Publications

Publications

Construction of a coronary atlas from CT angiography
Medrano-Gracia, P., et al. Construction of a coronary atlas from CT angiography. in Medical Image Computing and Computer Assisted Intervention. 2014. Boston, USA.

Describing the detailed statistical anatomy of the coronary artery tree is important for determining the ætiology of heart disease. A number of studies have investigated geometrical features and have found that these correlate with clinical outcomes, e.g. bifurcation angle with major adverse cardiac events.

These methodologies were mainly two-dimensional, manual and prone to inter-observer variability, and the data commonly relates to cases already with pathology. We propose a hybrid atlasing methodology to build a population of computational models of the coronary arteries to comprehensively and accurately assess anatomy including 3D size, geometry and shape descriptors.

A random sample of 122 cardiac CT scans with a calcium score of zero was segmented and analysed using a standardised protocol. The resulting atlas includes, but is not limited to, the distributions of the coronary tree in terms of angles, diameters, centrelines, principal component shape analysis and cross-sectional contours. This novel resource will facilitate the improvement of stent design and provide a reference for hemodynamic simulations, and provides a basis for large normal and pathological databases.

A Study of Coronary Bifurcation Shape in a Normal Population

Medrano-Gracia, P., et al., A Study of Coronary Bifurcation Shape in a Normal Population. Journal of Cardiovascular Translational Research, 2016: p. 1-9.

 

Abstract

During percutaneous coronary intervention, stents are placed in narrowings of the arteries to restore normal blood flow. Despite improvements in stent design, deployment techniques and drug-eluting coatings, restenosis and stent thrombosis remain a significant problem.

Population stent design based on statistical shape analysis may improve clinical outcomes. Computed tomographic (CT) coronary angiography scans from 211 patients with a zero calcium score, no stenoses and no intermediate artery, were used to create statistical shape models of 446 major coronary artery bifurcations (left main, first diagonal and obtuse marginal and right coronary crux). Coherent point drift was used for registration.

Principal component analysis shape scores were tested against clinical risk factors, quantifying the importance of recognised shape features in intervention including size, angles and curvature. Significant differences were found in (1) vessel size and bifurcation angle between the left main and other bifurcations; (2) inlet and curvature angle between the right coronary crux and other bifurcations; and (3) size and bifurcation angle by sex.

Hypertension, smoking history and diabetes did not appear to have an association with shape. Physiological diameter laws were compared, with the Huo-Kassab model having the best fit. Bifurcation coronary anatomy can be partitioned into clinically meaningful modes of variation showing significant shape differences.

A computational atlas of normal coronary bifurcation shape, where disease is common, may aid in the design of new stents and deployment techniques, by providing data for bench-top testing and computational modelling of blood flow and vessel wall mechanics.

 

 

What are bifurcations really like anatomically?
Medrano-Gracia, P., et al., What are bifurcations really like anatomically? Powerpoint
Overcoming spatio-temporal limitations using dynamically scaled in vitro PC-MRI — A flow field comparison to true-scale computer simulations of idealized, stented and patient-specific left main bifurcations
A new method to quantify coronary flow conditions using dynamically scaled in vitro phase contrast magnetic resonance imaging
Lecture Notes in Computer Science

Beier, S., et al., Lecture notes in Computer Science.

Abstract

Describing the detailed statistical anatomy of the coronary artery tree is important for determining the aetiology of heart disease. A number of studies have investigated geometrical features and have found that these correlate with clinical outcomes, e.g. bifurcation angle with major adverse cardiac events. These methodologies were mainly two-dimensional, manual and prone to inter-observer variability, and the data commonly relates to cases already with pathology.

We propose a hybrid atlasing methodology to build a population of computational models of the coronary arteries to comprehensively and accurately assess anatomy including 3D size, geometry and shape descriptors.

