Two steps procedure. A lung scintigraphy has two components. The ventilation part (v of v/q) which consists in inhalation of 99mtc-gaz or aerosols and the perfusion part (q of v/q) which involves the intravenous injection of 99mtc-maa which are used to see the perfusion of the lungs. Usually a lung scan is performed to detect pulmonary emboli (blood clots in the lungs) or to perform a preoperative evaluation.
Radiologyinfo. Org. Gives good overview.