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Improving Assessments of Hemodynamics and Vascular Disease
Linköping University, Department of Medical and Health Sciences, Division of Cardiovascular Medicine. Linköping University, Faculty of Medicine and Health Sciences. Linköping University, Center for Medical Image Science and Visualization (CMIV). (CMR)ORCID iD: 0000-0001-9184-9234
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Blood vessels are more than simple pipes, passively enabling blood to pass through them. Their form and function are dynamic, changing with both aging and disease. This process involves a feedback loop wherein changes to the shape of a blood vessel affect the hemodynamics, causing yet more structural adaptation. This feedback loop is driven in part by the hemodynamic forces generated by the blood flow, and the distribution and strength of these forces appear to play a role in the initiation, progression, severity, and the outcome of vascular diseases.

Magnetic Resonance Imaging (MRI) offers a unique platform for investigating both the form and function of the vascular system. The form of the vascular system can be examined using MR-based angiography, to generate detailed geometric analyses, or through quantitative techniques for measuring the composition of the vessel wall and atherosclerotic plaques. To complement these analyses, 4D Flow MRI can be used to quantify the functional aspect of the vascular system, by generating a full time-resolved three-dimensional velocity field that represents the blood flow.

This thesis aims to develop and evaluate new methods for assessing vascular disease using novel hemodynamic markers generated from 4D Flow MRI and quantitative MRI data towards the larger goal of a more comprehensive non-invasive examination oriented towards vascular disease. In Paper I, we developed and evaluated techniques to quantify flow stasis in abdominal aortic aneurysms to measure this under-explored aspect of aneurysmal hemodynamics. In Paper II, the distribution and intensity of turbulence in the aorta was quantified in both younger and older men to understand how aging changes this aspect of hemodynamics. A method to quantify the stresses generated by turbulence that act on the vessel wall was developed and evaluated using simulated flow data in Paper III, and in Paper V this method was utilized to examine the wall stresses of the carotid artery. The hemodynamics of vascular disease cannot be uncoupled from the anatomical changes the vessel wall undergoes, and therefore Paper IV developed and evaluated a semi-automatic method for quantifying several aspects of vessel wall composition. These developments, taken together, help generate more valuable information from imaging data, and can be pooled together with other methods to form a more comprehensive non-invasive examination for vascular disease.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2019. , p. 64
Series
Linköping University Medical Dissertations, ISSN 0345-0082 ; 1675
National Category
Medical Image Processing Biomedical Laboratory Science/Technology
Identifiers
URN: urn:nbn:se:liu:diva-156311DOI: 10.3384/diss.diva-156311ISBN: 9789176850985 (print)OAI: oai:DiVA.org:liu-156311DiVA, id: diva2:1304518
Public defence
2019-05-31, Hugo Theorell, Norra Entrén, Campus US, Linköping, 13:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 2013-06077Swedish Research Council, 2017-03857Region Östergötland, LIO-752951Available from: 2019-04-17 Created: 2019-04-12 Last updated: 2019-04-24Bibliographically approved
List of papers
1. Visualizing and quantifying flow stasis in abdominal aortic aneurysms in men using 4D flow MRI
Open this publication in new window or tab >>Visualizing and quantifying flow stasis in abdominal aortic aneurysms in men using 4D flow MRI
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2019 (English)In: Magnetic Resonance Imaging, ISSN 0730-725X, E-ISSN 1873-5894, Vol. 57, p. 103-110Article in journal (Refereed) Published
Abstract [en]

Purpose: To examine methods for visualizing and quantifying flow stasis in abdominal aortic aneurysms (AAA) using 4D Flow MRI. Methods: Three methods were investigated: conventional volumetric residence time (VRT), mean velocity analysis (MVA), and particle travel distance analysis (TDA). First, ideal 4D Flow MRI data was generated using numerical simulations and used as a platform to explore the effects of noise and background phase-offset errors, both of which are common 4D Flow MRI artifacts. Error-free results were compared to noise or offset affected results using linear regression. Subsequently, 4D Flow MRI data for thirteen (13) subjects with AAA was acquired and used to compare the stasis quantification methods against conventional flow visualization. Results: VRT (R-2 = 0.69) was more sensitive to noise than MVA (R-2 = 0.98) and TDA (R-2 = 0.99) at typical noncontrast signal-to-noise ratio levels (SNR = 20). VRT (R-2 = 0.14) was more sensitive to background phase-offsets than MVA (R-2 = 0.99) and TDA (R-2 = 0.96) when considering a 95% effective background phase-offset correction. Qualitatively, TDA outperformed MVA (Wilcoxon p amp;lt; 0.005, mean score improvement 1.6/5), and had good agreement (median score 4/5) with flow visualizations. Conclusion: Flow stasis can be quantitatively assessed using 4D Flow MRI. While conventional residence time calculations fail due to error accumulation as a result of imperfect measured velocity fields, methods that do not require lengthy particle tracking perform better. MVA and TDA are less sensitive to measurement errors, and TDA generates results most similar to those obtained using conventional flow visualization.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE INC, 2019
Keywords
Abdominal aortic aneurysm; Hemodynamics; 4D flow MRI; Flow stasis
National Category
Medical Laboratory and Measurements Technologies
Identifiers
urn:nbn:se:liu:diva-154524 (URN)10.1016/j.mri.2018.11.003 (DOI)000458096100012 ()30445146 (PubMedID)
Available from: 2019-02-20 Created: 2019-02-20 Last updated: 2019-04-17
2. Age-Related Vascular Changes Affect Turbulence in Aortic Blood Flow
Open this publication in new window or tab >>Age-Related Vascular Changes Affect Turbulence in Aortic Blood Flow
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2018 (English)In: Frontiers in Physiology, ISSN 1664-042X, E-ISSN 1664-042X, Vol. 9, article id 36Article in journal (Refereed) Published
Abstract [en]

