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Fast and Accurate 4D Flow MRI for Cardiovascular Blood Flow Assessment
Linköping University, Department of Medical and Health Sciences, Division of Cardiovascular Medicine. Linköping University, Center for Medical Image Science and Visualization (CMIV). Linköping University, Faculty of Health Sciences.
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The study of blood flow is essential in understanding the physiology and pathophysiology of the cardiovascular system. Small disturbances of the blood flow may over time evolve and contribute to cardiovascular pathology. While the blood flow in a healthy human appears to be predominately laminar, turbulent or transitional blood flow is thought to be involved in the pathogenesis of several cardiovascular diseases. Wall shear stress is the frictional force of blood on the vessel wall and has been linked to the pathogenesis of atherosclerosis and aneurysms. Despite the importance of hemodynamic factors, cardiovascular diagnostics largely relies on the indirect estimation of function based on morphological data.

Time-resolved three-dimensional (3D) phase-contrast magnetic resonance imaging (MRI), often referred to as 4D flow MRI, is a versatile and non-invasive tool for cardiovascular blood flow assessment. The use of 4D flow MRI permits estimation of flow volumes, pressure losses, wall shear stress, turbulence intensity and many other unique hemodynamic parameters. However, 4D flow MRI suffers from long scan times, sometimes over 40 minutes. Furthermore, the accuracy of the many different 4D flow MRI-based applications and estimates have not been thoroughly examined.

In this thesis, the accuracy of 4D flow MRI-based turbulence intensity mapping and wall shear stress estimation was investigated by using numerical simulations of MRI flow measurements. While the results from the turbulence intensity mapping agreed well with reference values from computational fluid dynamics data, the accuracy of the MRI-based wall shear stress estimates was found to be very sensitive to different parameters, especially to spatial resolution, and wall shear stress values over 5 N/m2 were not well resolved.

To reduce the scan time, a 4D flow MRI sequence using spiral k-space trajectories was implemented and validated in-vivo and in-vitro. The scan time of 4D flow MRI was reduced by more than two-fold compared to a conventional Cartesian acquisition already accelerated using SENSE factor 2, and the data quality was maintained. For a 4D flow scan of the human heart, the use of spiral k-space trajectories resulted in a scan time of around 13 min, compared to 30 min for the Cartesian acquisition. By combining parallel imaging and spiral trajectories, the total scan time of a 4D flow measurement of the entire heart may be further reduced. This scan time reduction may also be traded for higher spatial resolution.

Numerical simulation of 4D flow MRI may act as an important tool for future optimization and validation of the spiral 4D flow sequence. The scan-time reductions offered by the spiral k-space trajectories can help to cut costs, save time, reduce discomfort for the patient as well as to decrease the risk for motion artifacts. These benefits may facilitate an expanded clinical and investigative use of 4D flow MRI, including larger patient research studies.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2013.
Series
Linköping University Medical Dissertations, ISSN 0345-0082 ; 1380
National Category
Medical Engineering
Identifiers
URN: urn:nbn:se:liu:diva-100146DOI: 10.3384/diss.diva-100146ISBN: 978-91-7519-506-3 (print)OAI: oai:DiVA.org:liu-100146DiVA: diva2:661759
Public defence
2013-12-13, Berzeliussalen, Campus Valla, Linköpings universitet, Linköping, 09:00 (English)
Opponent
Supervisors
Available from: 2013-11-07 Created: 2013-10-29 Last updated: 2013-11-19Bibliographically approved
List of papers
1. Simulation of phase contrast MRI of turbulent flow
Open this publication in new window or tab >>Simulation of phase contrast MRI of turbulent flow
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2010 (English)In: Magnetic Resonance in Medicine, ISSN 0740-3194, E-ISSN 1522-2594, Vol. 64, no 4, 1039-1046 p.Article in journal (Refereed) Published
Abstract [en]

Phase contrast MRI is a powerful tool for the assessment of blood flow. However, especially in the highly complex and turbulent flow that accompanies many cardiovascular diseases, phase contrast MRI may suffer from artifacts. Simulation of phase contrast MRI of turbulent flow could increase our understanding of phase contrast MRI artifacts in turbulent flows and facilitate the development of phase contrast MRI methods for the assessment of turbulent blood flow. We present a method for the simulation of phase contrast MRI measurements of turbulent flow. The method uses an Eulerian-Lagrangian approach, in which spin particle trajectories are computed from time-resolved large eddy simulations. The Bloch equations are solved for each spin for a frame of reference moving along the spins trajectory. The method was validated by comparison with phase contrast MRI measurements of velocity and intravoxel velocity standard deviation (IVSD) on a flow phantom consisting of a straight rigid pipe with a stenosis. Turbulence related artifacts, such as signal drop and ghosting, could be recognized in the measurements as well as in the simulations. The velocity and the IVSD obtained from the magnitude of the phase contrast MRI simulations agreed well with the measurements.

