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Radiochemistry for Preclinical Imaging Studies
Published in George C. Kagadis, Nancy L. Ford, Dimitrios N. Karnabatidis, George K. Loudos, Handbook of Small Animal Imaging, 2018
Table 16.6 gives a brief summary of popular PET radioisotopes including their properties and production. Fluorine-18 is probably the most-favored radionuclide for PET imaging. This is due to its half-life of about 2 h, which is optimal for a range of applications. In addition, fluorine-18 has optimal radionuclear properties with emitted positrons of relatively low energy and high abundance. Fluorine-18 is usually obtained by cyclotron irradiation of an oxygen-18 enriched target. The common production method provides n.c.a. [18F] fluoride in water. Alternatively, although less widespread, [18F]F2 gas can be generated. The achievable concentration of 18F does not allow the formation of [18F]F2 molecules consisting of two 18F atoms. Therefore, this process depends on the addition of stable fluorine-19 carrier, reducing the specific radioactivity. Many PET applications demand a high specific radioactivity of the tracer to allow binding to molecular targets of low concentration such as brain receptors. Thus, the n.c.a.-based radionuclide production will be usually the method of choice.
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Published in Splinter Robert, Illustrated Encyclopedia of Applied and Engineering Physics, 2017
[atomic, biomedical, computational, general, imaging, solid-state] Therapeutic applications that use radioactive isotopes to create selective modifications to the biological DNA or cause cell death as well as rely on the selective incorporation of radioactive isotopes in a metabolic event to be used for imaging purposes. In imaging, the use of iodine isotope is one of the mechanisms that allow for monitoring the activity of the thyroid by means of a gamma camera. Another application in positron emission tomography imaging is the use of fludeoxyglucose isotope (active component: positron emitting fluorine-18), known as “fluorodeoxyglucose” as a radiopharmaceutical used as a glucose replacement for metabolic imaging. Cancer treatment uses radiopharmaceuticals that are designed to go directly to the organ considered for treatment, for example rhodium (105Rh) and rhenium-188, mostly used as beta emitters to inflict cell death. In therapeutic applications, the success will hinge on the target to nontarget differentiation of uptake of the peptide or other molecular structure (see Figure N.64).
Images from Radioactivity: Radionuclide Scans, SPECT, and PET
Published in Suzanne Amador Kane, Boris A. Gelman, Introduction to Physics in Modern Medicine, 2020
Suzanne Amador Kane, Boris A. Gelman
Just as with other radionuclide imaging procedures, in PET the person being scanned must ingest a low dose of a radiopharmaceutical. Because of their short half-lives, these radiopharmaceuticals must be artificially produced and the desired tracer molecule synthesized. Most of the radionuclides used in PET must be produced onsite using an expensive particle physics accelerator called a cyclotron (Figure 6.12a). Cyclotrons use electrical fields that alternate in polarity to accelerate protons (hydrogen nuclei) to extremely high energies. These protons are confined to a circular orbit by magnetic fields while they are boosted to high energies on the order of several MeV. The very energetic protons then are allowed to exit the cyclotron toward a target. The target contains stable nuclei of other elements that can undergo a nuclear reaction when bombarded with very energetic protons from the cyclotron. This process transforms the original nuclei into heavier, positron-emitting radioisotopes. Carbon-11, nitrogen-13, oxygen-15, and fluorine-18 all require a cyclotron for their production. The resulting radioisotopes can be transferred to a dedicated biosynthesizer unit, which swiftly performs any chemical reactions required to produce the desired radiopharmaceutical. The expense and complexity of a dedicated cyclotron facility once restricted the availability of PET. However, fluorine-18 has the additional significant advantage that its half-life is long enough to allow it to be distributed commercially, rather than produced onsite with a cyclotron. This fact, and the wide utility of fluorine-18 labeled radiotracers, has greatly expanded the scope of PET scanning.
Development of Tracer Particles for Positron Emission Particle Tracking
Published in Nuclear Science and Engineering, 2023
Thomas Leadbeater, Andy Buffler, Michael van Heerden, Ameerah Camroodien, Deon Steyn
Fluorine-18 has a 109.8-min half-life decaying by positron emission at 97% intensity with no associated gamma photon emissions.50 It forms very strong covalent bonds with carbon compounds and can be incorporated into a wide variety of organic molecules. The most widely used radiotracer in medical PET by far is 18FDG, having proven to be of great utility in the rate measurement of glycolytic metabolism.48 A different set of ion exchange reactions from those used for 68Ga enables different materials to be labeled with positron activity such that both the range of possible tracer particles and the useful lifetime are extended.