Numerical Modeling of Blood Flow Problems in Coronary Arteries: Patient-specific Processing, from Stented Geometries to Fluid Dynamics Open Access

Yang, Boyi (2015)

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Coronary stents expand the vessel to alleviate blockage, and they have been used for decades to save lives from coronary heart disease. Technological development has driven the evolution of stenting from bare or drug-coated metallic stents to the newly invented bioresorbable vascular stent (BVS). Rather than leaving a permanent metallic structure in the vessel, a BVS can dissolve in the body after opening the blocked artery, restoring the diseased coronary artery to its natural state. The BVS compensates for its decreased radial strength by having thicker struts that could cause disturbed blood flow, resulting in delayed healing and other devastating complications. Computational fluid dynamics (CFD) simulations had been used to analyze the potential risk of the BVS, but those simulations were conducted on either arbitrary geometry or partially patient-specific geometry with overly smoothed or virtually deployed stents [1]. In this thesis, we present a novel methodology to reconstruct true patient-specific CFD simulations: the geometry of a deployed stent inside a living patient is reconstructed from optical coherence tomography images; the shape and the curvature of the stented vessel are obtained from angiography; and a real pulsatile flow rate profile can also be prescribed as inflow condition, when it is available. With patient-specific geometry, the CFD results reflect the true hemodynamics after stent deployment and describe the wall shear stress and other quantities. We also aim to make the reconstruction and simulation process automatic, so that a large number of patients can be processed in a short time in order to draw statistically convincing conclusions. In fact, the entire process of the computational patient-specific analysis is expected to become a routine in clinical trials and activities. This has created significant challenges in many components of the methodological approach, ranging from image analysis, image processing, computational geometry to fluid dynamics. Our work witnesses the different challenges of this multi-component procedure. We hope the patient-specific reconstruction and simulation study can further the understanding of the BVS, improve its design, and ultimately expedite the tedious validation process so that it can soon become a soldier for us in the battle against coronary heart disease.

Table of Contents

1 Introduction 1

1.1 Coronary heart disease 2

1.2 The evolution of stenting treatment 3

1.3 Bioresorbable vascular stent (BVS) 5

1.4 Medical imaging technology 8

1.4.1 Optical coherence tomography (OCT) 9

1.4.2 Angiography 9

1.4.3 Thesis outline 10

2 Background 13

2.1 Comparison to related work 13

2.2 Previous work on patient-specic assessment of bioresorbable stent 23

2.3 Motivations and objectives 26

3 Preliminary assessment with idealized geometry 27

3.1 Overview on simulations with idealized geometry 28

3.2 Method 30

3.2.1 Geometric reconstruction of the stented vessel 30

3.2.2 Meshing of geometry 32

3.2.3 Computational fluid dynamics 32

3.2.4 Boundary conditions 32

3.2.5 Post-processing 34

3.3 Results 34

3.4 Conclusion and discussion 38

4 Automatic bioresorbable stent strut detection in OCT images 42

4.1 Method 43

4.1.1 Preprocessing 43

4.1.2 Main detection algorithm 45

4.1.3 Elimination of false positives 49

4.1.4 Supplementary detection 50

4.1.5 Final elimination of false positives 51

4.1.6 Patching and correction 52

4.2 Results and validation 53

4.3 Conclusion 54

5 Geometrical reconstruction of implanted bioresorbable stent 57

5.1 3D Strut-point cloud formation 57

5.2 Connectivity recovery 58

5.2.1 2D projection 58

5.2.2 Interactive pattern interpretation in 2D 63

5.2.3 3D categorization 64

5.3 Stent volume reconstruction 69

5.3.1 Wire-frame creation 69

5.3.2 Registration of the strut cross-sections 73

5.3.3 Assembly of ring and beam facets 74

5.3.4 Formation of joints facets and creation of a continuous surface 75

5.4 Results and discussion 77

6 Reconstruction of stented coronary artery 79

6.1 Lumen reconstruction 79

6.2 Application of curvature to the stent and lumen 80

6.3 Finding the Euclidean dierence of the lumen and the stent 81

6.4 Geometry repair 82

7 The CFD simulations 88

7.1 A priori adaptive meshing 88

7.2 Computational fluid dynamics 90

7.3 CFD results and discussion 93

7.3.1 Results of Case 1-3 with curvature 93

7.3.2 Results of Case 2 without curvature 94

7.3.3 Comparison between straight and curved Case 2 results 100

8 Extension to metallic stent reconstruction 102

8.1 Automatic detection of metallic stent strut from OCT images 103

8.2 Metallic stent reconstruction on the detected strut point cloud 105

9 Conclusion and limitations 109

9.1 Conclusion 109

9.2 Limitations 110

10 Future directions 111

10.1 Finding the ultimately missed struts 111

10.2 Saved 2D stent pattern 112

10.3 Bypass the geometry repair 112

10.4 Stress simulation with the existing geometry 113

10.5 More automatic, efficient, accurate reconstruction and simulation pipeline 114

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