Patent Publication Number: US-2022237864-A1

Title: Reduced order model for computing blood flow dynamics

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of U.S. Provisional Application No. 62/867,041 filed Jun. 26, 2019, the full content of which is incorporated herein by reference in its entirety. 
    
    
     GOVERNMENT SUPPORT 
     This invention was made with government support under Grant Number R01-HL103419 awarded by the National Institutes of Health. The government has certain rights in the invention. 
    
    
     BACKGROUND 
     Image-based computational fluid dynamics (CFD) is the current standard to study global and local blood flow features in the cardiovascular system. Applications of image-based CFD are numerous, and include assessment of pathologies, such as vascular stenosis, aneurysms, and dissection, as well as for surgical planning and the evaluation of medical devices. This technology has grown considerably over the last decade, gaining recent FDA approval and clinical adoption. Although image-based CFD enables comprehensive analysis of hemodynamics, it is computationally expensive, prone to numerical instabilities, and can be sensitive to numerical-method parameters. These factors have limited broader translation of image-based CFD to clinical settings, and have limited the ability to perform parametric analyses in cardiovascular applications such as uncertainty quantification, sensitivity analysis, data assimilation, optimization, parameter tuning, etc. Such ensemble-based studies are becoming crucial as hemodynamics modeling matures from a tool for exploratory research, to a paradigm for improving healthcare decisions. These factors motivate the need for reduced-order models (ROMs) of blood flow to be used in place of or in conjunction with fully-resolved CFD modeling. 
     ROMs can roughly be divided into data-fitting methods (e.g. polynomial chaos expansion, machine learning, etc.) and theory-based methods. The main focus here is on theory-based methods. The most basic ROMs are lumped parameter network (LPN) models. Like the Navier-Stokes equation, LPNs describe a mathematical relationship between flow, pressure and their derivatives; however, the relationship is given by an ordinary differential equation (ODE) and algebraic equation (AE). Mostly, LPNs can be interpreted as electrical circuit analogs. The most basic example being a vascular network (or single vessel) modeled is a resistor, where flow rate is linearly proportional to pressure drop across the network (vessel). The 3-element “RCR” is a popular and simple LPN, but more elaborate LPNs are also widely employed. LPN models are generally used to describe complete vascular territories, and by construction, continuous spatial variations in flow and pressure are not resolved. 
     In order to resolve continuous spatial variations in flow and pressure, the most common reduced-order modeling approach is to cross-sectionally average the Navier-Stokes equations to generate 1D (or quasi-1D) equations. This approach has been used extensively for decades to understand vascular fluid dynamics. Similarly, 2D models have been proposed to generalize 1D models while still reducing the computational cost of CFD simulation. Both 1D and 2D models are described by partial differential equations (PDEs), and are thus more computationally expensive than LPN models and more prone to numerical artifacts and instability. 
     The basic idea of lumped parameter modeling can be used to develop quasi-distributed pressure-flow models. For example, in prior works blood flow through a network of cerebrovascular arteries was modeled by assigning a net viscous resistance to each segment in the model and coupled with expected pressure losses at bifurcations. The underlying assumptions of Poiseuille flow break down in larger arteries and veins and other sources of energy dissipation such as from flow unsteadiness, kinetic effects, and vessel curvature can be significant. 
     SUMMARY 
     Implementations of the disclosed technology are generally directed to methods and systems for non-invasive assessment of blood flow dynamics (hemodynamics) information based on features extracted from angiographic medical image data. The proposed techniques generally rely on principles of fluid mechanics to develop a subject-specific lumped parameter network (LPN) of resistors (a surrogate model) describing expected energy losses along vascular segments, including from viscous dissipation, unsteadiness, flow separation, vessel curvature and vessel bifurcations. The surrogate model incorporates additional LPNs or mathematical relations that can describe upstream or downstream physiologic effects. Compared to computational fluid dynamics (CFD) methods that have traditionally been used to compute hemodynamics data, this methodology presents a significant reduction in computational effort and complexity. 
     This approach could be used to rapidly compute blood flow or pressure information from medical images or image-based computer models. Relevant information could include prediction of pressure drop across stenotic vascular segments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example of modeling steps of a framework  100  for computing blood flow and pressure in a representative patient-specific coronary model using a distributed LP method in accordance with the disclosed technology. 
