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PET Imaging Overview
The Origin of the PET Signal

Positron emission tomography (PET) is a functional nuclear medicine imaging modality based upon the detection of energy emitted when a positron produced by decay of a radioactive tracer is annihilated in a  collision with an electron. This energy is emitted in the form of two gamma rays emitted at 180o to each other with an energy of 511 keV.  The gamma rays are detected with a dedicated PET camera system comprised of a ring of detectors (Figure 1).

 Figure 1. Schematic representation of the principles of PET

The ring design exploits the fact that that two photons detected in close temporal proximity by detectors separated by 180o in the ring are likely to have originated from a single annihilation event in the sample somewhere along a line between the two detectors. This simultaneous detection is termed a “coincidence” event. All of the coincidence events detected during an imaging period are recorded and the raw data is reconstructed to produce cross-sectional images.

PET Isotopes

Some positron emitters and their half lives are given in Table 1. It is clear from Table 1 that isotopes that decay be positron emission are typically very short lived, meaning that efficicient operation of  PET imaging facility requires local production of isotopes. Isotopes for the Small Animal And Materials Imaging Core Facility are produced at the Winnipeg Cyclotron Facility on the Health Sciences Centre campus. 

 

Isotope

t1/2

15O

123 sec

13N

9.96 min

11C

20.4 min

68Ga

70 min

18F

110 min

64Cu
12.7 h
86Y
14.7 h
76Br
16.2 h
89Zr
78.5 h
124I
100.3 h

 

 Table 1. Positron emitting radionuclides

PET isotopes may be incorporated into a variety of tracers, depending upon the nature of the isotope and the physiological process under investigation (See Table 2). The most common clinical application of PET is the detection of increased metabolic activity by imaging the distribution of the PET tracer 18F-fluorodeoxyglucose (18F-FDG). FDG is a non-metabolisable analogue of glucose that is taken up by cells by the same transport mechanisms used to to take up glucose. FDG therefore accumulates in metabolically active tissues. Imaging the distribution of the PET analogue of FDG, 18F-FDG, allows investigators to asses variations in metabolism in various tissues. Such imaging has proven valuable in localising elevated metabolic activity associated with proliferating tumours and inflammatory responses

 

Process Type Tracer
Blood flow/perfusion

Diffusible

H215O

Blood Volume

11CO

Metabolism

Glucose 18F-fluorodeoxyglucose (FDG)
Osteoblastic activity 18F-

Proliferation

18F-fluorothymidine (FLT)

Receptor

Dopaminergic

18F-fluoro-L-dopa,

11C-raclopride

Serotonergic

11C-altanserin

 

 Table 2. Physiologically useful PET tracers.

PET isotopes incorporated into ligands for clinically important cellular receptors can be used to determine the distribution and density of such receptors, as well as to assess the affinity of ligands for receptors For example 18F labelled L-Dopa can be used to localise dopaminergic receptors. In addition to diagnosis, PET imaging can therefore also be used to study the efficacy and pharmacokinetics of therapeutics.