[Positron emission tomography].

Positron Emission Tomography (PET) is a method for determining biochemical and physiological processes in vivo in a quantitative way by using radiopharmaceuticals labeled with positron emitting radionuclides as C, N, O and F and by measuring the annihilation radiation using a coincidence technique. This includes also the measurement of the pharmacokinetics of labeled drugs and the measurement of the effects of drugs on metabolism. Also deviations of normal metabolism can be measured and insight in biological processes responsible for diseases can be obtained. 1. General introduction The idea of in vivo measurement of biological and/or biochemical processes was already envisaged in the 1930's when the first artificially produced radionuclides, which decay under emission of externally detectable radiation, of the biological important elements carbon, nitrogen and oxygen were discovered with help of the then recently developed cyclotron. These radionuclides decay by pure positron emission and the annihilation of positron and electron results in two 511 keV γ-quanta under a relative angle of 180 which are then measured in coincidence. This idea of PET could only be realized when the inorganic scintillation detectors for the detection of γ-radiation, the electronics for coincidence measurements and the computer capacity for data acquisition and image reconstruction became available. For this reason Positron Emission Tomography is a rather recent development in functional in vivo imaging. PET employs mainly short-lived positron emitting radiopharmaceuticals. The radionuclides employed most widely are: C (t1⁄2 = 20 min), N (t1⁄2 = 10 min), O (t1⁄2 = 2 min) and F (t1⁄2 = 110 min). Carbon, oxygen, nitrogen and hydrogen are the elements of life and the building stones of nearly every molecule of biological importance. However, hydrogen has no radioactive isotope decaying with emission of radiation which can be detected outside the human body. For this reason a fluorine isotope is often used as a replacement for a hydrogen atom in a molecule. Due to these short half-lives the radionuclides have to be produced in house, preferably with a small, dedicated cyclotron. Since the chemical form of the produced radionuclides can only be simple, input from organicand radiochemistry is essential for synthesis of the desired complex molecule. Input from pharmacy is required for the final formulation and pharmacokinetic studies and medical input is evident and required for application. Longer lived positron emitting radionuclides are sometimes commercially available or obtainable from research facilities with larger accelerators. Some examples of longer liver positron emitting radionuclides are Fe (t1⁄2 = 8.3 h), Co (t1⁄2 =17.5 h) and I (t1⁄2 = 4.2 d). Sometimes also positron emitting radionuclides can be obtained from a generator system. Examples are Rb (t1⁄2 = 76 s) from Sr (t1⁄2 = 25.5 d) and Ga (t1⁄2 = 68 m) from Ge (t1⁄2 = 270 d). Although all these radionuclides are used, the isotopes of the biological most important elements receive most attention. At the moment small dedicated cyclotrons are a commercially available product. These