In radiotherapy, the lung is considered one homogenous organ at risk during planning. However, avoiding dose to functional lung could help preserve lung function after treatment. This could be possible with ventilation mapping, which is traditionally done with SPECT imaging. Having an accurate, CT-based approach could allow for ventilation mapping within the radiotherapy workflow. In this study, we aim to compare a novel, CT-based ventilation mapping approach to SPECT-based ventilation images. We acquired both CT-based and SPECT-based ventilation scans for one patient. For the SPECT-based images, the patient inhaled 8.8 mCi Tc-99m DTPA aerosol before scan acquisition with a dual headed SPECT/CT. For the CT-based approach, we acquired 25 fast-helical free-breathing CTs as part of our 5DCT protocol with simultaneous monitoring of the breathing pattern using a respiratory bellows surrogate. We chose one scan as the reference scan, and deformably registered the other 24 scans to it. The Jacobian expansion of each registration was calculated for each voxel and linearly related to the bellows amplitude. The slope of the linear fit, dJ/dA, represented the ventilation for each voxel. To compare the resulting ventilation maps from each method, we first calculated the normalized cross-correlation (NCC) and Spearman correlation coefficient. We also created masks of ventilation greater than various thresholds to see how we could accurately define high and low functioning lung. For example, greater than 50th percentile ventilation being 1 (high functioning) and less than 50th percentile being 0 (low functioning). We calculated Dice scores to compare these threshold masks between the CT-based and SPECT-based ventilation images. The NCC between the ventilation images was 0.810, and the Spearman correlation between them was 0.587. When comparing volumes of ventilation greater than 50th percentile, the Dice score was 0.730. However, when the threshold was increased, the Dice scores were reduced. Qualitatively, we noted that the lobar ventilation agreed in general, but there was more detail throughout the CT-based image, as opposed to the more precisely defined, high ventilating areas in the SPECT image. We were able to compare our novel, CT-based ventilation approach against a more conventional technique in ventilation SPECT imaging. The resulting ventilation images were qualitatively similar. Analyzing different thresholds showed that we can reasonably separate high and low functioning tissue into greater than or less than 50th percentile ventilation, respectively. In future work, we will aim to expand this study over a larger cohort of patients and also examine longitudinal changes to ventilation after SBRT and lung brachytherapy.