Although there had been no acceptable validation of the use of perfusion scans in the diagnosis of pulmonary embolism, thousands of patients had been evaluated for pulmonary embolism based on perfusion scanning often using methods of imaging now considered to be inadequate. In the early 1980s, clinical practice interpreted a negative perfusion scan as overwhelming evidence against the presence of pulmonary emboli. This interpretation had not been adequately tested either in a prospective study or by long-term follow-up of patients to determine clinical outcome. With regards to positive perfusion scans, there were data to suggest that as many as two-thirds of positive perfusion scans could not subsequently be confirmed by pulmonary angiography. Prospective studies in which timely angiograms using selective injections and improved imaging techniques were needed to evaluate the usefulness of positive perfusion scans.