 A defect in the perfusion images requires a mismatched ventilation defect to indicate pulmonary embolism.In the ventilation phase of the test, a gaseous radionuclides such as xenon-133, krypton-81m, or technetium-99m DTPA in an aerosol form is inhaled by the patient through a mouthpiece or mask that covers the nose and mouth.Ventilation imaging can also be performed using a Technegas machine which produces an aerosol of radioactive nanoparticles, specifically carbon nanoparticles containing technetium-99m.