The synthesis, evaluation, and molecular modeling of a series of 18F-labeled 4- anilidopiperidines with high affinities for the μ-opioid receptor (μ-OR) are reported. On the basis of the high brain uptake and selective retention in brain regions that contain a high concentration of the μ-OR, combined with a good metabolic stability,[18F] fluoro-pentyl carfentanil ([18F] 4) and 2-(±)[18F] fluoropropyl-sufentanil ([18F] 6) were selected as the ...