3D geometric reconstruction of thoracic aortic aneurysms

Abstract

Background: The thoracic aortic aneurysm (TAA) is a pathology that involves an expansion of the aortic diameter in the thoracic aorta, leading to risk of rupture. Recent studies have suggested that internal wall stress, which is affected by TAA geometry and the presence or absence of thrombus, is a more reliable predictor of rupture than the maximum diameter, the current clinical criterion. Accurate reconstruction of TAA geometry is a crucial step in patient-specific stress calculations.

Methods: In this work, a novel methodology was developed, which combines data from several sets of magnetic resonance (MR) images with different levels of detail and different resolutions. Two sets of images were employed to create the final model, which has the highest level of detail for each component of the aneurysm (lumen, thrombus, and wall). A reference model was built by using a single set of images for comparison. This approach was applied to two patient-specific TAAs in the descending thoracic aorta.

Results: The results of finite element simulations showed differences in stress pattern between the coarse and fine models: higher stress values were found with the coarse model and the differences in predicted maximum wall stress were 30% for patient A and 11% for patient B.

Conclusion: This paper presents a new approach to the reconstruction of an aneurysm model based on the use of several sets of MR images. This enables more accurate representation of not only the lumen but also the wall surface of a TAA taking account of intraluminal thrombus.

Figure 1

Image segmentation samples. Examples of the MR images and segmented contours of Set A1 (figure 1a) and Set A2 (figure 1b) for patient A.

Figure 2

Segmentation results. From left to right: (a) 3D MR angiogram of the TAA of Patient A; (b) aneurysm shape from the segmentation of the image set A1 (wall contours c_W in blue and lumen contours c_LH in red); (c) lumen of the aneurysm from the segmentation of image set A2 (lumen contours c_LM in magenta) (Units in mm).

Figure 3

centreline realignment. Realignment of the centreline (Patient A) (Units in mm).

Figure 4

Lumen shape comparison. Comparison of the lumen shape from set A1 (contour c_LH, in red) and set A2 (contour c_LM, in magenta) (Patient A) (Units in mm).

Figure 5

Wall thickness retrieval. Example of the wall thickness measurement (Patient B).

Figure 6

Lumen surface for the two different models. Comparison of the lumen surface from set A1 (figure 6a) and set A2 (figure 6b) for Patient A.

Figure 7

CAD models. TAA models for Patient A (figure 7a) and Patient B (figure 7b). The contours are from the MATLAB segmentation, and the surface rendering are from the CAD models, with the wall domain shown in blue and the thrombus domain shown in yellow (Units in mm).

Figure 8

Patient A, stress pattern on the outer wall. Comparison of wall stress patterns on the outer wall for Patient A (A_H in figure 8a, A_HM in figure 8b); the dotted lines show the sections where transverse stress distributions are resented in figure 9, the triangles show the points of maximum stress (Units in kPa).

Figure 9

Patient A, stress pattern on a horizontal section. Comparison of cross-sectional wall stress distributions at the locations of maximum stress for Patient A (A_H in figure 9a, A_HM in figure 9b); the triangles show the point of maximum stress (Units in kPa).

Figure 10

Patient B, stress pattern on the outer wall. Comparison of wall stress patterns on the outer wall for Patient B (B_H in figure 10a, B_HM in figure 10b); the dotted lines show the sections where transverse stress distributions are presented in figure 11, the triangles show the points of maximum stress (Units in kPa).

Figure 11

Patient B, stress pattern on horizontal sections. Comparison of cross-sectional wall stress distributions at two locations for Patient B; sections from model B_H are given on the in figure 11a and 11c and sections from model B_HM are in figure 11b and 11d. The triangles show the points of maximum stress at the sections (Units in kPa).

Figure 12

Patient B, intra-operator dependence of the segmentation. Comparison of lumen and wall areas from the segmentations performed at three different times (Patient B).