13N-ammonia is used to measure what in PET?

• A. Blood flow/perfusion
• B. Glucose metabolism
• C. Protein synthesis
• D. Calcium deposition

Answer: (A)

Measuring tissue blood flow, or perfusion, with PET relies on tracers whose uptake reflects how much blood is reaching a region. 13N-ammonia behaves this way: it rapidly crosses into cells from the bloodstream, and in the heart it becomes effectively trapped as it is converted to glutamine. The amount of ammonia taken up in each region mirrors the local blood flow during the imaging window, making it a good tool for assessing myocardial perfusion and detecting ischemia or infarction. This is different from tracers used to image metabolism, like FDG for glucose use, or tracers used for protein synthesis or calcium-related processes, which rely on different biological pathways. The short half-life of 13N (about 10 minutes) also means it must be produced on-site at a cyclotron facility for timely imaging.