Radiopharmaceuticals for PET studies

Positron emission tomography (PET) or, with another interpretation, positron emitting tracers, is a tracer technique which provides a new means for measuring biological functions or biochemistry in vivo in research animals and humans. The technique utilizes short lived accelerator produced radionuclides, typically C-11, N-13, O-15 and F-18, with half-lives of 20.3, 10, 2 and 110 min, respectively. These radionuclides can be incorporated position specifically into a variety of tracer molecules. Upon PC emission, two annihilation photons are emitted at 180 degrees from each other, making it possible to measure quantitatively the radioactivity concentration, for example by a positron camera. Since some of these are radionuclides of elements common in biomolecules, the potential is very great for developing many tracer molecules by synthetic labelling chemistry. They are all produced by nuclear reactions using charged particle accelerators with protons or deuterons. Despite the short half-life, synthetic methods and techniques are available to produce radiopharmaceuticals labelled with some of these radionuclides in a controlled way, allowing routine production of a large number of radiotracers with application potential in many areas. For example, starting with the nuclear reaction N-14(p, a)C-11 using a proton beam on a nitrogen gas target, labelled C-11-carbon dioxide can be produced in high specific radioactivity of the order of several Ci/mu mol, which allows radiotracer applications in vivo at very low concentrations. The selection of the tracers has of course to be made in relation to the question addressed. The tracers can on some occasions be specifically designed to interpret the obtained PET data in a quantitative manner or to validate the models used in the quantification. Here, considerations regarding stereochemistry, the position of the label or the use of multiple labelling can give the most valuable information. The high specific radioactivity and the short half-lives allow the design of experimental protocols utilizing combinations of such short lived positron emitting tracer molecules in multitracer studies. In the paper some of the above mentioned points are discussed and examples given to illustrate that PET is really a technique of using positron emitting tracer molecules.