         FIG. 2  illustrates a first example  200  of an aorta model  202  that may be used in connection with implementations of the disclosed technology and corresponding time-varying inflow waveform  204 . 
         FIG. 3  illustrates a second example  300  of an aorta model  302  that may be used in connection with implementations of the disclosed technology and corresponding time-varying inflow waveform  304 . 
         FIG. 4  illustrates a third example  400  of an aorta model  402  that may be used in connection with implementations of the disclosed technology and corresponding time-varying inflow waveform  404 . 
         FIG. 5  illustrates a fourth example  500  of an aorta model that may be used in connection with implementations of the disclosed technology and corresponding time-varying inflow waveform  504 . 
         FIG. 6  illustrates an example  600  of errors of mean flow rate between the DLP model ( Q   0D ,  P   0D ) and the full 3Dt CFD simulation ( Q   3D ,  P   3D ) at inlets and outlet of the aortic models  202 ,  302 ,  402 , and  502  of  FIGS. 2-5 , respectively. 
         FIG. 7  illustrates an example  700  of errors of pressure between the DLP model ( Q   0D ,  P   0D ) and the full 3Dt CFD simulation ( Q   3D ,  P   3D ) at inlets and outlet of the aortic models  202 ,  302 ,  402 , and  502  of  FIGS. 2-5 , respectively. 
         FIG. 8  illustrates an example  800  of mean pressure errors between the 3Dt CFD and a DLP model assigning a Poiseuille resistance to each vascular segment ( P   pois ), of the aortic models  202 ,  302 ,  402 , and  502  of  FIGS. 2-5 , respectively. 
         FIG. 9  illustrates a first example  900  of an aorto-femoral model  902  that may be used in connection with implementations of the disclosed technology and corresponding time-varying inflow waveform  904 . 
         FIG. 10  illustrates a second example  1000  of an aorto-femoral model  1002  that may be used in connection with implementations of the disclosed technology and corresponding time-varying inflow waveform  1004 . 
         FIG. 11  illustrates a third example  1100  of an aorto-femoral model  1102  that may be used in connection with implementations of the disclosed technology and corresponding time-varying inflow waveform  1104 . 
         FIG. 12  illustrates a fourth example  1200  of an aorto-femoral model  1202  that may be used in connection with implementations of the disclosed technology and corresponding time-varying inflow waveform  1204 . 
         FIG. 13  illustrates an example  1300  of errors of mean flow rate between the DLP model ( Q   0D ,  P   0D ) and the full 3Dt CFD simulation ( Q   3D ,  P   3D ) at inlets and outlets of the aorto-femoral models  902 ,  1002 ,  1102 , and  1202  of  FIGS. 9-12 , respectively. 
         FIG. 14  illustrates an example  1400  of errors of pressure between the DLP model ( Q   0D ,  P   0D ) and the full 3Dt CFD simulation ( Q   3D ,  P   3D ) at inlets and outlets of the aorto-femoral models  902 ,  1002 ,  1102 , and  1202  of  FIGS. 9-12 , respectively. 
         FIG. 15  illustrates an example  1500  of mean pressure errors between the 3Dt CFD and a DLP model assigning a Poiseuille resistance to each vascular segment ( P   pois ), of the aorto-femoral models  902 ,  1002 ,  1102 , and  1202  of  FIGS. 9-12 , respectively. 
         FIG. 16  illustrates a first example  1600  of a coronary model  1602  that may be used in connection with implementations of the disclosed technology  1604 . 
         FIG. 17  illustrates a second example  1700  of a coronary model  1702  that may be used in connection with implementations of the disclosed technology. 
         FIG. 18  illustrates a third example  1800  of a coronary model  1802  that may be used in connection with implementations of the disclosed technology. 
         FIG. 19  illustrates a fourth example  1900  of a coronary model  1902  that may be used in connection with implementations of the disclosed technology. 
         FIG. 20  illustrates an example  2000  of a typical time-varying inflow waveform, left myocardium pressure waveform, and right myocardium pressure waveform prescribed at the ascending aorta of the models  1602 ,  1702 ,  1802 , and  1902  of  FIGS. 16-19 , respectively. 
         FIG. 21  illustrates an example  2100  of errors of mean flow rate between the DLP model ( Q   0D ,  P   0D ) and the full 3Dt CFD simulation ( Q   3D ,  P   3D ) at inlets and outlets of the coronary models of  FIGS. 16-19 . 
         FIG. 22  illustrates an example  2200  of errors of pressure between the DLP model ( Q   0D ,  P   0D ) and the full 3Dt CFD simulation ( Q   3D ,  P   3D ) at inlets and outlets of the coronary models of  FIGS. 16-19 . 
         FIG. 23  illustrates an example  2300  of mean pressure errors between the 3Dt CFD and a DLP model assigning a Poiseuille resistance to each vascular segment ( P   pois ), of the coronary models of  FIGS. 16-19 . 
         FIG. 24  illustrates a first example  2400  of a cerebrovascular model  2402  that may be used in connection with implementations of the disclosed technology and corresponding time-varying inflow waveform  2404  prescribed at the left and right vertebral arteries. 