Place, publisher, year, edition, pages
John Wiley and Sons, Ltd, 2010
Keyword
phase contrast magnetic resonance imaging, computer simulation, turbulent flow, constriction, turbulence intensity
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-60696 (URN)10.1002/mrm.22494 (DOI)000282477100012 ()
Available from: 2010-11-01 Created: 2010-10-22 Last updated: 2017-12-12Bibliographically approved
2. Assessment of the accuracy of MRI wall shear stress estimation using numerical simulations
Open this publication in new window or tab >>Assessment of the accuracy of MRI wall shear stress estimation using numerical simulations
2012 (English)In: Journal of Magnetic Resonance Imaging, ISSN 1053-1807, E-ISSN 1522-2586, Vol. 36, no 1, 128-138 p.Article in journal (Refereed) Published
Abstract [en]

Purpose: To investigate the accuracy of wall shear stress (WSS) estimation using MRI. Specifically, to investigate the impact of different parameters and if MRI WSS estimates are monotonically related to actual WSS. Materials and Methods: The accuracy of WSS estimation using methods based on phase-contrast (PC) MRI velocity mapping, Fourier velocity encoding (FVE) and intravoxel velocity standard deviation mapping were studied using numerical simulations. The influence of spatial resolution, velocity encoding, wall segmentation, and voxel location were investigated over a range of WSS values. Results: WSS estimates were found to be sensitive to parameter settings in general and spatial resolution in particular. All methods underestimated WSS, except for the FVE-based method, which instead was extremely sensitive to voxel position relative to the wall. Methods using PC-based WSS estimation with wall segmentation showed to be accurate for low WSS, but were sensitive to segmentation errors. Conclusion: Even in the absence of noise and for relatively simple velocity profiles, MRI WSS estimates cannot always be assumed to be linearly or even monotonically related to actual WSS. High WSS values cannot be resolved and the estimates depend on parameter setting. Nevertheless, distinguishing areas of low and moderate WSS may be feasible.

Place, publisher, year, edition, pages
Wiley-Blackwell, 2012
Keyword
wall shear stress; phase contrast magnetic resonance imaging; atherosclerosis; hemodynamics; Fourier velocity encoding; numerical simulations
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-79790 (URN)10.1002/jmri.23610 (DOI)000305185700011 ()
Available from: 2012-08-17 Created: 2012-08-14 Last updated: 2017-12-07Bibliographically approved
3. Four-dimensional flow MRI using spiral acquisition
Open this publication in new window or tab >>Four-dimensional flow MRI using spiral acquisition
2012 (English)In: Magnetic Resonance in Medicine, ISSN 0740-3194, E-ISSN 1522-2594, Vol. 68, no 4, 1065-1073 p.Article in journal (Refereed) Published
Abstract [en]

Time-resolved three-dimensional phase-contrast MRI is an important tool for physiological as well as clinical studies of blood flow in the heart and vessels. The application of the technique is, however, limited by the long scan times required. In this work, we investigate the feasibility of using spiral readouts to reduce the scan time of four-dimensional flow MRI without sacrificing quality. Three spiral approaches are presented and evaluated in vivo and in vitro against a conventional Cartesian acquisition. In vivo, the performance of each method was assessed in the thoracic aorta in 10 volunteers using pathline-based analysis and cardiac output analysis. Signal-to-noise ratio and background phase errors were investigated in vitro. Using spiral readouts, the scan times of a four-dimensional flow acquisition of the thoracic aorta could be reduced 23-fold, with no statistically significant difference in pathline validity or cardiac output. The shortened scan time improves the applicability of four-dimensional flow MRI, which may allow the technique to become a part of a clinical workflow for cardiovascular functional imaging.

