An indole-based in vivo imaging agent is provided by the present invention that binds with high affinity to PBR, has good uptake into the brain following administration, and which has good selective binding to PBR. The invention also includes a precursor compound useful in the synthesis of the in vivo imaging agent of the invention, as well as a method for synthesis of said in vivo imaging agent comprising use of said precursor compound, and a kit for carrying out said method. Also provided is a cassette for automated synthesis of the in vivo imaging agent. Further aspects of the invention include a radiopharmaceutical composition comprising the in vivo imaging agent of the invention, and methods for the use of said in vivo imaging agent.