         FIG. 25  illustrates a second example  2500  of a cerebrovascular model  2502  that may be used in connection with implementations of the disclosed technology and corresponding time-varying inflow waveform  2504  prescribed at the left and right vertebral arteries. 
         FIG. 26  illustrates a third example  2600  of a cerebrovascular model  2602  that may be used in connection with implementations of the disclosed technology and corresponding time-varying inflow waveform  2604  prescribed at the left and right vertebral arteries. 
         FIG. 27  illustrates a fourth example  2700  of a cerebrovascular model  2702  that may be used in connection with implementations of the disclosed technology and corresponding time-varying inflow waveform  2704  prescribed at the left and right vertebral arteries. 
         FIG. 28  illustrates an example  2800  of errors of mean flow rate between the DLP model ( Q   0D ,  P   0D ) and the full 3Dt CFD simulation ( Q   3D ,  P   3D ) at inlets and outlets of the cerebrovascular models  2402 ,  2502 ,  2602 , and  2702  of  FIGS. 24-27 , respectively. 
         FIG. 29  illustrates an example  2900  of errors of pressure between the DLP model ( Q   0D ,  P   0D ) and the full 3Dt CFD simulation ( Q   3D ,  P   3D ) at inlets and outlets of the cerebrovascular models  2402 ,  2502 ,  2602 , and  2702  of  FIGS. 24-27 , respectively. 
         FIG. 30  illustrates an example  3000  of mean pressure errors between the 3Dt CFD and a DLP model assigning a Poiseuille resistance to each vascular segment ( P   pois ), of the cerebrovascular models  2402 ,  2502 ,  2602 , and  2702  of  FIGS. 24-27 , respectively. 
         FIG. 31  illustrates a first example  3100  of a pulmonary model  3102  that may be used in connection with implementations of the disclosed technology and corresponding time-varying pulmonary blood flow waveform  3104 . 
         FIG. 32  illustrates a second example  3200  of a pulmonary model  3202  that may be used in connection with implementations of the disclosed technology and corresponding time-varying pulmonary blood flow waveform  3204 . 
         FIG. 33  illustrates a third example  3300  of a pulmonary model  3302  that may be used in connection with implementations of the disclosed technology and corresponding time-varying pulmonary blood flow waveform  3304 . 
         FIG. 34  illustrates a fourth example  3400  of a pulmonary model  3402  that may be used in connection with implementations of the disclosed technology and corresponding time-varying pulmonary blood flow waveform  3404 . 
         FIG. 35  illustrates an example  3500  of errors of mean flow rate between the DLP model ( Q   0D ,  P   0D ) and the full 3Dt CFD simulation ( Q   3D ,  P   3D ) at inlets and outlets of the pulmonary models  3102 ,  3202 ,  3302 , and  3402  of  FIGS. 31-34 , respectively. 
         FIG. 36  illustrates an example  3600  of errors of pressure between the DLP model ( Q   0D ,  P   0D ) and the full 3Dt CFD simulation ( Q   3D ,  P   3D ) at inlets and outlets of the pulmonary models  3102 ,  3202 ,  3302 , and  3402  of  FIGS. 31-34 , respectively. 
         FIG. 37  illustrates an example  3700  of mean pressure errors between the 3Dt CFD and a DLP model assigning a Poiseuille resistance to each vascular segment ( P   pois ), of the pulmonary models  3102 ,  3202 ,  3302 , and  3402  of  FIGS. 31-34 , respectively. 
         FIG. 38  illustrates a first example  3800  of a congenital heart disease model  3802  that may be used in connection with implementations of the disclosed technology and corresponding time-varying inflow waveform  3804  prescribed at the inlets. 
         FIG. 39  illustrates a second example  3900  of a congenital heart disease model  3902  that may be used in connection with implementations of the disclosed technology and corresponding time-varying inflow waveform  3904  prescribed at the inlets. 
         FIG. 40  illustrates a third example  4000  of a congenital heart disease model  4002  that may be used in connection with implementations of the disclosed technology and corresponding time-varying inflow waveform  4004  prescribed at the inlets. 
         FIG. 41  illustrates a fourth example  4100  of a congenital heart disease model  4102  that may be used in connection with implementations of the disclosed technology and corresponding time-varying inflow waveform  4104  prescribed at the inlets. 
         FIG. 42  illustrates an example  4200  of errors of mean flow rate between the DLP model ( Q   0D ,  P   0D ) and the full 3Dt CFD simulation ( Q   3D ,  P   3D ) at inlets and outlets of the congenital heart disease models  3802 ,  3902 ,  4002 , and  4102  of  FIGS. 38-41 , respectively. 
         FIG. 42  illustrates an example  4300  of errors of pressure between the DLP model ( Q   0D ,  P   0D ) and the full 3Dt CFD simulation ( Q   3D ,  P   3D ) at inlets and outlets of the congenital heart disease models  3802 ,  3902 ,  4002 , and  4102  of  FIGS. 38-41 , respectively. 
         FIG. 44  illustrates an example  4400  of mean pressure errors between the 3Dt CFD and a DLP model assigning a Poiseuille resistance to each vascular segment ( P   pois ), the congenital heart disease models  3802 ,  3902 ,  4002 , and  4102  of  FIGS. 38-41 , respectively. 
     
