In the blood, fibrinogen is converted into fibrin by the action of thrombin. Fibrin then spontaneously polymerizes into strands of fibrin that form the substance of the clot. The receptor binds fibrinogen which promotes platelet aggregation. Imaging is usually delayed 6 to 24 hours and therefore does not provide timely clinical information. The degree of platelet uptake is dependent on the age of the thrombus.
Less activity is seen in older thrombi as they contain predominantly fibrin and entrapped red cells.
4- Small thrombi can be missed due to the relatively high background blood pool activity. 5- Heparin and warfarin can interfere with platelet uptake, however, positive images may be obtained if the anticoagulant drugs are discontinued at least 4 hours prior to injection of the tracer. Small peptides with platelet affinity labeled with Tc-99m are also being evaluated. These small peptides with specific amino acid sequences are more advantageous than complex, large molecular weight antibodies since they can be produced synthetically, have faster blood clearance, do not elicit a HAMA response, and are less expensive. Additionally, fibrinogen is not antigenic.