Human lungs have not been visualized by photoscanning until recently, for lack of a metabolizable, particulate test material which is safe to inject intravenously. A method for preparing such an agent, namely 5–25 micro macro-radioalbumin aggregates (MRAA), is described. Experimental evidence in animals insures a tremendous factor of safety (∼1000 fold) for injecting the minute quantities needed for scanning. Experience from scanning more than 100 patients has verified the safety and revealed the diagnostic value of the procedure. It has the unique capacity to detect regional pulmonary ischemia caused by emboli and other diseases which obstruct pulmonary arterial blood flow before signs of these lesions are recognizable in routine chest X-ray films.