    
    
     DETAILED DESCRIPTION 
     Implementations of the disclosed technology are generally directed to a distributed lumped parameter (DLP) modeling framework arranged to rapidly compute blood flow and pressure in vascular domains. This can be achieved by developing analytical expressions describing expected energy losses along vascular segments, including from viscous dissipation, unsteadiness, flow separation, vessel curvature and vessel bifurcations. This methodology can be applied to solve for unsteady blood flow and pressure in a variety of complex 3D image-based vascular geometries, which are typically approached using time-dependent computational fluid dynamics (CFD) simulations. 
     Implementations of the DLP framework disclosed herein generally demonstrate consistent agreement with CFD simulations for a range of flow conditions and vascular anatomies, with mean errors in flow rate and pressure less than 7% over the range models considered. The computational cost of the DLP framework is orders of magnitude lower than the computational cost of CFD, which opens new possibilities to use hemodynamics modeling for timely decision support in clinical settings, or to improve the ability to perform data assimilation, optimization, parameter tuning, uncertainty quantification and sensitivity analysis for cardiovascular modeling applications. 
     Implementations of the disclosed technology are generally directed to a distributed lumped parameter (DLP) framework to compute temporal flow and pressure in different vasculature models by taking into account various sources of energy dissipation. This approach results in ODEs that are significantly easier to solve than the aforementioned 1D and 2D PDE-based models. This approach can be even more accurate depending on the types of energy losses considered. 
     Most ROM approaches described above are reasonable in smaller vessels where the flow becomes 1D and fully developed. However, most ROMs produce erroneous results for flow in larger arteries, as typically derived from medical imaging, where flow conditions are known to be complex and unsteady. 
     Therefore, to evaluate the accuracy of the proposed DLP methodology, this framework is applied to a diverse range of healthy and diseased patient-specific cardiovascular anatomies including aortic, aorto-femoral, cerebrovascular, coronary, pulmonary and congenital heart disease models, and have compared the DLP results to the “gold-standard” from 3D time-dependent (“3Dt”) CFD simulations. The ability of this framework to be automated provides many advantage for both exploratory and translational applications. 
     Similar to most fluid mechanic models, a relationship can be developed between flow and pressure from conservation of mass and balance of momentum principles. Namely, the formulation can be in terms of cross-sectionally averaged pressure P(x) and volumetric flow rate Q(x), where x denotes the axial coordinate along a given vessel. The cross-sectionally averaged Navier-Stokes equations, under suitable assumptions, reduce upon linearization to 
     
       
         
           
             
               
                 
                   
                     
                       
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     where A 0  is the reference cross-sectional area, ρ is fluid density and f characterizes viscous dissipation. These equations have been widely used to describe flow and pressure in vascular domains, but are generally inaccurate in realistic geometries that contain energy loss due to curvature, flow separation, sudden expansions, etc. Therefore, as a basic adaptation, the dissipation term is redefined on the right hand side of Eq. (2) to account for losses neglected in deriving this equation. Namely, for a vascular segment of length L and (spatially varying) radius R, a momentum balance is considered of the form 
     
       
         
           
             
               
                 
                   
                     
                       
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     An appropriate vascular resistance can be defined by considering expected sources of energy dissipation, that when used in Eq. (3) and along with Eq. (1) and appropriate boundary conditions, provides accurate estimation of hemodynamics in realistic vascular geometries, particularly those derived from medical imaging and normally approached by CFD simulation. 
     Viscous Dissipation 
     Assuming Poiseuille flow, the pressure drop across a cylindrical vessel of length L is given by ΔP=R v Q, where R v =8 μL/πR 4  is the hydraulic resistance, with μ being the blood viscosity and R being the vessel radius. An integral form of this equation is considered to better account for potential variations of radius along the length of a vessel as 
     
       
         
           
             
               
                 
                   
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     For vessels of non-circular cross-section, R(x)=√{square root over (A(x)/π)} denotes the effective radius (henceforth radius) at local axial coordinate x∈(0, L). 
     The Poiseuille law, ΔP=8 μLQ/πR 4 , can be viewed a special case of the Darcy-Weisbach equation 
     
       
         
           
             
               
                 
                   
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     where V=Q/πR 2  is the average fluid velocity, and λ s  is the viscous friction factor. The viscous friction factor of a straight vessel is λ s =64/Re, which yields Poiseuille&#39;s law, where Re denotes the Reynolds number. This viewpoint can be used next to incorporate pressure loss due to vessel curvature. 
     Curvature 
     The presence of centrifugal forces in a curved vessel results in a skewed velocity profile, which generally increases viscous dissipation as compared to flow through a straight vessel. Assuming steady and fully developed flow, the viscous friction factor of a curved vessel (λ c ) can be related to the viscous friction factor of a straight vessel (λ s ) with similar length and radius 
     
       
         
           
             
               
                 
                   
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     where K=Re√{square root over (R/a)} is the Dean number and a is the vessel curvature. Therefore, Eq. (4) can be modified to 
     
       
         
           
             
               
                 
                   
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     Note that R, a and K generally vary as a function of x. 
     Expansions 
     The energy dissipation due to sudden expansions can be taken into account by using a semi-empirical model 
     
       
         
           
             
               
                 
                   
                     
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     where A s  and A 0  are cross-sectional areas describing the expansion as described below, and |Q| denotes the absolute value of flow rate (in case of reverse flow). Eq. (8) with K t =1 can be derived from a control volume energy balance of inviscid flow through a sudden expansion. K t  is added an empirical correction factor to account for losses from flow separation. A previous study showed that K t =1.52 produced consistently accurate prediction of pressure drop compared to 3D simulations in realistic vascular (coronary) stenoses. 
     In this study, the local minima and maxima of R(x) were computed along each vessel segment. Then A s  was computed at each local minimum. For each local minimum, A 0  was computed from the mean value of the radius at the local maxima immediately proximal and distal to the corresponding local minimum. For vascular segments with multiple sudden expansions, these losses were added in series 
     
