9-(4-(18F)Fluoro-3-(hydroxymethyl)butyl)guanine
18F-FHBG is a fluorine-18-labeled acycloguanosine derivative substrate for herpes simplex virus type-1 thymidine kinase (HSV1-tk). 18F-FHBG is used as a reporter probe to image the expression of the herpes simplex virus type-1 thymidine kinase (HSV1-tk) gene in gene transfer therapy. HSV1-tk and HSV1-tk-metabolized 18F-FHBG co-localize, allowing positron emission tomography (PET) localization of HSV1-tk gene-transfected tissue and the assessment of gene transfer efficiency.