An imaging agent which comprises an 18 F-labelled compound of Formula I: where: X 1, X 2 and X 3 are independently Br, Cl, 19 F or 18 F, with the proviso that at least one of X 1, X 2 and X 3 is 18 F; M is Ga 3+, In 3+, Sc 3+ or Y 3+; Z 1 is of Formula Z a: where each R 3 is independently H, C 1-4 alkyl, C 2'-4 alkoxyalkyl, C 1-4 hydroxyalkyl, a Y group or a Q group;each f is 1; p is 1 or 2; y is 0; Y is independently -(A 1) x -Y 1 or -(A 1) x -Y 3 -Q where each A 1 is independently -CH 2 - or -O-, provided that Y does not comprise any -O-O- bonds; wherein x is an integer of value 1 to 6; and Y' is -NHR a, -NH(CH 2) 2 NHR a, -NH(CH 2) 3 NHR a, -(C=O)NHR a, -NH(C=O)R a, -NH(C=NH)NHR a, -OR a, a Y 2 group or a Y 3 group; Y 2 is: Y 3 is Arg, Lys, Asn, Gln, Ser, Thr or Tyr; wherein R a is independently H, C 1-4 alkyl, C 2-4 alkenyl, C 2-4 alkynyl, C 2-4 alkoxyalkyl or C 1-4 hydroxyalkyl; and wherein each R 1 is independently C 1-4 alkyl, C 2-4 alkoxyalkyl or C 1-4 hydroxyalkyl; and R 2 is independently H, C 1-4 alkyl or Si(C 1-4 alkyl) 3; Q is -L-; L is a synthetic linker group of formula - m - wherein each A is independently - CR 2 -,-CR=CR-, -C≡C-, -CR 2 CO 2 -, -CO 2 CR 2 -, -NRCO-, -CONR-, -CR=N-O-, - NR(C=O)NR-, -NR(C=S)NR-, -SO 2 NR-, -NRSO 2 -, -CR 2 OCR 2 -, -CR 2 SCR 2 -, - CR 2 NRCR 2 -, a C 4-8 cycloheteroalkylene group, a C 4-8 cycloalkylene group, -Ar-, - NR-Ar-, -O-Ar-, -Ar- -, an amino acid or a monodisperse polyethyleneglycol building block, wherein each R is independently chosen from H, C 1-4 alkyl, C 2-4 alkenyl, C 2-4 alkynyl, C 2-4 alkoxyalkyl or C 1-4 hydroxyalkyl; m is an integer of value 1 to 20 when the linker group comprises PEG or a peptide chain, otherwise L has a backbone chain of linked atoms which make up the - m moiety of 2 to 10 atoms; each Ar is independently a C 5-12 arylene group, or a C 3-12 heteroarylene group; BTM is a 3-100 mer peptide. X 1, X 2 and X 3 are independently Br, Cl, 19 F or 18 F, with the proviso that at least one of X 1, X 2 and X 3 is 18 F; M is Ga 3+, In 3+, Sc 3+ or Y 3+; Z 1 is of Formula Z a: where each R 3 is independently H, C 1-4 alkyl, C 2'-4 alkoxyalkyl, C 1-4 hydroxyalkyl, a Y group or a Q group;each f is 1; p is 1 or 2; y is 0; Y is independently -(A 1) x -Y 1 or -(A 1) x -Y 3 -Q where each A 1 is independently -CH 2 - or -O-, provided that Y does not comprise any -O-O- bonds; wherein x is an integer of value 1 to 6; and Y' is -NHR a, -NH(CH 2) 2 NHR a, -NH(CH 2) 3 NHR a, -(C=O)NHR a, -NH(C=O)R a, -NH(C=NH)NHR a, -OR a, a Y 2 group or a Y 3 group; Y 2 is: Y 3 is Arg, Lys, Asn, Gln, Ser, Thr or Tyr; wherein R a is independently H, C 1-4 alkyl, C 2-4 alkenyl, C 2-4 alkynyl, C 2-4 alkoxyalkyl or C 1-4 hydroxyalkyl; and wherein each R 1 is independently C 1-4 alkyl, C 2-4 alkoxyalkyl or C 1-4 hydroxyalkyl; and R 2 is independently H, C 1-4 alkyl or Si(C 1-4 alkyl) 3; Y' is -NHR a, -NH(CH 2) 2 NHR a, -NH(CH 2) 3 NHR a, -(C=O)NHR a, -NH(C=O)R a, -NH(C=NH)NHR a, -OR a, a Y 2 group or a Y 3 group; Y 2 is: Y 3 is Arg, Lys, Asn, Gln, Ser, Thr or Tyr; wherein R a is independently H, C 1-4 alkyl, C 2-4 alkenyl, C 2-4 alkynyl, C 2-4 alkoxyalkyl or C 1-4 hydroxyalkyl; and wherein each R 1 is independently C 1-4 alkyl, C 2-4 alkoxyalkyl or C 1-4 hydroxyalkyl; and R 2 is independently H, C 1-4 alkyl or Si(C 1-4 alkyl) 3; Q is -L-; L is a synthetic linker group of formula - m - wherein each A is independently - CR 2 -,-CR=CR-, -C≡C-, -CR 2 CO 2 -, -CO 2 CR 2 -, -NRCO-, -CONR-, -CR=N-O-, - NR(C=O)NR-, -NR(C=S)NR-, -SO 2 NR-, -NRSO 2 -, -CR 2 OCR 2 -, -CR 2 SCR 2 -, - CR 2 NRCR 2 -, a C 4-8 cycloheteroalkylene group, a C 4-8 cycloalkylene group, -Ar-, - NR-Ar-, -O-Ar-, -Ar- -, an amino acid or a monodisperse polyethyleneglycol building block, wherein each R is independently chosen from H, C 1-4 alkyl, C 2-4 alkenyl, C 2-4 alkynyl, C 2-4 alkoxyalkyl or C 1-4 hydroxyalkyl; m is an integer of value 1 to 20 when the linker group comprises PEG or a peptide chain, otherwise L has a backbone chain of linked atoms which make up the - m moiety of 2 to 10 atoms; wherein each R is independently chosen from H, C 1-4 alkyl, C 2-4 alkenyl, C 2-4 alkynyl, C 2-4 alkoxyalkyl or C 1-4 hydroxyalkyl; m is an integer of value 1 to 20 when the linker group comprises PEG or a peptide chain, otherwise L has a backbone chain of linked atoms which make up the - m moiety of 2 to 10 atoms; each Ar is independently a C 5-12 arylene group, or a C 3-12 heteroarylene group; BTM is a 3-100 mer peptide.