The clinical diagnosis of pulmonary embolism was subject to a high frequency of false positives and false negatives. The most definitive diagnostic procedure was pulmonary angiography, an invasive, expensive procedure which was not without risk. It required specialized equipment and highly trained personnel both for performance and for interpretation. Therefore, it was used to diagnose pulmonary embolism only in the major medical centers. Another technique utilized as a method to diagnose pulmonary embolism involved a combination of perfusion and ventilation scanning; this method was only minimally invasive. A normal perfusion scan was thought to be of considerable value because it essentially excluded the diagnosis of pulmonary embolism. In selected patient populations, abnormal perfusion scans combined with normal ventilation scans were of substantial help in diagnosis.