quantitatively. Positron-emitting isotopes depend on a cyclotron for production and their half-life is short, thus PET scanners only exist on adjacent sites. This limits availability for routine clinical use but PET scanners provide valuable research information.

Each decaying positron results in the release of two photons in diametric opposition; these activate two coincidental detectors. Multiple pairs of detectors and computer processing techniques enable quantitative determination of local radioactivity (and density of the labelled compound) for each 'voxel' (a cube of tissue) within the imaged field. Reconstruction using similar imaging techniques to CT scanning produces the positron emission scan.

Each detector is linked to several others in a fan shaped distribution few millimetres

— positron release interacts

Y photon

with electron

Compound labelled with positron V photon emitting isotope

Radiation - ^rrrrr^ Each decayin9 P°sitron detectors activates only two

Radiation - ^rrrrr^ Each decayin9 P°sitron detectors activates only two

Each detector is linked to several others in a fan shaped distribution

coincidental detectors e.g. A and B

----Rejected by AB but coincident for CD

coincidental detectors e.g. A and B

----Rejected by AB but coincident for CD

Isotope

Binding compound

Measurement under study

"Oxygen "Oxygen '"Fluorine "Oxygen

"Carbon "Carbon

Carbon monoxide Water

Fluorodeoxyglucose Oxygen

Drug, e.g. phenytoin Methyl spiperone inhalation Cerebral blood volume (CBV)

■ i.v. bolus Cerebral glucose metabolism (CMRgl) inhalation Cerebral oxygen utilisation (CMR02)

Oxygen extraction factor (OEF)

■ i.v. bolus Dopamine binding site

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