## Silicon-[\({}^{18}\)F]Fluorine Radiochemistry: Basics, Applications and Challenges  
**[PERSON] 1,*, [PERSON] 2, [PERSON] 2, [PERSON] 2, [PERSON] 3 and [PERSON] 2,*
**  
**Abstract**  
The \(\beta^{+}\) emitting radionuclide (e.g., fluorine-18) is attached to a biomolecule. After the \(\beta^{+}\) particle is emitted, it travels within the tissue, depending on its \(\beta^{+}\) energy. After coming to rest, the \(\beta^{+}\) forms a hydrogen atom-like system (with a positron instead of a proton center) with an electron from the surrounding environment and finally disintegrates under emission of two diametrically opposed gamma rays of 511 keV. These photons can be detected in \"coincidence\" (meaning at the same time within a certain time frame) and used for reconstruction of a 3D image of the radioactive decay and thus tracer accumulation. **PET principle**  
A positron is the antimatter counterpart of an electron and it can annihilate with an electron of the environment under full conversion of the positron-electron masses into energy. The conversion energy which is equivalent to 2 electron masses (a positron weighs the same as an electron) is 1,022 keV. As a result of the conservation of energy and momentum at annihilation, two gamma rays of 511 keV are emitted in an angle of 180\({}^{\circ}\) which are capable of penetrating most materials, including living tissue (Figure 1). These can be detected simultaneously by two radioactivity detectors outside of the object/subject within a certain time window. A PET camera consists of a multitude of such detectors (usually bismuth germanium oxide (BGO) crystal detectors) arranged in a circular shape stretching  
Figure 1: A \(\beta^{+}\) emitting radionuclide (e.g., fluorine-18) is attached to a biomolecule. After the \(\beta^{+}\) particle is emitted, it travels within the tissue, depending on its \(\beta^{+}\) energy. After coming to rest, the \(\beta^{+}\) forms a hydrogen atom-like system (with a positron instead of a proton center) with an electron from the surrounding environment and finally disintegrates under emission of two diametrically opposed gamma rays of 511 keV. These photons can be detected in “coincidence” (meaning at the same time within a certain time frame) and used for reconstruction of a 3D image of the radioactive decay and thus tracer accumulation. over many layers and a subject (e.g., a human) can be placed inside that cavity. A computer system hooked up to these detectors registers coincidence detection of two gamma rays between two independent detectors at a time. In the end, after processing many of those events and subsequent data reconstructions, a 3D image of the object is obtained. The PET technique has been proven very useful in the detection of cancer, cardiovascular disease, metabolic disorders and for imaging of certain structures in the human brain as receptors or proteins which are associated with known afflictions, such as [PERSON]'s and Alzheimer's disease. One of the most useful radionuclides for PET imaging is \({}^{18}\)F. With a half-life of 109 min and low positron energy of 640 keV, \({}^{18}\)F can yield PET images of high resolution when PET scanners of the newest generation are used. In order to take advantage of the favorable physical characteristics of \({}^{18}\)F, methods have to be found to stably introduce \({}^{18}\)F into any useful molecule serving a biological purpose such as binding to a specific brain receptor or accumulating in tumor tissue. Until recently, \({}^{18}\)F was only incorporated into organic compounds by electrophilic or nucleophilic reactions resulting in the formation of a carbon-\({}^{18}\)F bond [2, 3, 4]. The problem is that both electrophilic (e.g., using [\({}^{18}\)F]F\({}_{2}\)) and nucleophilic (using [\({}^{18}\)F]F\({}^{\prime}\)) reactions are not 100% specific and other radioactive products besides those intended are formed.