Biomedical Applications of Emission Tomography - Correction Techniques in Emission Tomography - page 15

FIGURE 2.1: (See color insert.) The tracer principle. Molecular targets
are typically addressed and visualized by injection of tracers, which are con-
sisting of a drug (light green) to which a flag (dark green; radioactivity, flu-
orescent dyes, quenched optical dyes, etc.) is attached. The tracer arrives in
organs via blood vessels, diffuses into the extracellular space and can bind to
externalized targets (orange; receptors, etc.) on the cell surface or cross the
cell membrane to bind to intracellular targets. By its flag the tracer emits light
or gamma rays to be detected from outside the organism by SPECT, PET or
optical imaging. Examples of isotopes and dyes used for SPECT, PET and
optical imaging are listed.
Many of the tracer approaches make use of amplification strategies to
optimize the target-to-non-target ratio. A prominent example is the tracer
fluorine-18-2-deoxy-2D-glucose (FDG) which is the most common and clini-
cally established [ 18 F] labeled tracer. It suces for many applications, with
the majority of studies performed for tumor imaging and imaging of inflam-
matory and neuronal pathologies as well as imaging of cardiovascular diseases.
Since FDG is a glucose analogue its uptake by cells is correlated to the rate
of glycolysis. Upon intravenous injection FDG follows the initial biochemical
route of glucose being taken up intracellularly by glucose transporters. As glu-
cose, FDG is phosphorylated by hexokinase to FDG-6-P in the cell. However,
in contrast to glucose, FDG-6-P is not further metabolized since it does not
undergo metabolism in the citrate cycle. As a result FDG is trapped intracel-
lularly. Since a single hexokinase enzyme can react \serially" with many FDG
molecules, the imaging signal is amplified.