       
         
           
             
               
                 
                   
                     
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     where n is the number of sudden expansion locations over the given vascular segment. Aggregate losses due to sudden expansions were added in series to the viscous pressure loss derived above. 
     Bifurcations 
     Energy losses at vascular junctions were determined from geometric (branch angle) and hydraulic (flow split) considerations. Namely, the amount of pressure drop due to a vascular bifurcation was calculated as 
     
       
         
           
             
               
                 
                   
                     
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     where Q dat  and A dat  are the flow rate and cross-sectional area of the datum supplier vessel, respectively. λ j =Q j /Q dat  defines the volumetric flow split, ψ j =A dat /A j  defines the area split and φ j =3(π−θ j )/4 is defined from the angle θ j  between a datum supplier and child branch. Hence the nonlinear resistance 
     
       
         
           
             
               
                 
                   
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     can be added in series to the resistances due to viscous and sudden expansion effects in the child branch. 
     Unsteadiness 
     Flow unsteadiness affects the momentum balance in Eq. (3) in two main ways. First, it explicitly leads to a change in fluid inertia as captured in the first term. Second, it implicitly changes the cross-sectional profile of the flow, which in turn changes the shear rate and viscous dissipation captured in the second term. To account for the later, the viscous resistance may be modified according to the Womersley number. 
     The Womersley number, α=R√{square root over (ρω/μ)}, quantifies the relative importance between pulsatile inertial effects and viscous effects. The heart rate is generally the dominant frequency and thus used to define ω. For image-based vascular models, ω, ρ and μ vary minimally and thus intra- and inter-model variations of a are due to variations in vessel radius R. 
     As the Womersley number increases the velocity profile changes from parabolic to flat, increasing viscous losses. Based on the well-known Womersley&#39;s solution for pulsatile flow in a straight, rigid vessel, wall shear stress can be computed and then used to compute the (viscous) resistance due to Womersley flow. This can be used to define a function ζ(α) describing the viscous resistance ratio between Womersley and Poiseuille flow. Hence, the (nominally Poiseuille) viscous resistance introduced in Eq. (4) can be modified to 
     
       
         
           
             
               
                 
                   
                     
