Fluorine-18 is a positron-emitting radioisotope. The application of 18F-FDG was aimed for mapping glucose metabolism in tumors considered to be highly glucose consuming. The major use of 18F-FDG subsequently emerged in the detection, staging, and treatment response monitoring of various types of cancers. Currently, in PET studies, the use of 18F-FDG accounts for the majority of all images performed with radiopharmaceuticals. A number of other fluorine-18-labeled radiopharmaceuticals have been developed, and many more are under clinical investigations.

Increasing clinical demand for 18F-FDG has triggered technological advances in various fields such as accelerator technology, radiochemistry, automated processing modules, detector systems, and imaging software.

Dispensers and injectors perform the most critical steps in the radiopharmaceutical handling processes. In Fig. 6.1 the equipment for loading, dispensing, and calibrating the dose of F18-FDG ready for injection for PET/CT imaging is presented.

The use of 18F-FDG in endocrinology was not particularly interesting, until the experience brought the necessary knowledges, especially in endocrine cancers. Nevertheless PET studies are still indicated in well-defined clinical situations, due to some less invasive and more accessible methods that might be used in the frequent pathologies, such as those of the thyroid gland; regarding the diagnosis of the endocrine tumors, the role of PET/CT is clear and will be discussed in the following chapters of the book.

F18-Fluorocholine PET/CT appears to be a promising, effective imaging method for localization of hyperfunctioning parathyroid tissue. The performance of 18F-FCH PET/CT was superior to Tc-99m MIBI standard methods, particularly in patients with multiple lesions or hyperplasia. The tracer was first used in prostate cancer, where the mechanism consists in the following mechanism: FCH enters cell through choline transporters with accumulation in tumors in part due to malignancy-induced overexpression of choline kinase (CK) that catalyzes the phosphorylation of choline to form phosphorylcholine followed by generation of phosphatidylcholine in the tumor cell membrane.

The lack of availability and the costs are limiting seriously the use of this tracer in parathyroid pathology, being necessary larger studies to conclude on the superiority of the method, over the standard MIBI imaging.

Gallium-67 is produced by bombardment of Zn-68 in a cyclotron. Ga-67 decays by electron capture to Zn-67, with a half-life of about 78 h. There are three gamma emissions at 93, 185, and 300 keV. Following intravenous injection, gallium distributes widely in the body. It binds to circulating proteins, fact that leads to long persistency in the plasma. Gallium has a wide normal distribution throughout the soft tissues, mainly in the liver, spleen, bone, bone marrow, and bowel. There is a rapid uptake in the zones with high rate of inflammation, due mainly to the increased capillary permeability occurring in these processes.

Somatostatin receptors are expressed by many neuroendocrine and non-neuroendocrine cells of the body, so different organs may be imaged by somatostatin receptor scintigraphy, including the liver, spleen, pituitary gland, thyroid, kidneys, adrenal glands, salivary glands, stomach wall, and bowel.

Ga-67 citrate was considered the master radiopharmaceutical for tumor imaging and detection of inflammatory sites. Its role is highly affected after the introduction of PET tracers.

Gallium-68 (halftime is 68 min) is available from the Ge-68/Ga-68 generator. In terms of radiochemistry, labeling with metallic nuclides is based on chelating systems which are coupled to biomolecules or which have interesting biological properties themselves. A new tracer is Ga-68 DOTA (Ga-68-labeled 1,4,7,10-tetraazacyclododecane-N, N′,N″,N‴-tetraacetic acid). Ga-68 DOTA-conjugated peptides (DOTA-TOC, DOTA-NOC, DOTA-TATE) are rapidly cleared from the blood. Arterial activity elimination is bi-exponential, and no radioactive metabolites are detected within 4 h in serum and urine. Excretion is almost entirely through the kidneys.

Just recently the Ge-68/Ga-68 generators and the DOTA-conjugated peptide obtained a marketing authorization, until now, and they have to be prepared taking into account national regulations and good radiopharmaceutical practices (GRPP) as outlined in specific European Association of Nuclear Medicine (EANM) guidelines. PET with Ga-68-DOTA-conjugated peptides has brought dramatic improvement in spatial resolution and is being used more and more often in specialized centers. All of them can bind to some of the somatostatin receptors, and they also present different affinity profile for other somatostatin receptor subtypes. In particular, PET clearly offers higher resolution and improved pharmacokinetics as compared to somatostatin receptor scintigraphy, with promising results for the detection of somatostatin receptor expressing tumors, and provides prognostic information and selection for targeted specific treatments.