Clinical Instrumentation
PET Camera
The PET is based on imaging through
the detection of positron-emitting elements. Positrons are positively charged
electrons emitted from the nucleus of some radioactive atoms.
Within nanoseconds these positrons
migrate a short distance into the
tissues and
their power dissipates,
losing its kinetic energy and annihilate colliding with negatrons.
This distance is the called Positron Range.
The destruction process converts the
positron - electron into two 511 keV photons each,
emerging in opposite
directions (180°) along a line. However, a small deviation of 0.5° to the
common geometric, can occur due to the kinetic energy remaining in the positron
at the time of annihilation. The range of positrons, reduces the resolution
because the annihilation occurs in a location other than the location of the
radionuclide.
Most of those positron-emitting elements
with clinical
applications are produced in a cyclotron. These positron-emitting marking
specific molecules are administered to patients intravenously and concentrate in
the organs and systems of interest within a short time. There are a variety of
radiotracers that can be used for these purposes. The most important are
presented in Table 1.
The most
widely used is labeled fluorodeoxyglucose fluorine-18 (F-18 FDG). FDG
is a glucose analogue molecule and differs chemically from it because it takes
into position 2 a F-18 atom instead of an OH group.
FDG is taken up by the
cell transporters and phosphorilized, but not metabolized. In this way its
concentration inside the cell is very
significant.
Also in recent years, the use of Ga-68 DOTATATE for neuroendocrine tumors has
grown increasingly. The DOTATATE is a
peptide analogue of somatostatin, whose receptors are found in high numbers in
the cells of these tumors.
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