PET scanning: the principles

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PET scanning: the principles

Positron emission tomography (PET) is a relatively recently introduced

technique whose full potential is probably yet to be realised. A PET camera

performs a similar function to a gamma camera in localising the distribution

of gamma rays introduced into the body. However, in contrast to gamma

cameras which work in two dimensional (2D) or three dimensional (3D) mode,

PET cameras operate only in 3D mode.

Positron emitting radionuclides are a special class of radionuclides whose

atomic nucleus lacks a balancing number of negatively charged electrons, so

that positron particles are continually being shed. When this occurs within

body tissues, the escaping positron is rapidly (usually within 1-2 mm of

travel) captured by spare electrons. In this process, called annihilation,

two gamma rays of identical energy are emitted at 180 degrees to each other.

Opposing detector systems within a circular array are coupled electronically

to register only gamma ray events that coincide with those on the opposite

side of the ring. This is called coincidence counting. The source of

radioactivity can then be pinponted to a straight line between the opposing

detectors. With a circular array of detectors the X and Y co-ordinates of

the radioactive emissions can be mapped with precision. Many common

biological molecules (oxygen, carbon, nitrogen and fluorine) have

positron-emitting isotopes, but an on-site cyclotron is required to produce

most of the radionuclides.

The strength of PET scanning is the ability to quantify the distribution of

radioactivity in terms of MBq per ml or grams of tissue. An increasingly

common clinical application is the use of PET scannning with

fluorine-labelled deoxyglucose (18FDG) in the diagnosis of lung cancer. This

is a sensitive screening test for malignancy with a high negative predictive

value in the assessment of lung nodules. The Hammersmith Hospital was the

first hospital in the UK to apply PET scanning in a clinical setting.

Research at the Hammersmith has pioneered clinical and academic studies of

PET, the results of which are now being applied throughout the country. It

continues to lead the way in national and international research in this

area.

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