The investigators found a cyclic variation in the apparent mass of the pulmonary parenchyma. The investigators hypothesize this variation is due to changes in pulmonary perfusion from respiratory-induced variation in cardiac output. The investigators hypothesize this respiratory induced blood mass change (RIBMC) will allow the identification of hypoperfused lung regions. The investigators did a prelimary study by creating 4DCT RIBMC images from cases with hypoxia induced vasoconstriction, patients with malignant airway constriction. The resulting images compare well with 99mTc-MAA SPECT images. It is unknown, however, if this process works to detect perfusion defects due to PE where the perfusion is obstructed and breathing normal.