In a study comparing CT attenuation with SPECT perfusion defects, patients were found to have hypo-attenuated pulmonary regions corresponding to regions with decreased perfusion in 57 percent of acute pulmonary emboli and 88 percent of chronic emboli cases. In the same study, hyper-attenuated regions were found to correspond to regions with hyperperfusion. A method to measure pulmonary perfusion based on subtraction digital fluoroscopy without contrast has been reported. In that study, subtraction images were generated between chest projection images at systole and diastole generating an image representing the perfusion difference. These perfusion projection images were correlate with 99mTc-MAA scintigraphy. Thus, changes in the amount and distribution of pulmonary perfusion throughout the respiratory cycle can be expected and these changes may be apparent on dynamic CT.