A random sample of 122 cardiac CT scans with a calcium score of zero was segmented and analysed using a standardised protocol. The resulting atlas includes, but is not limited to, the distributions of the coronary tree in terms of angles, diameters, centrelines, principal component shape analysis and cross-sectional contours.

This novel resource will facilitate the improvement of stent design and provide a reference for hemodynamic simulations, and provides a basis for large normal and pathological databases.
Lecture Notes in Computer Science. Available from: https://www.researchgate.net/publication/269282821_Lecture_Notes_in_Computer_Science [accessed Jul 23, 2017].

A new method to quantify coronary flow conditions using dynamically scaled in vitro phase contrast magnetic resonance imaging
A Computational Atlas of Normal Coronary Artery Anatomy

Medrano-Gracia, P., et al., A Computational Atlas of Normal Coronary Artery Anatomy. EuroIntervention, 2016. EIJ-D- 16-00111(in press).

Aims

The aim of this study was to define the shape variations, including diameters and angles, of the major coronary artery bifurcations.

Method and results

Computed tomographic angiograms from 300 adults with a zero calcium score and no stenoses were segmented for centreline and luminal models. A computational atlas was constructed enabling automatic quantification of 3D angles, diameters and lengths of the coronary tree. The diameter (mean±SD) of the left main coronary was 3.5±0.8 mm and the length 10.5±5.3 mm. The left main bifurcation angle (distal angle or angle B) was 89±21° for cases with, and 75±23° for those without an intermediate artery (p<0.001). Analogous measurements of diameter and angle were tabulated for the other major bifurcations (left anterior descending/diagonal, circumflex/obtuse marginal and right coronary crux). Novel 3D angle definitions are proposed and analysed.

Conclusions

A computational atlas of normal coronary artery anatomy provides distributions of diameter, lengths and bifurcation angles as well as more complex shape analysis. These data define normal anatomical variation, facilitating stent design, selection and optimal treatment strategy. These population models are necessary for accurate computational flow dynamics, can be 3D printed for bench testing bifurcation stents and deployment strategies, and can aid in the discussion of different approaches to the treatment of coronary bifurcations.

Impact of bifurcation angle and other anatomical characteristics on blood flow–A computational study of non-stented and stented coronary arteries.

Beier, S., et al., Impact of bifurcation angle and other anatomical characteristics on blood flow–A computational study of non-stented and stented coronary arteries. Journal of biomechanics, 2016. 49(9): p. 1570-1582.

Abstract

The hemodynamic influence of vessel shape such as bifurcation angle is not fully understood with clinical and quantitative observations being equivocal. The aim of this study is to use computational modeling to study the hemodynamic effect of shape characteristics, in particular bifurcation angle (BA), for non-stented and stented coronary arteries. Nine bifurcations with angles of 40°, 60° and 80°, representative of ±1 SD of 101 asymptomatic computed tomography angiogram cases (average age 54±8 years; 57 females), were generated for (1) a non-stented idealized, (2) stented idealized, and (3) non-stented patient-specific geometry.

Only the bifurcation angle was changed while the geometries were constant to eliminate flow effects induced by other vessel shape characteristics. The commercially available Biomatrix stent was used as a template and virtually inserted into each branch, simulating the T-stenting technique. Three patient-specific geometries with additional shape variation and ±2 SD BA variation (33°, 42° and 117°) were also computed.

Computational fluid dynamics (CFD) analysis was performed for all 12 geometries to simulate physiological conditions, enabling the quantification of the hemodynamic stress distributions, including a threshold analysis of adversely low and high wall shear stress (WSS), low time-averaged WSS (TAWSS), high spatial WSS gradient (WSSG) and high Oscillatory Shear Index (OSI) area.

The bifurcation angle had a minor impact on the areas of adverse hemodynamics in the idealized non-stented geometries, which fully disappeared once stented and was not apparent for patient geometries. High WSS regions were located close to the carina around peak-flow, and WSSG increased significantly after stenting for the idealized bifurcations.

