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Three-Dimensional Assessment of Cardiovascular Blood Flow

Blood flow dictates the form and function of the heart and blood vessels. Despite this, assessment of the cardiovascular system is predominantly based on anatomical descriptions rather than flow. Turbulent blood flow dominates the flow dynamics near clinically relevant regions such as valvular and vascular stenoses and prosthetic heart valves, but has been nearly overlooked in medical imaging. This incongruity is due mainly to inadequacies in imaging tools leading to a lack of insight into this topic. This project aims to create new tools and obtain new insights addressing persistent gaps in our understanding of complex cardiovascular macroflow by in-vivo assessment of laminar and turbulent blood flow in health and disease. Utilizing an innovative approach to phase-contrast magnetic resonance imaging combined with novel post-processing methods based on constitutive equations from fluid mechanics, we anticipate providing new methods for the assessment of stenoses and prosthetic heart valves. By studying patients with specific cardiovascular diseases, we expect to obtain new insights into the design of prosthetic heart valves, and the unique effects of different surgical approaches to valve replacement and to valvular and ventricular reconstruction.

  • Principal Investigator:
    Jan Engvall, Tino Ebbers
  • Main Supervisor:
    Tino Ebbers, Jan Engvall
  • Medical Area:
    Cardiovascular System
  • Technical Area:
    Data Acquisition and Reconstruction
    Segmentation, Classification and Quantification
    Visualization and Image Enhancement
  • Modality:
    Magnetic Resonance Imaging
  • Medical Activity:
    Research
  • Technical Activity:
    Research
  • Grants:
    2762 kSEK
  • Financial Body:
    Swedish Research Council
    Swedish Heart-Lung Foundation
    Linköping University
    Knowledge Foundation
    Vårdal Foundation
    Foundation for Strategic Research
    VINNOVA
    Invest in Sweden Agency
  • Financial Support:
    National
    Local
  • Man Months:
    39
  • Project Duration:
    2005/01/01 - 2008/12/31
  • Former Staff:
  • Project Description:
  • Phase-contrast (PC) magnetic resonance imaging (MRI) is the most accurate non-invasive method for measurement of in-vivo velocities for quantification of flow. This method is versatile, accurate, and the MRI method of choice in the diagnosis of valvular regurgitation and complex congenital malformations. We have developed a method for the assessment of three-dimensional (3D) cardiovascular flow using PC-MRI (1-4). Our previous work has provided new insights regarding normal cardiovascular blood flow, such as the existence and behavior of vortical flow in the human left atrium (5) and in the aortic sinuses of Valsalva (6), and the dynamic and inhomogeneous three-dimensional relative pressure field in the heart (7).

    In this project we aim to 1) improve the assessment of cardiovascular flow by development of new acquisition, quantification and visualization methods 2) increase our understanding of the diseased heart by utilizing this technique.

     

    1)     Recently, we have finished the implementation of an improved free-breathing time-resolved 3D velocity MRI acquisition method on our Philips Achieva 1.5T research MRI system. Compared with the previous acquisition method used in Linköping, our new approach represents an enormous improvement in temporal and spatial resolution, compensation for breathing artifacts, and, most important, greater flexibility. This new acquisition strategy allows us to study the respirophasic blood flow patterns in the right heart as we have done on the left side previously. We redesigned the data handling pipeline, allowing more extensive and in-depth patient studies. We can now automatically perform all necessary corrections and transformations. Subsequent visualization using a virtual reality theater constructed in the heart of the hospital allows researchers and surgeons dramatically improved and intuitive comprehension of the results in 3D space (Fig. 1). See also Particle trace visualization of intracardiac flow (movie) and Fly through visualization of intracardiac blood flow (movie).

    Particle trace visualization of intracardiac flow

    Fig. 1. Particle trace visualization of blood flow patterns in the left (red) and right (blue) side of the heart. Here presented on our virtual reality theater.

    These techniques have demonstrated that there are flow structures and behaviors inherent to normal cardiovascular blood flows that have been missing from our understanding of human physiology. However, turbulent blood flow poses a great challenge for these 3D techniques in their present form.

    Blood flow in the healthy human body seems to be remarkably free of turbulence (8). Blood flow downstream from vascular or valvular stenoses and heart valve prostheses does contains turbulent components, however (8-10). The occurrence of turbulence in a fluid flow decreases the transport efficiency dramatically and can damage the surroundings. In the cardiovascular system, turbulence manifests as a pressure drop over a constriction, and may explain the relation between disturbed, fluctuating blood flow and the pathogenesis of atherosclerosis (11). Developing new techniques and adapting previous techniques for turbulent flow is therefore a vital task that may inform new methods of diagnosis, fundamental knowledge, treatment and device design. Recently, we have presented a method to measure the standard deviation of the blood flow velocity distribution within a voxel (intravoxel velocity standard deviation (IVSD)) (12). This method gives us access to a completely new class of data, creating a long list of potential applications. For example, flow can be assessed in terms of kinetic energy of the fluctuating velocity field (13, 15) (Figs. 2, 3).

    Quantification of mean and fluctuating flow

    Fig. 2. Visualization of the flow in a phantom with a 75 % area reduction stenosis at Reynolds number 500 (left), 1000 (right). a) Streamline visualization of the mean velocity field, b) kinetic energy of the mean velocity field, c) kinetic energy of the fluctuating velocity field. The flow direction is left to right in the images.

    Quantification of mean and fluctuating flow in a bileaflet valve phantom

    Fig. 3. Flow velocity patterns (top), kinetic energy of the mean velocity field (middle), and kinetic energy of the fluctuating velocity field (bottom) in a bileaflet valve (Sorin BicarbonÒ 27 mm) phantom.

