To establish a new method of preparation of 99mTc -labelled SZ-51 for radioimmunoimaging of thrombi and to evaluate the value of 99mTc-SZ-51 in detecting pulmonary embolism and distinguishing fresh and old thrombi. SZ-51 was modified with 2-iminotholane and labelled with 99mTc-GH using ligand exchange method. Labelling yield and immunoreactivity were determined by instant thin-layer chromatography ( ITLC) and enzyme linked immunosorbent assay ( ELISA) respectively. The kit was produced using the optimal labelling conditions. 8 dogs with pulmonary embolism were used for radioimaging. Using the best modulation ratio of 2-IT and McAb SZ-51

radiolabelling yield was 90%-95% as determined by ITLC. The results of ELISA showed that immunoactivity was not affected. Radioimaging results showed that 99mTc-SZ-51 concentrated in the pulmonary embolism 1 hour after injection

and the image became clear gradully within 3 h. Quantitative analysis 3 h after injection showed that the ratios of radioactive uptake between the thrombus and surrounding lung were 5.3±0.5 in vivo imaging and 7.0±1.2 in vitro imaging. Higher uptake was found in fresh pulmonary thrombus than in old one

The ratio of them was 2.65 in vitro measurement.