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     Implementation 
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     and Eq. (3) along with conservation of mass can be used to solve for flow rate through each vascular segment and pressure at vascular junctions. 
     Certain embodiments include an in-house Python framework to completely automate the DLP modeling procedure based on image-based geometry. Namely, the only input to the framework might be the geometric model (e.g. STL surface mesh) and specification of boundary conditions. Automated procedures can arranged to compute vessel lengths, bifurcation angles, and the variation of cross-sectional area and curvature along each vascular segment. A variety of boundary conditions can be implemented typical to current state-of-the-art image-based modeling, including specification of a time-varying flow or pressure waveform, or coupled LPN models at the inlet or outlets. 
     The end result is generally a system of nonlinear AEs and ODEs that describe the basic conservation laws and coupled boundary conditions. An implicit Euler scheme may be used to handle time-stepping. This results in a set of nonlinear algebraic equations to be solved each time-step, which is handled by iterative linearization. The solution of this system includes flow rate at all vascular segments and pressure values at all vascular junctions and boundaries. 
     In certain implementations, the DLP framework may be applied to a variety of image-based vascular models and the results were compared to those derived from CFD. An example included constructing a 3D model of the arteries from corresponding image data, employing and tuning boundary conditions representative of downstream and upstream physiology, finite element simulation of the 3D time-dependent Navier-Stokes equations, and post-processing to compute the flow rate and pressure at locations for comparison with the DLP model. All CFD simulations were run assuming rigid vessel walls. The domain was discretized using linear tetrahedral elements that included boundary layer meshing for all simulations. Mesh independence studies were undertaken for all 3D simulations to ensure that the results in the final meshes were not significantly affected by inadequate mesh resolution. Solutions were run until the pressure fields at the inlets and outlets did not change more than 1.0% compared to the solutions at the same time point in the previous cardiac cycle. Fluid properties (blood density ρ=1.06 g/cm 3  and blood viscosity ρ=0.04 dyn/(cm 3  s)) were assumed common among all models. 
     The DLP model is not tuned to the 3D CFD results. To ensure a consistent comparison, the same inlet and outlet boundary conditions were used. To quantify the comparison, the relative errors were calculated between the mean values of the temporal flow rate and pressure from the DLP model against those derived from CFD simulations at the inlets and outlets of each model. Values at the outlets were chosen because this evaluates the ability of the DLP model to serve as a surrogate for the entire 3D domain in applications such as UQ, parameter tuning, etc., and because these locations are generally where integrated differences become compounded (i.e., error is expected to accumulate as greater segments of vasculature is traversed). Nonetheless, the temporal variations in the flow rate and pressure computed from the DLP model were also compared with those from the 3D CFD simulations at several locations in the interior of the domain. For perspective, there was a comparison against results from a basic DLP model for which the Poiseuille resistance was assumed. This model can be viewed as the baseline model (cf. Eq. (2)) for which the corrections to the dissipation term proposed herein are neglected. 
     In the example, the DLP framework, on a single CPU core, took about 334±49 seconds to complete steps  102 - 110  of  FIG. 1  over the range of models presented below. 
     Framework 
       FIG. 1  illustrates an example of modeling steps of a framework  100  for computing blood flow and pressure in a representative patient-specific coronary model using a distributed LP method in accordance with the disclosed technology. At  102 , geometry of a vessel, e.g., an artery of a human subject is segmented in accordance with the techniques disclosed herein. At  104 , one or more centerlines corresponding to portions of the vessel are generated. At  106 , geometric features of the vessel are extracted. At  108 , an LPN of resistors is constructed and, at  110 , a system of equations are solved. At  112 , postprocess simulation results are provided and may be provided as output, e.g., to a visual display, storage device, or both. 
     Aortic Models 
     Four aortic models  202 ,  302 ,  402 , and  502  were considered including one normal, two coarctation (moderate and severe), and one aneurysmal, as illustrated by  FIGS. 2-5 , respectively.  FIGS. 6 and 7  respectively illustrate relative errors of mean flow rates  600  and pressures  700  between the DLP model ( Q   0D ,  P   0D ) and CFD ( Q   3D ,  P   3D ) at the inlet and all outlets, demonstrating that the DLP model provides accurate prediction of mean flow rate and pressure in all cases when compared with CFD simulations. The mean value of the errors is less than 1% for all cases and the maximum error is ˜1.5%. For comparison, when the baseline Poiseuille resistance is assigned to each vascular segment as illustrated by  FIG. 8 , the mean values of pressure ( P   pois ) are significantly inaccurate, particularly for the cases with coarctation. 
     For all cases, phase-contrast (PC) MRI was used to measure flow in the ascending aorta. The respective volumetric flow waveforms from PC-MRI for each patient was mapped to the inlet of the CFD models using a time varying parabolic flow profile as the inflow boundary condition. Three-element “RCR” Windkessel models were coupled at all outlets. Regional mesh refinement was used for cases with aortic coarctation to resolve the complex flow features in the stenotic regions. 
     Aorto Femoral Models 
     Four aorto-femoral models  902 ,  1002 ,  1102 , and  1202  were considered as illustrated by  FIGS. 9-12 . Model A ( 902 ) represents a non-diseased vasculature of significant spatial extent, which spanned from the aortic root to the femoral arteries. Model B ( 1002 ) had moderate aneurysms in both right and left common iliac arteries. Models C ( 1102 ) and D ( 1202 ) had relatively complex abdominal aortic aneurysms, with notable aortic curvature. 
       FIGS. 13 and 14  respectively illustrates the errors in mean flow rate  1300  and pressure  1400  between the DLP and CFD models, demonstrating that the results from the DLP model are in good agreement with CFD simulations. The mean value of the errors is less than 2% for all cases. A maximum error of 4% occurred at the right renal artery of Model C. 