    2)     Simultaneously with the technical development we are utilizing our techniques to investigate cardiovascular flow patterns and mixing, turbulent kinetic energy, relative pressure fields, and their interaction in both patients and healthy volunteers. By investigating laminar and turbulent flow and the pressure field in the vicinity of moderate aortic coarctations we expect to obtain new insights into the phenomenon of poststenotic dilatation and its relationship to true aortic and pulmonary aneurysms  (see movie below). Patients with dilated cardiomyopathy have disorganized flow within the left ventricle. Little is known about its effect on pumping efficiency. By investigating patients before, shortly after and 2 years after valve surgery we expect to elucidate the effects of various reconstructive surgical strategies.

    The methods for flow assessment in this project can also be used for many other applications. Our methods could facilitate research in the early evaluation of various pharmaceutical treatments and surgical procedures for many cardiovascular disorders. The choice of treatment often impacts the patient’s prognosis and quality of life. Evaluation of new therapies is often incomplete and laborious, leading to delays and loss of life. The addition of a new and accurate tool, applicable in vivo and without risk, may expedite the transition of novel treatments from bench to bedside.

    Movies

    1. Particle trace visualization of intracardiac flow (movie): Particle trace visualization of blood flow patterns in the left (red) and right (blue) side of the heart.

    2. Fly through visualization of intracardiac blood flow (movie): Fly through visualization of blood flow patterns in the left (red) and right (blue) side of the heart.

    3. Visualization of Turbulence in Aortic Coarctation (movie):Time-resolved visualization of turbulent kinetic energy (TKE), a direction-independent measure of turbulence intensity, in a patient with an aortic coarctation. A 3D iso-surface rendering of contrast enhanced MRA data outlines the geometry. Sparse 3D streamlines, emitted from the center of the coarctation, outlines the instantaneous mean velocities near the coarctation at each time frame.  The highest values of TKE can be observed downstream from the coarctation in the areas surrounding the flow jet that emerges from the coarctation. Elevated values of TKE can also be observed in the aortic root due to a subaortic membrane.

    References

    1.         Wigström L, Ebbers T, Fyrenius A, Karlsson M, Engvall J, Wranne B, Bolger AF. Particle trace visualization of intracardiac flow using time resolved 3D phase contrast MRI. Magn Reson Med 1999;41(4):793-799.

    2.         Heiberg E, Ebbers T, Wigström L, Karlsson M. Three dimensional flow characterization using vector pattern matching. IEEE Trans Vis Comp Graphics 2003;9:313 - 319.

    3.         Ebbers T, Wigström L, Bolger AF, Engvall J, Karlsson M. Estimation of relative cardiovascular pressures using time-resolved three-dimensional phase contrast MRI. Magn Reson Med 2001;45(5):872-879.

    4.         Fyrenius A, Wigström L, Bolger AF, Ebbers T, Öhman KP, Karlsson M, Wranne B, Engvall J. Pitfalls in Doppler evaluation of diastolic function: insights from three-dimensional magnetic resonance imaging. J Am Soc Echocardiogr 1999;12(10):817-826.

    5.         Fyrenius A, Wigstrom L, Ebbers T, Karlsson M, Engvall J, Bolger AF. Three dimensional flow in the human left atrium. Heart 2001;86(4):448-455.

    6.         Kvitting JP, Ebbers T, Wigstrom L, Engvall J, Olin CL, Bolger AF. Flow patterns in the aortic root and the aorta studied with time-resolved, 3-dimensional, phase-contrast magnetic resonance imaging: Implications for aortic valve-sparing surgery. J Thorac Cardiovasc Surg 2004;127(6):1602-1607.

    7.         Ebbers T, Wigstrom L, Bolger AF, Wranne B, Karlsson M. Noninvasive measurement of time-varying three-dimensional relative pressure fields within the human heart. J Biomech Eng 2002;124(3):288-293.

    8.         Stein PD, Sabbah HN. Turbulent blood flow in the ascending aorta of humans with normal and diseased aortic valves. Circulation research 1976;39(1):58-65.

    9.         McDonald DA. Blood Flow in Arteries. Southamptom: The Camelot Press Ltd; 1974.

    10.        Ghalichi F, Deng X, De Champlain A, Douville Y, King M, Guidoin R. Low Reynolds number turbulence modeling of blood flow in arterial stenoses. Biorheology 1998;35(4-5):281-294.

    11.        Brooks AR, Lelkes PI, Rubanyi GM. Gene expression profiling of vascular endothelial cells exposed to fluid mechanical forces: relevance for focal susceptibility to atherosclerosis. Endothelium 2004;11(1):45-57.

    12.        Dyverfeldt P, Sigfridsson A, Kvitting JP, Ebbers T. Quantification of intravoxel velocity standard deviation and turbulence intensity by generalizing phase-contrast MRI. Magn Reson Med 2006;56(4):850-858.

    13.        Ebbers T, Dyverfeldt P, Sigfridsson A, Kvitting JP. Quantification of Mean and Fluctuating Blood Flow.; 2006 13-16 July; New York, USA.

    14.        Bolger AF, Heiberg E, Karlsson M, Wigstrom L, Engvall J, Sigfridsson A, Ebbers T, Kvitting JP, Carlhall CJ, Wranne B. Transit of blood flow through the human left ventricle mapped by cardiovascular magnetic resonance. J Cardiovasc Magn Reson 2007;9(5):741-747.

    15.       Dyverfeldt P, Kvitting JPE, Sigfridsson A, Engvall J, Bolger AF, Ebbers T. Assessment of Fluctuating Velocities in Disturbed Cardiovascular Blood Flow: In-Vivo Feasibility of Generalized Phase-Contrast MRI. J Magn Reson Imaging 2008;28(3):655-663.

     

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