A c-Met binding cyclic peptide for use in an in vivo imaging method to assist 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 which comprises 18 F-radiolabelled said c-Met binding cyclic peptide; (ii) imaging using positron emission tomography at least one site of said cancer following step (i); (iii) making a determination from the imaging of step (ii) whether or not there is elevated uptake of said imaging agent at said site; (iv) when the determination of step (iii) shows elevated uptake, the cancer is deemed to overexpress c-Met, and anti-Met therapy is determined to be suitable for said patient; (v) when the determination from step (iii) shows no elevated uptake, the cancer is deemed not to overexpress c-Met, and anti-Met therapy is determined not to be suitable for said patient; wherein said c-Met binding cyclic peptide is an 18 to 30-mer cyclic peptide of Formula I: Z 1 -[cMBP]-Z 2 where: cMBP is of Formula II: - x -Q-(A') y - where Q is the amino acid sequence (SEQ-1): -Cys a -X 1 -Cys c -X 2 -Gly-Pro-Pro-X 3 -Phe-Glu-Cys d -Trp-Cys b -Tyr-X 4 -X 5 -X 6 - wherein X 1 is Asn, His or Tyr; X 2 is Gly, Ser, Thr or Asn; X 3 is Thr or Arg; X 4 is Ala, Asp, Glu, Gly or Ser; X 5 is Ser or Thr; X 6 is Asp or Glu; and Cys a-d are each cysteine residues such that residues a and b as well as c and d are cyclised to form 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 and y are independently integers of value 0 to 13, and are chosen such that [x + y] = 1 to 13; Z 1 is attached to the N -terminus of cMBP, and is H or M IG; Z 2 is attached to the C-terminus of cMBP and is OH, OB c, or M IG, where B c is a biocompatible cation; each M IG is independently a metabolism inhibiting group which is a biocompatible group which inhibits or suppresses in vivo metabolism of the cMBP peptide; wherein cMBP is labelled at the Lys residue of the A or A' groups with 18 F and wherein the anti-Met therapy functions by direct inhibition of the receptor c-Met, by interfering with HGF binding to c-Met or by inhibiting c-Met kinase activity. (i) administering to said individual patient an imaging agent which comprises 18 F-radiolabelled said c-Met binding cyclic peptide; (ii) imaging using positron emission tomography at least one site of said cancer following step (i); (iii) making a determination from the imaging of step (ii) whether or not there is elevated uptake of said imaging agent at said site; (iv) when the determination of step (iii) shows elevated uptake, the cancer is deemed to overexpress c-Met, and anti-Met therapy is determined to be suitable for said patient; (v) when the determination from step (iii) shows no elevated uptake, the cancer is deemed not to overexpress c-Met, and anti-Met therapy is determined not to be suitable for said patient; cMBP is of Formula II: - x -Q-(A') y - where Q is the amino acid sequence (SEQ-1): -Cys a -X 1 -Cys c -X 2 -Gly-Pro-Pro-X 3 -Phe-Glu-Cys d -Trp-Cys b -Tyr-X 4 -X 5 -X 6 - wherein X 1 is Asn, His or Tyr; X 2 is Gly, Ser, Thr or Asn; X 3 is Thr or Arg; X 4 is Ala, Asp, Glu, Gly or Ser; X 5 is Ser or Thr; X 6 is Asp or Glu; and Cys a-d are each cysteine residues such that residues a and b as well as c and d are cyclised to form 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 and y are independently integers of value 0 to 13, and are chosen such that [x + y] = 1 to 13; wherein X 1 is Asn, His or Tyr; X 2 is Gly, Ser, Thr or Asn; X 3 is Thr or Arg; X 4 is Ala, Asp, Glu, Gly or Ser; X 5 is Ser or Thr; X 6 is Asp or Glu; and Cys a-d are each cysteine residues such that residues a and b as well as c and d are cyclised to form 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 and y are independently integers of value 0 to 13, and are chosen such that [x + y] = 1 to 13; Z 1 is attached to the N -terminus of cMBP, and is H or M IG; Z 2 is attached to the C-terminus of cMBP and is OH, OB c, or M IG, where B c is a biocompatible cation; each M IG is independently a metabolism inhibiting group which is a biocompatible group which inhibits or suppresses in vivo metabolism of the cMBP peptide; where B c is a biocompatible cation; each M IG is independently a metabolism inhibiting group which is a biocompatible group which inhibits or suppresses in vivo metabolism of the cMBP peptide; wherein cMBP is labelled at the Lys residue of the A or A' groups with 18 F and wherein the anti-Met therapy functions by direct inhibition of the receptor c-Met, by interfering with HGF binding to c-Met or by inhibiting c-Met kinase activity.