99mTc-MAA imaged with single photon emission computed tomography (SPECT) is considered the standard method for the quantitative determination of pulmonary perfusion. Magnetic resonance imaging (MRI) with contrast agents have been utilized experimentally to image the pulmonary vasculature and tissue perfusion. Quantification of SPECT images requires correction of the acquired data for attenuation and attenuation correction, which has lead to the development of SPECT/CT scanners. The low-dose CT can be utilized to evaluate the lung airway architecture, lung parenchyma, and pleural space in conjunction with the registered perfusion images rivaling CTA in sensitivity and specificity.