# Forward Computational Modeling of Respiratory Airflow
## 3 Image-Based Structural and Spatial Geometric Models
### Airway Segmentation  
After lobar segmentation, the airway tree is automatically segmented out of the whole lung structure, else manual or semi-automatic airway tree segmentation is required. A given segmented airway tree is converted into a 3D geometric structure for CFD simulations by conforming the model geometry to the surface data of the airways and/or creating a 1D centerline airway tree in 3D space [5]. To utilize 1D centerline airway for CFD Simulation, diameters must be defined throughout the entire structure based on image measurements of the airway in a plane orthogonal to the central axis [5]. Such 3D geometric structures can be utilized directly if saved in a format recognized by CFD simulation software (e.g., Ansys, SOLIDWORKS, COMSOL Multiphysics, among others).  
The geometric accuracy of airway tree models generated from 3D medical images is dependent on spatial resolution. Hence, segmentations are only feasible for about four to five generations. To overcome this limitation, [PERSON] et al. [43] developed an image-based volume-filling branching (VFB) method to create an entire airway tree model with high anatomic fidelity. This method was based on the modification of the Monte Carlo technique for a distributive system as discussed above. The generated geometric model accounts for the asymmetry between parent and daughter airway segment lengths, diameters, and branching angles. As opposed to utilizing the methodology of [PERSON] [20] for the generation of the central airway geometry with ends that served as the initial points for the VFB method [28], the central airway tree is semi-automatically segmented out to the fifth generation from images obtained at total lung capacity (TLC). Lung lobes are segmented as well, serving as host volumes which are filled with uniformly spaced grids of seed points. Each seed represents the location of an acinus in space. Centerline information is extracted from the segmented central airway, and the end of the centerline serves as the initial point while the surface of the host volume (lobar surface) serves as the boundary. The seed containing volumes are split recursively into subregions as described earlier, and the seed points are associated with the closest terminal branch. This iterative process repeats until the length of a branch is less than a predefined terminal length or only one seed point left in the subregion, leaving room for alveoli. The VFB hence generates a 1D-structural, 3D-spatial geometry for the whole lung (Figure 3).  
[PERSON] and co-workers [16; 45; 46] used this VFB methodology to develop a multiscale image-based model for the whole lung. It is multiscale because it incorporates both 3D and 1D geometric structures in a single model. After the generation of the entire 1D structural airway tree using the VFB method, any airway branch pathway from the trachea to a terminal bronchiole can be selected and converted to a 3D structure. The 3D mesh is only created for the pathway, while the rest of the tree can be described as a 1D structure. This 3D-1D structural combination allows for air flow simulations and analyses in specific regions of the lung that cannot be obtained by 1D simulations alone.