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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