A preposition is a word that tells you where or when something is in relation to something else. Examples of prepositions include words like 'after', 'before', 'on', 'under', 'inside' and 'outside'.

compound close compound sentenceA sentence which has at least two main clauses joined together with a conjunction. The two, or more, clauses of the sentence still make sense on their own.

The positron emission tomography (PET) ligand, Pittsburgh Compound-B (PiB-C11), is the most well studied amyloid imaging agent, but the short half-life of carbon-11 limits its clinical viability.