Deformable image registration of heterogeneous human lung incorporating the bronchial tree

Abstract

Purpose: To investigate the effect of the bronchial tree on the accuracy of biomechanical-based deformable image registration of human lungs.

Methods: Three dimensional finite element models have been developed using four dimensional computed tomography image data of ten lung cancer patients. Each model is built of a body, left and right lungs, tumor, and bronchial trees. Triangular shell elements are used for the bronchial trees while tetrahedral elements are used for other components. Hyperelastic material properties based on experimental investigation on human lungs are used for the lung parenchyma. Different material properties are assigned for the bronchial tree using five values for the modulus of elasticity of 0.01, 0.12, 0.5, 10, and 18 MPa. Lungs are modeled to slide inside chest cavities by applying frictionless contact surfaces between each lung and corresponding chest cavity. The accuracy of the models is examined using an average of 40 bronchial bifurcation points identified on inhale and exhale images. Relative accuracy is evaluated by comparing the displacement of all nodes within the lungs as well as the dosimetric difference at the exhale position predicted by the model.

Results: There is no significant effect of bronchial tree on the model accuracy based on the bifurcation points analysis. However, on the local level, using an average of 38 000 nodes, there is a maximum difference of 8.5 mm in the deformation of the bronchial trees, as the modulus of elasticity of the bronchial trees increases from 0.01 to 18 MPa; however, more than 96% of nodes are within a 2.5 mm difference in each direction. The average dose difference at the predicted exhale position is less than 35 cGy between the models.

Conclusions: The bronchial tree has little effect on the global deformation and the accuracy of deformable image registration of lungs. Hence, the homogenous model is a reasonable assumption. Since there are some local deformation differences between nodes as the material properties of the bronchial tree change that may affect the accuracy of dosimetric results, heterogeneity may be required for a smaller scale modeling of lungs.

Figure 1

Model development starts by acquiring CT images for both exhale and inhale phases. 3D surface meshes are created and used for the projection of the inhale and exhale surfaces to find the boundary conditions. The inhale surface mesh of all components, except the bronchial tree, is tetrameshed, leaving the bronchial trees as shell structures. After applying contact surface on the lungs, finite element analysis are conducted.

Figure 2

Bifurcation points in inhale and exhale images. The error in the location of bifurcation points represents the difference in location between the estimated FEM location and the image-based location in the exhale breathing phase.

Figure 3

The percentage of bifurcation points with absolute registration error less than 2.5 mm in the LR direction using different elastic modulus of the bronchial tree (E_b) and compared to the homogeneous model.

Figure 4

The percentage of bifurcation points with absolute registration error less than 2.5 mm in the AP direction using different elastic modulus of the bronchial tree (E_b) and compared to the homogeneous model.

Figure 5

The percentage of bifurcation points with absolute registration error less than 2.5 mm in the SI direction using different elastic modulus of the bronchial tree (E_b) and compared to the homogeneous model.

Figure 6

Histogram of displacement difference between bronchial tree with modulus of elasticity of 18 and 0.01 MPa in the LR, AP, and SI directions.

Figure 7

Displacement of the bronchial tree in the SI direction relative to its position in the SI direction using modulus of elasticity of the bronchial tree E_b tree of 0.5 and 10 MPa, in addition to the displacement difference using these two moduli.

Figure 8

Minimum and maximum displacement differences in the LR direction using different modulus of elasticity of the bronchial trees of 0.01, 0.5, and 18 MPa.

Figure 9

Minimum and maximum displacement differences in the AP direction using different modulus of elasticity of the bronchial trees of 0.01, 0.5, and 18 MPa.

Figure 10

Minimum and maximum displacement differences in the SI direction using different modulus of elasticity of the bronchial trees of 0.01, 0.5, and 18 MPa.

Figure 11

Tumor location relative to the bronchial tree.

Figure 12

Lung deformation from inhale to exhale for P9. The deformation is more confined to the area near diaphragm (dimensions in cm).