Additional shape variations affected the hemodynamic profiles, suggesting that BA alone has little effect on a patient׳s hemodynamic profile. Incoming flow angle, diameter and tortuosity appear to have stronger effects. This suggests that other bifurcation shape characteristics and stent placement/strategy may be more important than bifurcation angle in atherosclerotic disease development, progression, and stent outcome.

Vascular Hemodynamics with Computational Modeling and Experimental Studies

Beier, S., et al., Vascular Hemodynamics with Computational Modeling and Experimental Studies DOI: 10.1016/B978-0-12-811018-8.00009-6 . In book: Computing and Visualization for Intravascular Imaging and Computer-Assisted Stenting, pp.227-251

Abstract

This chapter discusses coronary artery flow assessment for atherosclerosis investigations. The overall goal is to foster the reader’s understanding of coronary flow assessment with CFD and experimental MRI, including advantages, shortcomings, and potential for clinical applicability. In Section 1 , we begin by introducing coronary artery disease and how it links to local blood flow and hemodynamic parameters, before introducing strategies to investigating coronary flow for risk assessment—computational modeling and experimental studies. Both of these need the artery geometry and embedded stents to be retrieved first, as detailed in Section 2 . Section 3 details the concepts of computational coronary flow modeling with computational fluid dynamics (CFD) including the governing equations, mesh discretization, and boundary and initial conditions. Section 4 introduces experimental approaches using in vitro flow sensitive magnetic resonance imaging (MRI), including dynamic scaling for steady or transient state considerations, creation of phantom, consideration of vessel compliance and motion, non-Newtonian blood properties, and the design of an experimental circuit. Postprocessing, analysis, and comparison of both methods are explained in Section 5 , before discussion of the accuracy and reliability of the results in Section 6 . Finally, current developments, particularly patient-specific profiling, are discussed in Section 7 .

Literature Review: Vascular Hemodynamics with Computational Modeling and Experimental Studies. Available from: https://www.researchgate.net/publication/312513133_Vascular_Hemodynamics_with_Computational_Modeling_and_Experimental_Studies [accessed Jul 23, 2017].

Dynamically Scaled Phantom Phase Contrast MRI Compared to True-Scale Computational Modeling of Coronary Artery Flow

 

Beier, S., et al., Dynamically Scaled Phantom Phase Contrast MRI Compared to True-Scale Computational Modeling of Coronary Artery Flow, Article · Apr 2016 · Journal of Magnetic Resonance Imaging.

 

Abstract

 

Purpose: To examine the feasibility of combining computational fluid dynamics (CFD) and dynamically scaled phantom phase-contrast magnetic resonance imaging (PC-MRI) for coronary flow assessment. Materials and methods: Left main coronary bifurcations segmented from computed tomography with bifurcation angles of 33°, 68°, and 117° were scaled-up ∼7× and 3D printed. Steady coronary flow was reproduced in these phantoms using the principle of dynamic similarity to preserve the true-scale Reynolds number, using blood analog fluid and a pump circuit in a 3T MRI scanner. After PC-MRI acquisition, the data were segmented and coregistered to CFD simulations of identical, but true-scale geometries. Velocities at the inlet region were extracted from the PC-MRI to define the CFD inlet boundary condition. Results: The PC-MRI and CFD flow data agreed well, and comparison showed: 1) small velocity magnitude discrepancies (2-8%); 2) with a Spearman’s rank correlation ≥0.72; and 3) a velocity vector correlation (including direction) of r(2) ≥ 0.82. The highest agreement was achieved for high velocity regions with discrepancies being located in slow or recirculating zones with low MRI signal-to-noise ratio (SNRv ) in tortuous segments and large bifurcating vessels. Conclusion: Characterization of coronary flow using a dynamically scaled PC-MRI phantom flow is feasible and provides higher resolution than current in vivo or true-scale in vitro methods, and may be used to provide boundary conditions for true-scale CFD simulations. J. Magn. Reson. Imaging 2016.

