The development of detector systems for in vivo imaging of compounds labeled with the accelerator-produced short-lived P+-emitting radionuclides nC, 15O, 13N, and 18F, applicable in clinical diagnosis, has been an incentive for the development of new tracer molecules. The sensitivity of the positron emission tomography (PET) technique and the possibility of performing non-invasive studies have thus opened up new ways of studying in vivo biochemistry and pharmacology in man.

In the past several years, commercial networks for the delivery of tracers such as 2-[18F]fluorodeoxyglu-cose (FDG) and other 18F-labeled compounds have increased the clinical usage of PET. However, it is clear that the PET technology has a wider potential, and that additional 18F-labeled tracers need to be developed and to be complemented by compounds labeled with other radionuclides. Carbon-11 especially, with a half-life short enough to allow repeated PET investigations on the same subject within short time intervals, but long enough to perform multi-step synthesis, has proven to be a useful alternative. There are, however, limitations for the development of PET technology related to tracer production with the short-lived nC, 15N and 15O for clinical applications. Today, tracers containing these radionuclides can be used only when there is access to in-house production facilities, and such sites benefit from the experience of a research-oriented background. There is thus a potential for further development of tracers and technology applicable in the clinical setting.

There is no doubt in our minds that the great potential of PET technology lies very much in the development of nC-labeled tracer molecules for routine applications because of the synthetic versatility of carbon. In this chapter we will illustrate some approaches to labeling synthesis and give examples of 11C tracers which have been applied in clinical PET studies. Many of the nC compounds used in clinical research have not been evaluated in clinical trials, and the future of the clinical use of PET technology will be dependent to some extent on the development of organizational structures where such trials can be performed routinely in an efficient way.

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