The preparation of dry monodisperse microspheres of (99mTc) albumin for lung ventilation imaging

2 μm microspheres of albumin were prepared and labelled with 99mTc according to published methods. However, when the labelling technique was repeated without the inclusion of microspheres, apparent labelling, which was attributed to the formation of a 99mTc tin colloid, was still observed. It was not possible to identify 99mTc tin colloid in the presence of 99mTc microspheres. To establish conditions under which there was no 99mTc tin colloid formation, labelling, with and without microspheres, was studied at low pH for a range of SnCl2·2H2O concentrations. A “kit” containing 0.4 mg of microspheres and 1 μg of SnCl2·2H2O in 1 ml of 25 mM HCl gave optimum labelling of 88.3% (SD 3.9, n = 38). Unlabelled and labelled kits were stable for 1 and 3 h respectively. Before administration, the microspheres were resuspended in ethanol, in which they were stable for 6 h. None of the preparations contained 99mTc tin colloid.