 

Hemodynamics in Idealized Stented Coronary Arteries: Important Stent Design Considerations
Coronary artery bifurcation haemodynamics - comparison between phase contrast MRI and computational fluid dynamics

Coronary artery bifurcation haemodynamics – comparison between phase contrast MRI and computational fluid dynamics. Poster presentation. Susann Beier1, John Ormiston2, Mark Webster2, John Cater3, Pau Medrano-Gracia1, Alistair Young1, Brett R Cowan1*From 17th Annual SCMR Scientific SessionsNew Orleans, LA, USA. 16-19 January 2014

Background

Coronary atherosclerosis is common at vessel bifurcations. A quantitative approach to measuring blood velocity, vorticity and more complex flow features at bifurcations would enhance the understanding of the mechanisms of atheroma development, and potentially predict vessels at highest risk. The aim of this work was to validate 4D phase contrast (PC) magnetic resonance imaging flow measurements using a simplified arterial model of the left main coronary bifurcation against computational fluid dynamic (CFD) modelling

Coronary artery bifurcation haemodynamics – comparison between phase contrast MRI and computational fluid dynamics (PDF Download Available).

Ex-Vivo Stented Coronary Artery Hemodynamics Using 4D Flow Measurements and Computational Flow Dynamics (CFD)

Ex-Vivo Stented Coronary Artery Hemodynamics Using4D Flow Measurements and Computational Flow Dynamics (CFD). Susann Beier 1,∗, John Ormiston 2, Mark Webster 2, JohnCater 3, Pau Medrano-Gracia 1, Alistair Young1, BrettCowan 11Auckland MRI Research Group, University of Auckland,Auckland, New Zealand2Mercy Angiography, Auckland, New Zealand3Department of Engineering Science, University of Auckland,Auckland, New Zealand

Abstract 

Design characteristics of coronary stents, such as strutthickness, influence clinical outcome, including stentthrombosis and restenosis. In-vivo testing is difficult toperform.The geometries of two common coronary stents, theOmega (Boston Scientific) and Biomatrix Flex (BiosensorsInternational Ltd) in a straight vessel, were computation-ally reconstructed from micro CT scan data.

The accounted strut thickness of the Omega and Biomatrix stents wasis 81 ␮and 120 ␮, and the mean distance between hoops was 144 and 160 respectively. An up-scaled, rigid 3D phantom was printed using rapid prototyping and con-nected to a steady flow circuit.

Flow was measured with aphase contrast sequence on a 3 T Siemens MRI scanner.A computational flow model was solved in ANSYS CFX.Both methods used an inlet flow velocity of 0.4 m/s andaccounted for dimensional 6-fold scaling.The MR flow data was co-registered, yielding a 3D vol-ume of more than 90 million velocity sample points againstwhich the CFX data was validated (the standard deviationof the difference in velocity was 0.05 m/s).

Significant differences between the two stent designs were found; the Omega stent created jet-like blood flow acceleration, whereas the Biomatrix flow boundary layer did not detach between stent cells, such that there was no overall acceleration of the blood stream. The peak velocity was lower for the Omega stent at 0.45m/s, comparedwith 0.51 m/s for the Biomatrix. The wall shear stress was lower for the Omega stent at 0.43 Pa, compared with 0.57 Pa for the Biomatrix. In both stents, the highest wall shear stresses were found around the stent connectors, reaching a peak of 1.61 Pa for the Biomatrix and 1.2 Pa for the Omega.This study demonstrates important differences between stent designs on vessel flow acceleration, peak velocity and wall shear stress.

Optimising stent design may lessen the stimuli for in-stent thrombosis and neo-intimal proliferation.

Ex-Vivo Stented Coronary Artery Hemodynamics Using 4D Flow Measurements and Computational Flow Dynamics (CFD) (PDF Download Available).

 

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