Introduction to PET Physics



[Contents] [Section 1] [Section 2] [Section 3] [Section 4] [Section 5] [Section 6]


1. Positron Emission Tomography (PET) in Medical Imaging.
 

Positron Emission Tomography (PET) is a radiotracer imaging technique, in which tracer compounds labelled with positron-emitting radionuclides are injected into the subject of the study. These tracer compounds can then be used to track biochemical and physiological processes in vivo. One of the prime reasons for the importance of PET in medical research and practice is the existence of positron-emitting isotopes of elements such as carbon, nitrogen, oxygen and fluorine which may be processed to create a range of tracer compounds which are similar to naturally occurring substances in the body. Some examples of these radio-tracers are shown in table 1, together with some typical clinical and research applications.

Isotope

Tracer compound

Physiological process or function

Typical application

Example reference

 11C

methionine

protein synthesis

oncology

Hellman et al (1994)

11C

flumazenil

benzodiazepine receptor antagonist

epilepsy

Burdette et al (1995)

 11C

raclopride

D2 receptor agonist

movement disorders

Antonini et al (1997)

13N

ammonia

blood perfusion

myocardial perfusion

Kuhle et al (1992)

15O

carbon dioxide

blood perfusion

brain activation studies

Kanno et al (1984)

 15O

water

blood perfusion

brain activation studies

Huang et al (1983)

 18F

Fluoro-deoxy-glucose

glucose metabolism

oncology, neurology, cardiology

Brock et al, 1997 (review)

18F

Fluoride ion

bone metabolism

oncology

Hawkins et al(1992)

 18F

Fluoro-mizonidazole

hypoxia

oncology - response to radiotherapy

Koh et al (1995)

Table 1. Examples of radiotracers and their applications.

While PET was originally used primarily as a research tool, in recent years it has come to have an increasingly important clinical role. The largest area of clinical use of PET is in oncology. The most widely used tracer in oncology is 18F-fluoro-deoxy-glucose (18F-FDG). 18F-FDG is relatively easy to synthesise with a high radiochemical yield (Hamacher et al 1986). It also follows a similar metabolic pathway to glucose in vivo, except that it is not metabolised to CO2 and water, but remains trapped within tissue. This makes it well suited to use as a glucose uptake tracer. This is of interest in oncology because proliferating cancer cells have a higher than average rate of glucose metabolism (Warburg 1931). 11C-methionine is also used in oncology, where it acts as a marker for protein synthesis.

PET has applications in cardiology, where 13N-NH3 is used as a tracer for myocardial perfusion. When 13N-NH3 and 18F-FDG scans of the same patient are interpreted together, PET can be used to distinguish between viable and non-viable tissue in poorly perfused areas of the heart (Marshall et al 1983). Such information can be extremely valuable in identifying candidates for coronary by-pass surgery.

In neurology, PET has been used in a range of conditions, and in particular in severe focal epilepsy, where it may be used to compliment Magnetic Resonance Imaging.

Another reason for the importance of PET lies in the fact that, unlike earlier radiotracer techniques it offers the possibility of quantitative measurements of biochemical and physiological processes in vivo. This is important in both research and in clinical applications. For example, it has been shown that semi-quantitative measurements of FDG uptake in tumours can be useful in the grading of disease (Strauss and Conti 1991). By modelling the kinetics of tracers in vivo it is also possible to obtain quantitative values of physiological parameters such as myocardial blood-flow in ml/min/g or FDG uptake in mmol/min/g providing the acquired data is an accurate measure of tracer concentration. Absolute values of myocardial blood flow can by useful in, for example, the identification of triple-vessel coronary artery disease and absolute values of FDG uptake can be useful in studies of cerebral metabolism.

 
 
 
 
 
 
 

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Last revised by:

Ramsey Badawi

Revision date:

12 Jan 1999