99Tcm-labelled aerosol ventilation and 99Tcm-macroaggregate albumin (99Tcm-MAA) perfusion can be performed in the diagnosis of pulmonary embolism (PE). If both examinations are performed on the same day, the superposition of activity from the first scintigraphic examination might mask defects in the second. In this study, 106 examinations were carried out for suspected PE. Aerosol ventilation was performed first with 20 to 30 mCi 99Tcm-labelled rhenium sulphur (four views, 200000 counts). Immediately afterwards, with the patient remaining in the same position, 5 to 7 mCi of 99Tcm-MAA were injected (four views, 400000 counts). After normalization, aerosol activity was subtracted from perfusion images and unprocessed perfusion (UP) and ventilation subtraction perfusion (SP) images were compared. Interobserver diagnostic reproducibility between three readers was calculated both for UP and SP images. Intraobserver reproducibility between UP and SP images was calculated for each reader. Interobserver reproducibility was comparable for SP and UP images. Intraobserver reproducibility was good. Thus, whether ventilation was subtracted or not from perfusion images, there was no appreciable effect on perfusion defect detection. However, some perfusion abnormalities showed up more clearly on SP images. Perfusion can be performed immediately after aerosol ventilation; the images thus obtained are reliable for interpretation, and subtraction of ventilation is not necessary.
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