thiol - containing peptides can be radiolabeled with fluorine - 18 by reacting a peptide comprising a free thiol group with an 18 f - bound labelling reagent which also has a group that is reactive with thiols . the resulting 18 f - labeled peptides may be targeted to a tissue of interest using bispecific antibodies or bispecific antibody fragments having one arm specific for the 18 f - labeled peptide or a low molecular weight hapten conjugated to the 18 f - labeled peptide , and another arm specific to the targeted tissue . the targeted tissue is subsequently visualized by clinical positron emission tomography .