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[13N]Ammonia.

Authors

Cheng KT.

Source

Molecular Imaging and Contrast Agent Database (MICAD) [Internet]. Bethesda (MD): National Center for Biotechnology Information (US); 2004-2013.
2005 Dec 05 [updated 2007 Dec 04].

Excerpt

[13N]Ammonia ([13N]NH3) is a useful 13N-labeled compound that has been developed as a positron emission tomography (PET) imaging agent for assessing regional blood flow in tissues (1-3). The compound is labeled with 13N which is a positron emitter with a physical t½ of 9.965 min. [13N]NH3 was approved by the United States Food and Drug Administration in 2000 for PET imaging of the myocardium under rest or pharmacologic stress conditions to evaluate myocardial perfusion in patients with suspected or existing coronary artery disease. Ammonia is important in many metabolic activities of various organs and is involved in biochemical pathways leading to the production of amino acids, purines, and urea (4). Ammonia is produced in the body from the deamination of amino acids and the deamidation of amides (5). About 20% of urea produced in the body is converted to ammonia and carbon dioxide in the gut. Ammonia is absorbed and converted back to urea in the liver. It also plays a significant role in glutamine synthesis. Ammonia is produced from glutamine and other amino acids in the kidney. NH3, as a nonionic form, is freely permeable to all cell membranes (6). In an acidic environment, NH3 accepts a proton and exists as NH4+. With a dissociation constant (pKa) of 9.3, NH4+ constitutes about 99% of the total ammonia (NH3 + NH4+) concentration in the pH range of body fluids. As an ionized form, NH4+ is a relatively impermeable cation to cell membranes. The mechanism of cellular localization of [13N]NH3 is not entirely known. One known mechanism is cellular membrane diffusion of [13N]NH3 and then metabolic trapping of radioactivity with the conversion of ammonia to glutamine, glutamic acid and carbamyl phosphate (6-8). Ammonia labeled with 13N was first produced by Joliot and Curie (9, 10). The short t½ of 13N requires on-site cyclotron production of 13N and a short synthesis time of [13N]NH3. After i.v. intravenous injection, [13N]NH3 rapidly clears from the circulation. It is taken up mainly by the myocardium, brain, liver, kidneys, and skeletal muscle (2, 6, 11, 12). In the myocardium and brain, [13N]NH3 is removed from the blood and metabolically trapped within the tissues. The apparent linear relationship between distribution of [13N]NH3 and the regional blood perfusion makes feasible the use of this radiotracer for imaging and measuring cerebral and myocardial blood flows.

PMID:
20641280
[PubMed]
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