The present invention provides radiolabeled trimethoprim which are useful in imaging tests such as PET scans. The compounds show robust bacterial uptake in vitro and identify infections from inflammation or tumor when administered to a subject. The compounds show rapid and sensitive detection of Ec DHFR containing tumors from control tumors and background tissue. In one aspect, a compound having the structure of formula (I) is provided or a pharmaceutically acceptable salt or prodrug thereof, wherein R is defined herein. Also provided are compositions containing these compounds, positron emission tomography reporter probe comprising these compounds, and methods of imaging a bacterial infection, tracking or monitoring bacteria, distinguishing a bacterial infection from inflammation or tumor, monitoring genetically fused protein expression, and monitoring genetically engineered cells in clinical scenarios such as immunotherapy for cancer treatment.