Place, publisher, year, edition, pages
Wiley-Blackwell, 2012
Keyword
cine three-dimensional flow, phase contrast, spiral, pathline
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-84888 (URN)10.1002/mrm.23297 (DOI)000309203500008 ()
Note

Funding Agencies|Swedish Research Council||Swedish Heart-Lung Foundation||

Available from: 2012-11-01 Created: 2012-10-26 Last updated: 2017-12-07Bibliographically approved
4. Retrospectively Gated Intra-cardiac 4D Flow MRI using Spiral Trajectories
Open this publication in new window or tab >>Retrospectively Gated Intra-cardiac 4D Flow MRI using Spiral Trajectories
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2016 (English)In: Magnetic Resonance in Medicine, ISSN 0740-3194, E-ISSN 1522-2594, Vol. 75, no 1, 196-206 p.Article in journal (Refereed) Published
Abstract [en]

Background: Four-dimensional (4D) flow MRI is a powerful tool for the quantification of blood flow and enables calculation of a range of unique hemodynamic parameters. However, the application of cardiac 4D flow MRI is limited by long scan times (20-40 minutes). The high efficiency of spiral readouts can be used to reduce scan times without sacrificing SNR. The aim of this work was to develop and validate a retrospectively gated 4D flow MRI sequence using spiral readouts for the measurement of intra-cardiac velocities.

Methods: A retrospectively ECG gated 4D flow sequence using stacks of spiral readouts was implemented on a clinical 1.5 T MRI scanner. The spiral 4D flow MRI sequence was validated in-vivo by comparisons with a two-dimensional (2D) through-plane velocity measurement and a conventional Cartesian 4D flow acquisition (SENSE factor 2) in 7 healthy volunteers (age 27 ± 3 years, four men) and 2 patients (age 19 and 52, women, only spiral 4D flow and 2D). Net volume flow was estimated from all three acquisition approaches and compared using one-way ANOVA. A quantitative pathline based validation was performed on the Cartesian and the spiral 4D flow MRI acquisitions by comparing the left ventricular inflow and outflow (two-tailed paired t-tests).

Results: The scan time of the spiral 4D flow sequence was 44±6% of the Cartesian counterpart. Compared to time-resolved 2D flow in the aorta, the spiral and Cartesian 4D flow acquisitions provided similarly good data, as there was no significant difference between the net volume flow for all acquisitions (Spiral: 89±14 ml, Cartesian: 93±11 ml, 2D: 93±18 ml, p=0.878). There was no significant difference between pathline-based calculations of inflow and outflow with either Cartesian (In: 88±15, Out: 85±16, p = 0.168) or spiral (In: 93±17 ml, Out: 84±18, p = 0.055) 4D flow acquisitions.

Conclusions: Retrospectively gated spiral cardiac 4D flow MRI permits more than two-fold reduction in scan time compared to conventional Cartesian 4D flow MRI without notable loss in data quality. The time-savings offered by spiral trajectories could provide a step towards the expanded clinical use of 4D flow MRI.

Place, publisher, year, edition, pages
Wiley-Blackwell, 2016
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-101103 (URN)10.1002/mrm.25612 (DOI)000367739200019 ()
Note

Funding agencies: European Research Council [310612]; Swedish Research Council; Swedish Heart and Lung foundation

Available from: 2013-11-19 Created: 2013-11-19 Last updated: 2017-05-03Bibliographically approved
5. Quantification of Stenotic Flow Using Spiral 3D Phase-Contrast MRI
Open this publication in new window or tab >>Quantification of Stenotic Flow Using Spiral 3D Phase-Contrast MRI
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2013 (English)Manuscript (preprint) (Other academic)
Abstract [en]

Purpose: To evaluate the feasibility of spiral 3D phase contrast MRI for the assessment of velocity, volume flow rate, peak velocity and turbulent kinetic energy in stenotic flow.

Materials and Methods: A-stack-of-spirals 3D phase contrast MRI sequence was evaluated in-vitro against a conventional Cartesian sequence. Measurements were made in a flow phantom with a 75% stenosis. Both spiral and Cartesian imaging were performed using different scan orientations and flow rates. Volume flow rate, peak velocity and turbulent kinetic energy (TKE) were computed for both methods. For further validation, the estimated TKE was compared to computational fluid dynamics (CFD) data.

Results: The volume flow rate, peak velocity and TKE obtained with spiral 4D flow MRI agreed well with Cartesian data and CFD data. As expected, the short echo time of the spiral sequence resulted in less prominent displacement artifacts compared to the Cartesian sequence. However, both spiral and Cartesian flow rate estimates were sensitive to displacement when the flow was oblique to the encoding directions.

Conclusion: Spiral 3D phase contrast MRI appears favorable for the assessment of stenotic flow. The spiral sequence was more than three times faster and less sensitive to displacement artifacts when compared to a conventional Cartesian sequence.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-101107 (URN)
Available from: 2013-11-19 Created: 2013-11-19 Last updated: 2013-12-17Bibliographically approved

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