A cyclic c-Met binding peptide for use in an in vivo imaging method to aid in the determination of whether an individual patient previously diagnosed with cancer is susceptible to treatment with anti-Met therapy, said method comprising: (i) administering to said individual patient an imaging agent comprising said cyclic c-Met binding peptide radiolabelled with 18F； (ii) create images using positron emission tomography (PET) at at least one site of said cancer after stage (i)； (iii) make a determination from the imaging of step (ii) of whether there is a high uptake of said imaging agent at said site or not； (iv) when the determination of stage (iii) shows high uptake, cancer is considered to be overexpressing c-Met, and it is determined that anti-Met therapy is suitable for said patient； (v) when the determination from step (iii) does not show high uptake, the cancer is considered not to overexpress c-Met, and it is determined that anti-Met therapy is not suitable for said patient； wherein said cyclic C-Met binding peptide is a cyclic peptide of 18 to 30 units of formula I: Z1- [cMBP] -Z2 (I) wherein: cMBP is of formula II - (A) xQ- (A ') and- (II) where Q is the amino acid sequence (SEQ-1): -Cysa-X1-Cysc-X2-Gly-Pro-Pro-X3-Phe-Glu-Cysd-Trp-Cysb-Tyr-X4- X5-X6- In which X1 is Asn, His or Tyr； X2 is Gly, Ser, Thr or Asn； X3 is Thr or Arg； X4 is Ala, Asp, Glu, Gly or Ser； X5 is Ser or Thr； X6 is Asp or Glu； and Cysa-d are each cysteine residues such that residues a and b, as well as c and d, cyclize into two separate disulfide bonds； A and A 'are independently any amino acid other than Cys, with the proviso that at least one of A and A' is present and is Lys； x e y are independently integers of value 0 to 13, and are chosen such that [x + y]>； = 1 to 13； Z1 is linked to the N-terminus of cMBP, and is H or MIG； Z2 is linked to the C-terminus of cMBP and is OH, OBc, or MIG, where BC is a biocompatible cation； each MIG is independently a metabolism inhibitor group, which
