In this study patients found to have new segmental, lobar or greater perfusion defects, will be imaged with 99mTc-MAA SPECT/CT and 4DCT to compare perfusion with RIBMC defects. This is a prospective imaging trial of 15 subjects diagnosed by CTA. Each will receive 99mTc-MAA SPECT/CT and 4DCT (x2) imaging on the same day. Respiratory induced blood mass change (RIBMC) images will be derived from the 4DCT image and quantitatively compared with the SPECT perfusion images.