The investigators hypothesize that respiratory induced blood mass change in the lungs will allow the identification of under-perfused lung regions. The investigators did a prelimary study and tested this theory in cases with low oxygen levels, narrowing of the blood vessels, airway obstruction, comparing 4DCT with SPECT scan. The images compared well. It is unknown, however, if this process works to detect perfusion defects due to PE where the perfusion is obstructed and breathing normal.