ligands which are useful for imaging and quantifying the pde10a enzyme using positron emission tomography application_number US 201113890705 A description the present invention relates to radiolabelled compounds their production and the use of said compounds for pet imaging in the following embodiments of the invention are disclosed the first embodiment is denoted e1 the second embodiment is denoted e2 and so forth comprising at least one isotope selected from the group consisting of c 15 o and 18 f or a salt or a hydrate form of the compound r1 is a radiolabelled group comprising at least on isotope selected from the group consisting of 11 c 15 o and 18 f and r2 r7 are individually selected from h f methyl or methoxy one or more of r2 r7 is a radiolabelled group comprising an isotope selected from the group consisting of 11 c 15 o and 18 f or one or more of r2 r5 is 18 f and r1 is selected from h c1 c6 alkyl such as methyl ethyl 1 propyl 2 propyl isobutyl c1 c6 alkyl c3 c8 cycloalkyl such as cyclopropylmethyl c1 c6 hydroxyalkyl such as hydroxyethyl fluoropropyl fluoroethyl and fluorobutyl e4 the compound of e1 wherein r3 r4 and r5 are.