Modified annexins having an N-terminal chelation site comprising an amino acid extension are disclosed. The amino acid extension includes an amino acid sequence X.sub.1-Gly-X.sub.2, where X.sub.1 and X.sub.2 are selected from Gly and Cys, where at least one of X.sub.1 or X.sub.2 is Cys. Radionuclides are readily chelated by the modified annexins to provide radiolabeled annexins useful in imaging vascular thrombi and apoptosis in vivo.