Positron emitters

Positron Imaging. Creating images of distributions of positron emitters requires a somewhat different type of apparatus. Positron cameras use many of the same technologies as do cameras for other isotopes, but there is a broader array of methods and physical arrangements. AH of these systems take advantage of the physical characteristics of positrons. 

The camera actually images the annihilation events, not the radioactive decay events directiy. Thus imaging of high energy positron emitters can have a limiting resolution owing to the range of the positron. 

Positron emission tomography (PET) makes use of a short-lived positron emitter such as fluorine-18 to image human tissue with a degree of detail not possible with x-rays. It has been used extensively to study brain function (see illustration) and in medical diagnosis. For example, when the hormone estrogen is labelled with fluorine-18 and injected into a cancer patient, the fluorine-bearing compound is preferentially absorbed by the tumor. The positrons given off by the fluorine atoms are quickly annihilated when they meet... 

C22-0113. One isotope of nitrogen and one isotope of fluorine are positron emitters with relatively long half-lives. 

Positron emission tomography (PET) is another form of imaging that uses positron emitters, such as nC, 13N, 150 and 18F. These isotopes are incorporated into chemicals that are taken up by tissue. When the isotopes decay, the emitted positron reacts with a nearby electron, giving off 2 gamma rays, which are detected and an image of the tissue is created. 

Keywords. [ F] Fluorine, [ F] Fluoride, Positron emitter. Synthesis, Radioligand... 

Physical Properties of Currently Used Positron Emitters. 204... 

The chemical reactions involving positron emitters have to be specially designed to take into account the short half life of the radionuclide, the limited number of labelled precursors and the sub-micromolar amounts of these precursors. Moreover, the reactions must be possible without any addition of the stable isotope (especially when ligands of receptors are synthesized). Several practical considerations that influence the design of positron-emitter radiotracers with a high specific radioactivity and their experimental handling have been reviewed [4,8,14-19]. 

Fig.1. PET Studies using positron emitters (number of hits per keyword)...

Eowler JS,Wolf AP (1986) Positron emitter-labelled compounds priorities and problems. In Phelps M, Mazziotta J, Schelbert H (eds) Positron emission tomography and anto-radiography principles and applications for the brain and heart. Raven Press. New York,... 

The radionuclide fluorine-18 and some general considerations concerning short-lived positron emitters... 

The position of fluorine-18 among short-lived positron emitters for PET... 

Design of radiotracers and radiopharmaceuticals labelled with a short-lived positron emitter The case of fluorine-18... 

Challenges in radiochemistry with short-lived positron emitters, including fluorine-18... 

THE RADIONUCLIDE FLUORINE-18 AND SOME GENERAL CONSIDERATIONS CONCERNING SHORT-LIVED POSITRON EMITTERS... 

From a chemical point of view, the half-life of fluorine-18 allows multi-step synthetic approaches that can be extended over hours. Fluorine-18 has therefore, in spite of its somewhat limited chemical repertoire, been effectively used for the labelling of numerous both relatively simple and complex bioactive chemical structures [3,5-9], including high-molecular-weight macromolecules such as peptides, proteins [10-13] and oligonucleotides [14-18]. General considerations on radiochemistry involving short-lived positron emitters will be discussed in Section 2.3. 

The design of radiotracers or radiopharmaceuticais labelled with short-lived positron emitters requires beside the inescapable selection of a chemical structure of interest to be labelled the choice of the radionuclide to be used. 

Most of the challenges associated with the handling of short-lived positron emitters are direct consequences of their physical properties, half-life and decay mode, from which also ensues very high maximum specific radioactivity and the associated practical minute amounts of material engaged in the radiosyntheses. 

J.S. Fowier, A.P. Woif, Positron-emitter-labeled compounds Priorities and problems, in M. Pheips, J. Mazziotta, H. Schelbert (Eds.), Positron Emission Tomography and Autoradiography Principies and Applications for the Brain and Heart, Raven Press, New York, 1986, pp. 391-450. 

Y. Shai, L. Kirk, M.A. Channing, B.B. Dunn, M.A. Lesniak, R.C. Eastman, R.D. Finn, J. Roth, K.A. Jacobson, F-labeled insulin A prosthetic group methodology for incorporation of a positron emitter into peptides and proteins, Biochem. 28 (1989) 4801 806. 

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