Development of a Dynamic Model for the Lung Lobes and Airway Tree in the NCAT Phantom

The four-dimensional (4-D) NCAT phantom was developed to realistically model human anatomy based on the visible human data and cardiac and respiratory motions based on 4-D tagged magnetic resonance imaging and respiratory-gated CT data from normal human subjects. Currently, the 4-D NCAT phantom does not include the airway tree or its motion within the lungs. Also, each lung is defined with a single surface; the individual lobes are not distinguished. The authors further the development of the phantom by creating dynamic models for the individual lung lobes and for the airway tree in each lobe. NURBS surfaces for the lobes and an initial airway tree model (~ 4 generations) were created through manual segmentation of the visible human data. A mathematical algorithm with physiological constraints was used to extend the original airway model to fill each lobe. For each parent airway branch inside a lobe, the algorithm extends the airway tree by creating two daughter branches modeled with cylindrical tubes. Parameters for the cylindrical tubes such as diameter, length, and angle are constrained based on flow parameters and available lung space. The bifurcating branches are propagated within a lung lobe until it is filled. Once each lobe is filled, the cylindrical tubes are converted into NURBS surfaces and blended with the original airway tree obtained through segmentation. The respiratory model previously developed using the respiratory-gated CT data is then applied to the surfaces of the lobes and airway tree to create the new 4-D respiratory model. This improved model will provide a useful tool in future studies researching the effects of respiratory motion on lung tumor imaging. It is also an important step in advancing the 4-D NCAT for applications in more high-resolution imaging modalities such as x-ray CT.