Respiratory correlated CT is commercially available, and we have implemented its routine clinical use in planning lung tumor patients. Its value is determined by the fidelity of the spatiotemporal data set after processing the acquired reconstructed slices. Retrospective sorting of reconstructed slices is based on respiratory phase. However, the existing commercial software inadequately models respiratory phase for about 30% of the patients, mainly due to irregularities in the respiratory cycle. We have developed software that improves phase determination and consequently leads to an improvement of retrospective data sorting to make 4DCT data acquisition feasible for routine clinical use. Peak inhalation and exhalation respiratory states are selected manually; intermediate phases are interpolated. Residual motion artifacts in the resulting 4DCT volumes are reduced and allow use of the 4D imaging studies for treatment planning.