     For Model A ( 902 ), an aortic flow waveform was adapted to have a mean cardiac output of 4.6 L/min (female). For Model B ( 1002 ) the supraceliac aorta blood flow waveform was derived from PC-MRI data. For Models C ( 1102 ) and D ( 1202 ) individualized inflow boundary conditions were determined based on the Baker equation, relating body surface area to cardiac output, and assuming that 70% of the cardiac output is distributed to the supraceliac aorta. The resulting mean flows were used to generate inflow waveforms by scaling a gender-matched representative supraceliac aortic flow waveform. Three element “RCR” Windkessel models were applied at each outlet. The values of the three parameters for each outlet are determined based on flow distributions to the outlets obtained from clinical PC-MRI measurements for Model B ( 1002 ), or from literature data for Models A ( 902 ), C ( 1102 ), and D ( 1202 ). 
     Coronary Models 
     Four patient-specific models  1602 ,  1702 ,  1802 , and  1902  of the proximal aorta and major coronary arteries were considered as illustrated by  FIGS. 16-19 , respectively. Model C ( 1802 ) represents a “healthy vasculature” with no observable coronary artery disease. Model B ( 1702 ) had a mild stenosis (˜50%) in the mid left anterior descending (LAD) artery. Model D ( 1902 ) had ˜50% diameter reduction in the right coronaries, at the bifurcation of the main and first marginal right coronary artery. Model A ( 1602 ) had a severe stenosis (˜80%) in the first diagonal branch as well as a mild stenosis (˜50%) in the beginning of the mid LAD. The mean value of the pressure errors were 5%, 2.6%, 0.7% and 0.9% for Models A ( 1602 ), B ( 1702 ), C ( 1802 ), and D ( 1902 ), respectively, as illustrated by  FIG. 22 . The maximum error of 10% was observed for one of the coronary branches of Model A ( 1602 ). This maximum error occurred in a healthy branch, and not in the complex stenotic region. For comparison,  FIG. 23  illustrates corresponding errors from assigning the baseline Poiseuille resistance to each vascular segment, demonstrating significantly higher errors. 
     In all cases, aortic flow was prescribed at the inlet, an “RCR” Windkessel of the systemic circulation was coupled at the aortic outlet, and coronary-specific LPNs that consider the time-dependent intramyocardial pressure were coupled at the coronary outlets (each outlet having a separate such LPN). The effect of intramyocardial pressure is modeled by scaling the typical left and right ventricle pressures to recover realistic coronary flow waveforms. Namely, the left coronary waveform is described by low systolic flow and high diastolic flow, while the right coronary flow demonstrates two peaks one in systole and one in diastole. The LPN parameters of the systemic and coronary outlets were tuned to match target pressure and flow splits to the aorta and systemic and coronary outlets. 
     Cerebrovascular Models 
     Four cerebrovascular models  2402 ,  2502 ,  2602 , and  2702  were considered as illustrated by  FIGS. 24-27 , respectively. All of these models include the left and right vertebral arteries as inlets of the computational domain. The DLP model predicts consistently accurate flow rate and pressure results ( FIGS. 28 and 29 , respectively) with mean error values less than 1% for all cases. When Poiseuille resistance is used as illustrated by  FIG. 30 , the maximum errors are increased by a factor of greater than 3-fold when compared to our DLP model (right panel). 
     For all models, a characteristic vertebral blood flow waveform was scaled to match time-averaged PC-MRI measurements in the vertebral arteries and used to prescribe an inflow boundary condition to the computational models assuming a parabolic profile. Resistance boundary conditions were used at the outlets. A total resistance was calculated and distributed amongst the outlets by assuming all outlets act in parallel with values inversely proportional to the outlet area. 
     Pulmonary Models 
     Four patient-specific pulmonary models  3102 ,  3202 ,  3302 , and  3402  extended from the main pulmonary artery to various levels of branching in the left and right pulmonary arteries (LPA, RPA) were considered as illustrated by  FIGS. 31-34 , respectively.  FIG. 36  illustrates that the mean value of pressure errors is less than 3% for all pulmonary models tested. Simple Poiseuille model are relatively accurate Model C ( 3302 ) and D ( 3402 ), however, greater deviation is observed for Models A ( 3102 ) and B ( 3202 ) with more junctions. For example, the maximum pressure error increased from ˜10% for our DLP model to ˜25% for the baseline Poiseuille model. 
     Pulmonary blood flow waveforms  3104 ,  3204 ,  3304 , and  3404  from PC-MRI are applied to the inlet of each model illustrated by  FIGS. 31-34 , respectively. The inflow waveforms were manipulated to have zero back flow to avoid numerical instability for 3D simulations. Resistance values were assigned at the outlet of each model based on the estimated mean pressure values of each patient, cross sectional area of the outlets, and left to right pulmonary flow split ratio that are obtained from PC-MRI data. 
     Congenital Heart Disease Models 
     Models  3802 ,  3902 ,  4002 , and  4102  from four pediatric patients with congenital heart disease were considered as illustrated by  FIGS. 38-41 , respectively. The DLP method maintained reliable agreement with 3D simulation results for blood flow simulations in models with congenital heart disease where the mean pressure errors are 4.7%, 3.9%, 5.8% and 1.6% for Models A ( 3802 ), B ( 3902 ), C ( 4002 ), and D ( 4102 ), respectively. The maximum error for all cases is &lt;10% except for Model C ( 4002 ) that has the maximum error of 15%. As illustrated by  FIG. 44 , this maximum error significantly increased to ˜40% when the baseline Poiseuille resistance is used. This also leads to higher mean errors of 5.8%, 7.0%, 20.2% and 2.8% for Models A ( 3802 ), B ( 3902 ), C ( 4002 ), and D ( 4102 ), respectively. 
     PC-MRI data was used to prescribe an inflow waveform to the inlets of computational domains. Inflow waveforms prescribed at the inferior and superior vena cava (IVC, SVC), internal jugular vein (IW) and brachiocephalic vein (BrS) are shown in  FIGS. 38-41  for all models. “RCR” boundary conditions were applied to each outlet. The “RCR” parameters are tuned to match the clinical target pressure values and assuming the LPA/RPA flow split ratio to be 45%/55% for all patients. 
     DISCUSSION 
     Implementations of the disclosed technology are generally directed to a DLP framework to predict temporal flow rate and pressure waveforms distributions in 3D image-based vascular models. The proposed DLP framework can provide consistent prediction with CFD simulations with mean errors &lt;7% in a range of cardiovascular modeling applications. This framework can be fully automated based on an input model geometry (and specification of boundary conditions), and generally requires less than 1/1000 of the computational cost of corresponding CFD simulations. 