Turbulent blood flow is implicated in the pathogenesis of several aortic diseases but the extent and degree of turbulent blood flow in the normal aorta is unknown. We aimed to quantify the extent and degree of turbulece in the normal aorta and to assess whether age impacts the degree of turbulence. 22 young normal males (23.7 +/- 3.0 y.o.) and 20 old normal males (70.9 +/- 3.5 y.o.) were examined using four dimensional flow magnetic resonance imaging (4D Flow MRI) to quantify the turbulent kinetic energy (TKE), a measure of the intensity of turbulence, in the aorta. All healthy subjects developed turbulent flow in the aorta, with total TKE of 3-19 mJ. The overall degree of turbulence in the entire aorta was similar between the groups, although the old subjects had about 73% more total TKE in the ascending aorta compared to the young subjects (young = 3.7 +/- 1.8 mJ, old = 6.4 +/- 2.4 mJ, p amp;lt; 0.001). This increase in ascending aorta TKE in old subjects was associated with age-related dilation of the ascending aorta which increases the volume available for turbulence development. Conversely, age-related dilation of the descending and abdominal aorta decreased the average flow velocity and suppressed the development of turbulence. In conclusion, turbulent blood flow develops in the aorta of normal subjects and is impacted by age-related geometric changes. Non-invasive assessment enables the determination of normal levels of turbulent flow in the aorta which is a prerequisite for understanding the role of turbulence in the pathophysiology of cardiovascular disease.

Place, publisher, year, edition, pages
FRONTIERS MEDIA SA, 2018
Keywords
turbulent kinetic energy (TKE); turbulent blood flow; aortic blood flow; aortic dilation; normal values; 4D flow MRI; phase contrast MRI
National Category
Physiology
Identifiers
urn:nbn:se:liu:diva-145129 (URN)10.3389/fphys.2018.00036 (DOI)000423400000001 ()29422871 (PubMedID)
Note

Funding Agencies|Swedish Research Council [2013-6077, 2014-6191]; Swedish Heart and Lung foundation [20140398]; Kangwon National University [D1001179-01-01]; Basic Science Research Program through the National Research Foundation of Korea (NRF) - Ministry of Education [2016R1A6A3A03006337]

Available from: 2018-02-19 Created: 2018-02-19 Last updated: 2019-04-17
3. Assessment of Turbulent Flow Effects on the Vessel Wall Using Four-Dimensional Flow MRI
Open this publication in new window or tab >>Assessment of Turbulent Flow Effects on the Vessel Wall Using Four-Dimensional Flow MRI
2017 (English)In: Magnetic Resonance in Medicine, ISSN 0740-3194, E-ISSN 1522-2594, Vol. 77, no 6, p. 2310-2319Article in journal (Refereed) Published
Abstract [en]

Purpose: To explore the use of MR-estimated turbulence quantities for the assessment of turbulent flow effects on the vessel wall. Methods: Numerical velocity data for two patient-derived models was obtained using computational fluid dynamics (CFD) for two physiological flow rates. The four-dimensional (4D) Flow MRI measurements were simulated at three different spatial resolutions and used to investigate the estimation of turbulent wall shear stress (tWSS) using the intravoxel standard deviation (IVSD) of velocity and turbulent kinetic energy (TKE) estimated near the vessel wall. Results: Accurate estimation of tWSS using the IVSD is limited by the spatial resolution achievable with 4D Flow MRI. TKE, estimated near the wall, has a strong linear relationship to the tWSS (mean R(2=)0.84). Near-wall TKE estimates from MR simulations have good agreement to CFD-derived ground truth (mean R-2=0.90). Maps of near-wall TKE have strong visual correspondence to tWSS. Conclusion: Near-wall estimation of TKE permits assessment of relative maps of tWSS, but direct estimation of tWSS is challenging due to limitations in spatial resolution. Assessment of tWSS and near-wall TKE may open new avenues for analysis of different pathologies. (C) 2016 International Society for Magnetic Resonance in Medicine

Place, publisher, year, edition, pages
WILEY, 2017
Keywords
phase contrast magnetic resonance imaging; wall shear stress; turbulence; turbulent kinetic energy; aorta
National Category
Medical Image Processing
Identifiers
urn:nbn:se:liu:diva-138232 (URN)10.1002/mrm.26308 (DOI)000401270900022 ()27350049 (PubMedID)
Note

Funding Agencies|Swedish Research Council; National Supercomputer Centre [SNIC2014-11-22]

Available from: 2017-06-14 Created: 2017-06-14 Last updated: 2019-04-17

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