Biomedical Engineering Reference
In-Depth Information
One of the objectives pursued in the radiolabeling is the preservation of the
physical and chemical characteristics of the molecule, in order to maintain the
physiological behaviour. Therefore the isotopic substitution of an atom or a
chemical group comes out as an obvious solution. However, if this is accessible to
PET technology that makes use of positron emitters that are isotopes of biological
elements such as Carbon, Oxygen and Nitrogen, it is practically infeasible for
SPECT that employs gamma emitters such as Technitium or Thalium. Contributing
to the choice of a particular radionuclide, the half-life period, the energy, its decay
mode and its production process are also of importance. However, the radionuclides
of low atomic number that are gamma emitters and that can be employed in the
above-mentioned isotopic substitution are not suitable since the half-life periods are
much too long for use in conventional nuclear medicine. This problem does not exist
in PET, since the isotopes are positrons emitters and, frequently, present adequate
features for clinical purposes. Note that very short half-life periods could hinder the
radiolabeling and long half-life periods have adverse dosimetric effects.
Regarding the gamma photon energy, it must lie between 70 and 250 keV in
order to maximize the efficiency of the gamma camera. In PET, the energy of the
annihilation photons is constant and equal to 511 keV.
Gamma photons with low energy will produce a very small number of photo-
electrons when they interact with the detector's scintillator crystal. Since this is a
random process governed by Poisson statistics, it generates a large uncertainty in
the amplitude of the electric signals that are produced in the photomultiplier tubes,
with consequent degradation of the image contrast.
On the other hand, high-energy photons are more likely to penetrate the
collimator, corrupting the SPECT image quality and degrading spatial resolution
and contrast.
The importance of the decay mode is mainly related with dosimetric aspects
(except for the loss of spatial resolution due to the path of the positrons before
thermalization), since the emission of particles prior to the gamma radiation
increases the dose in patients without any benefit for the formation of the image.
The most commonly used radionuclide in conventional nuclear medicine is
technetium-99 m ( 99m Tc), whereas for PET it is fluorodeoxyglucose (18 F-FDG).
Technetium is the lightest chemical element (Z
43) with no stable isotope; none
of the technetium isotopes has a longer half-life of more than 4.2 million years
and therefore it is not surprising that technetium is found in nature only in trace
amounts. Although predicted by Mendeliev and named as ekamanganese , its name
has changed in 1937 to Technetium (from Greek " o which means artificial)
after the synthesis of the isotope 97. The 99m Tc is a radioactive metastable isotope
that emits gamma radiation with 140 keV with a half-life of 6.01 h. These features,
combined with the fact that there are commercial kits available for production, make
it the preferred radionuclide in conventional nuclear medicine. There are several
examples of radiotracers used in clinical routine based on 99m Tc, some of them are
shown in Table 1 .
D
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