     The instant disclosure includes the most comprehensive comparisons of a ROM to the current state-of-the-art image-based CFD modeling. A broad range of vascular anatomies and variations in geometric features were considered. Namely, the two key non-dimensional parameters of Reynolds (Re) and Womersley (α) numbers varied from ˜100-3,000 and ˜1-20, respectively. Notably, these ranges in Re, α, and vessel diameter span the spectrum of expected values encountered in most image-based computational modeling of blood flow. In addition to the dynamic and geometric difference explored, the number of vascular junctions across the models varied from 4 to 96. Interestingly, the junctions include both converging and diverging flows, where converging flows sometimes result in a negative loss coefficient, and in presence of back flow, some bifurcations can have both diverging and converging flows at different time points of the cardiac cycle. Additionally, vascular expansions, as quantified by the area reduction ratio A s /A 0 , ranged up to ˜90%, and the mean curvature ratio δ=a/R ranged up to −0.5 in the considered models, which again cover a broad range of expected values. 
     To provide more general insight into the contribution of each source of energy dissipation, the modification due to flow separation downstream of expansions, when present, may have the highest contribution to accurately predict flow and pressure in the corresponding model. Viscous dissipation introduced by curvature effects generally has the second highest contribution in energy losses. This can be confirmed by comparing the second and third panels of error plots for coronary models with negligible unsteadiness effect, particularly for the healthy coronary Model C or Models B and D with mild stenoses. 
     Moreover, for cerebrovascular models the results are improved mostly by considering the curvature effects, since the Womersley number is ≈2 in these models and there is no sudden changes in cross-sectional area. The Womersley effect has the next highest impact on modifying Poiseuille resistance model, where viscous resistance can increase by a factor of 5 in for example aortic branches. The Womersley effect can be seen for example in the aorto-femoral models where the errors approximately tripled without considering this effect. Finally, the energetic losses due to bifurcations was found to be relatively negligible, except in cases where numerous junctions exist, such as pulmonary models. 
     Given the accuracy of the DLP approach, implementations of the disclosed technology can potentially replace CFD in applications where time is of the essence, thus opening the door to broader use of image-based modeling in, for example, clinical settings. A major impact of this approach is to applications where numerous CFD simulations would be desirable, such as for model parameter or boundary condition tuning, uncertainty quantification, sensitivity analysis, surgery/device design, data assimilation, machine learning training, etc. In such scenarios, it could be the de facto model, or used in conjunction with more expensive CFD simulations to reduce the number of full CFD model evaluations. This could be highly significant as the above types of engineering analyses are playing an increasing role in image-based modeling research and translational applications. 
     The following discussion is intended to provide a brief, general description of a suitable machine in which embodiments of the disclosed technology can be implemented. As used herein, the term “machine” is intended to broadly encompass a single machine or a system of communicatively coupled machines or devices operating together. Exemplary machines may include computing devices such as personal computers, workstations, servers, portable computers, handheld devices, tablet devices, and the like. 
     Typically, a machine includes a system bus to which processors, memory such as random access memory (RAM), read-only memory (ROM), and other state-preserving medium, storage devices, a video interface, and input/output interface ports can be attached. The machine may also include embedded controllers such as programmable or non-programmable logic devices or arrays, Application Specific Integrated Circuits (ASICs), embedded computers, smart cards, and the like. The machine may be controlled, at least in part, by input from conventional input devices such as keyboards and mice, as well as by directives received from another machine, interaction with a virtual reality (VR) environment, biometric feedback, or other pertinent input. 
     The machine may utilize one or more connections to one or more remote machines, such as through a network interface, modem, or other communicative coupling. Machines can be interconnected by way of a physical and/or logical network, such as an intranet, the Internet, local area networks, wide area networks, etc. One having ordinary skill in the art will appreciate that network communication may utilize various wired and/or wireless short range or long range carriers and protocols, including radio frequency (RF), satellite, microwave, Institute of Electrical and Electronics Engineers (IEEE) 545.11, Bluetooth, optical, infrared, cable, laser, etc. 
     Embodiments of the disclosed technology may be described by reference to or in conjunction with associated data including functions, procedures, data structures, application programs, instructions, etc. that, when accessed by a machine, may result in the machine performing tasks or defining abstract data types or low-level hardware contexts. Associated data may be stored in, for example, volatile and/or non-volatile memory, such as RAM and ROM, or in other storage devices and their associated storage media, which can include hard-drives, floppy-disks, optical storage, tapes, flash memory, memory sticks, digital video disks, biological storage, and other non-transitory, physical storage media. 
     Associated data may be delivered over transmission environments, including the physical and/or logical network, in the form of packets, serial data, parallel data, etc., and may be used in a compressed or encrypted format. Associated data may be used in a distributed environment, and stored locally and/or remotely for machine access. 
     Having described and illustrated the principles of the invention with reference to illustrated embodiments, it will be recognized that the illustrated embodiments may be modified in arrangement and detail without departing from such principles, and may be combined in any desired manner And although the foregoing discussion has focused on particular embodiments, other configurations are contemplated. 
     Consequently, in view of the wide variety of permutations to the embodiments that are described herein, this detailed description and accompanying material is intended to be illustrative only, and should not be taken as limiting the scope of the invention. What is claimed as the invention, therefore, is all such modifications as may come within the scope and spirit of the following claims and equivalents thereto.