4D-CT reconstruction based on pulmonary average CT values.

To date, commercial 4D-CT systems typically depend on an external respiratory monitoring device. Immobilizing patients in a thermoplastic mask while receiving radiotherapy may result in a failure of 4D-CT reconstruction. The aim of this study is to investigate the feasibility of 4D-CT reconstruction based on a method using pulmonary average CT values (ACV) without an external respiratory monitoring device. The ACV of the whole lung assumes cyclical variation during respiration. Phases of CT images were identified by calculating the ACV over time. Subsequently, five sets of 4D-CT images based on a Real-time Position Management (RPM) system were selected to verify the ACV method. The entire lung CT datasets of another sixteen free-breathing patients were acquired in Cine scan mode for multiple couch positions. The phase of every CT image was identified and re-sorted into different phase 4D-CT volumes by analyzing the time dependence of the corresponding ACVs. This paper demonstrates the ACV method using the 4D-CT data sets based on the RPM system. Convenient and reliable 4D-CT reconstruction can be accomplished without any external respiratory monitoring device using ACVs.