A bifunctional compound or bifunctional molecule for use in an in vivo method of diagnostic molecular imaging, wherein (i) the bifunctional compound is represented by the formula: or salts thereof; wherein one of X and Y is C= O and the other is NR; wherein each m and p are independently selected from 0 to 6; wherein R 1 is a chelating group capable of chelating a radionuclide and is selected from: wherein R, R 2, R 3 and R 4 are independently hydrogen or an optionally substituted C 1-7 alkyl group; B is a linker group for linking the chelating group to the reactive group, and is represented by the formula: wherein each Q is independently selected from a group consisting of -NR 5 -, -C NR 5 -, - C O-, -NR 5 C NR 5 -, -NR 5 C NR 5 - and -O-, each R 5 is independently hydrogen or an optionally substituted C 1-7 alkyl group, each q and s are independently selected from 0 to 6 and each r is independently selected from 1 to 6; A* is a reacted reactive group that is coupled to T, T being a targeting group capable of binding to a target of interest in the subject; and wherein (ii) the bifunctional molecule comprises a bifunctional compound as defined above and a radionuclide bound through a chelating group of the bifunctional compound; and the method comprises the steps of: (a) administering the bifunctional compound or bifunctional molecule to the subject so that the bifunctional compound or bifunctional molecule binds to the target of interest; and (b) where a bifunctional compound has been administered, further administering an imaging probe composition comprising a radionuclide to the subject, so that the chelating group R 1 of the bifunctional compound chelates the radionuclide at the target of interest; and (c) detecting the radionuclide to image the target of interest in the biological system. (i) the bifunctional compound is represented by the formula: or salts thereof; wherein one of X and Y is C= O and the other is NR; wherein each m and p are independently selected from 0 to 6; wherein R 1 is a chelating group capable of chelating a radionuclide and is selected from: wherein R, R 2, R 3 and R 4 are independently hydrogen or an optionally substituted C 1-7 alkyl group; B is a linker group for linking the chelating group to the reactive group, and is represented by the formula: wherein each Q is independently selected from a group consisting of -NR 5 -, -C NR 5 -, - C O-, -NR 5 C NR 5 -, -NR 5 C NR 5 - and -O-, each R 5 is independently hydrogen or an optionally substituted C 1-7 alkyl group, each q and s are independently selected from 0 to 6 and each r is independently selected from 1 to 6; A* is a reacted reactive group that is coupled to T, T being a targeting group capable of binding to a target of interest in the subject; and wherein wherein one of X and Y is C= O and the other is NR; wherein each m and p are independently selected from 0 to 6; wherein R 1 is a chelating group capable of chelating a radionuclide and is selected from: wherein R, R 2, R 3 and R 4 are independently hydrogen or an optionally substituted C 1-7 alkyl group; B is a linker group for linking the chelating group to the reactive group, and is represented by the formula: wherein each Q is independently selected from a group consisting of -NR 5 -, -C NR 5 -, - C O-, -NR 5 C NR 5 -, -NR 5 C NR 5 - and -O-, each R 5 is independently hydrogen or an optionally substituted C 1-7 alkyl group, each q and s are independently selected from 0 to 6 and each r is independently selected from 1 to 6; A* is a reacted reactive group that is coupled to T, T being a targeting group capable of binding to a target of interest in the subject; (ii) the bifunctional molecule comprises a bifunctional compound as defined above and a radionuclide bound through a chelating group of the bifunctional compound; (a) administering the bifunctional compound or bifunctional molecule to the subject so that the bifunctional compound or bifunctional molecule binds to the target of interest; and (b) where a bifunctional compound has been administered, further administering an imaging probe composition comprising a radionuclide to the subject, so that the chelating group R 1 of the bifunctional compound chelates the radionuclide at the target of interest; and (c) detecting the radionuclide to image the target of interest in the biological system.