How can they do a 'lung VQ scan'?

Compare 2 studies. First part of study is usually inhaling gas isotope such as xenon 133. Several breaths are taken and imaging of inhalation, exhalation, and equilibrium are performed.Second part of study is intravenous injection of isotope tc99m maa into peripheral vein and subsequent imaging of lung perfusion in multiple projections.Mostly used for detectiong of pulmonary embolism. Vent./ perfusion scan compared.