Image-Based Computational Fluid Dynamics to Compare Two Repair Techniques for Mitral Valve Prolapse

OBJECTIVE : The treatment of mitral valve prolapse involves two distinct repair techniques: chordal replacement (Neochordae technique) and leaflet resection (Resection technique). However, there is still a debate in the literature about which is the optimal one. In this context, we performed an image-based computational fluid dynamic study to evaluate blood dynamics in the two surgical techniques. METHODS : We considered a healthy subject (H) and two patients (N and R) who underwent surgery for prolapse of the posterior leaflet and were operated with the Neochordae and Resection technique, respectively. Computational Fluid Dynamics (CFD) was employed with prescribed motion of the entire left heart coming from cine-MRI images, with a Large Eddy Simulation model to describe the transition to turbulence and a resistive method for managing valve dynamics. We created three different virtual scenarios where the operated mitral valves were inserted in the same left heart geometry of the healthy subject to study the differences attributed only to the two techniques. RESULTS : We compared the three scenarios by quantitatively analyzing ventricular velocity patterns and pressures, transition to turbulence, and the ventricle ability to prevent thrombi formation. From these results, we found that the operative techniques affected the ventricular blood dynamics in different ways, with variations attributed to the reduced mobility of the Resection posterior leaflet. Specifically, the Resection technique resulted in turbulent forces, related with the risk of hemolysis formation, up to 640 Pa, while the other two scenarios exhibited a maximum of 240 Pa. Moreover, in correspondence of the ventricular apex, the Resection technique reduced the areas with low velocity to 15%, whereas the healthy case and the Neochordae case maintained these areas at 30 and 48%, respectively. Our findings suggest that the Neochordae technique developed a more physiological flow with respect to the Resection technique. CONCLUSION: Resection technique gives rise to a different direction of the mitral jet during diastole increasing the ability to washout the ventricular apex preventing from thrombi formation, but at the same time it promotes turbulence formation that is associated with ventricular effort and risk of hemolysis.

[4D flow MRI: value and clinical perspectives in patients with pathology of the heart and great vessels (part 2): A review]

The study of blood flow is becoming a new trend in cardiology and cardiovascular surgery. Based on the literature and our own data, a review is presented on the use of 4D flow in diseases of the heart and blood vessels. The main state of the question about the features of the application of the technique in various pathologies of the cardiovascular system is described in detail, the priorities, limitations and promising directions of the technique application are considered taking into account the goals of practical medicine. The review consists of two parts. The first is devoted to general issues, limitations of the technique, and issues of 4D flow mapping in patients with lesions of the great vessels. In the second part, the emphasis is on the use of 4D flow MRI in the study of intraventricular blood flow and the application of the technique in congenital heart and vascular diseases.

Quantitative evaluation of pulmonary hypertension using 4D flow MRI: A retrospective study

Background

Pulmonary hypertension (PH) is a severe vascular disorder that may affect 50 % of patients with heart failure. Currently, right-sided heart catheterization is required to definitively diagnose PH. However, this method is invasive and thus may not be appropriate for repeated, long-term monitoring of PH patients. This retrospective study's aim was to evaluate whether 4D flow magnetic resonance imaging (MRI) can be used to quantitively measure flow parameters to identify patients with PH.

Methods

The study cohort included 97 patients recruited from a single institution and divided into three groups based on echocardiographic estimate of pulmonary artery systolic pressure (PASP): normal group with PASP<36 mmHg, borderline PH group with PASP of 37-50 mmHg, and PH group with PASP>50 mmHg. 4D flow MRI was used to quantitively assess blood flow and velocity, regurgitation, wall shear stress (WSS) and kinetic energy in the pulmonary artery trunk, right main pulmonary artery, and left pulmonary artery. Two experienced radiologists independently analyzed the MR images, blinded to clinical details.

Results

We found a significant difference in WSS in the pulmonary artery trunk, right main pulmonary artery and left main pulmonary artery among the three patient groups. We also found significant differences in the kinetic energy and average through velocity in the pulmonary artery trunk and right main pulmonary artery, and significant differences in the flow rate in the right main pulmonary artery.

Conclusion

These data suggest that 4D flow MRI can quantitate pulmonary artery flow parameters and distinguish between patients with and without PH.

Acute vasodilator response testing in the adult Fontan circulation using non-invasive 4D Flow MRI: a proof-of-principle study

BACKGROUND

Pulmonary vasodilator therapy in Fontan patients can improve exercise tolerance. We aimed to assess the potential for non-invasive testing of acute vasodilator response using four-dimensional (D) flow MRI during oxygen inhalation.

MATERIALS AND METHODS

Six patients with well-functioning Fontan circulations were prospectively recruited and underwent cardiac MRI. Ventricular anatomical imaging and 4D Flow MRI were acquired at baseline and during inhalation of oxygen. Data were compared with six age-matched healthy volunteers with 4D Flow MRI scans acquired at baseline.

RESULTS

All six patients tolerated the MRI scan well. The dominant ventricle had a left ventricular morphology in all cases. On 4D Flow MRI assessment, two patients (Patients 2 and 6) showed improved cardiac filling with improved preload during oxygen administration, increased mitral inflow, increased maximum E-wave kinetic energy, and decreased systolic peak kinetic energy. Patient 1 showed improved preload only. Patient 5 showed no change, and patient 3 had equivocal results. Patient 4, however, showed a decrease in preload and cardiac filling/function with oxygen.

DISCUSSION

Using oxygen as a pulmonary vasodilator to assess increased pulmonary venous return as a marker for positive acute vasodilator response would provide pre-treatment assessment in a more physiological state - the awake patient. This proof-of-concept study showed that it is well tolerated and has shown changes in some stable patients with a Fontan circulation.

Effect of Aging on Intraventricular Kinetic Energy and Energy Dissipation

In recent years, analysis of kinetic energy (KE) and the rate of kinetic energy dissipation (KED) or energy loss (EL) within the cardiac chambers, obtained by cardiac imaging techniques, has gained increasing attention. Thus, there is a need to clarify the effect of physiological variables, specifically aging, on these energetic measures. To elucidate this aspect, we reviewed the literature on this topic. Overall, cardiac magnetic resonance and echocardiographic studies published so far indicate that aging affects the energetics of left and right intraventricular blood flow, although not all energy measures during the cardiac cycle seem to be affected by age in the same way. Current studies, however, have limitations. Additional large, multicenter investigations are needed to test the effect of physiological variables on intraventricular KE and KED/EL measures.

Time domain adaptation of left ventricular diastolic intraventricular pressure in elite female ice hockey athletes

Background

Ice hockey is a high-intensity dynamic sport for which competitive athletes train for longer than 20 hours each week for several years. The cumulative time of myocardial exposure to hemodynamic stress affects cardiac remodeling. However, the intracardiac pressure distribution of the elite ice hockey athletes' heart during adaptation to long-term training remains to be explored. This study aimed to compare the diastolic intraventricular pressure difference (IVPD) of the left ventricle (LV) between healthy volunteers and ice hockey athletes with different training times.

Methods

Fifty-three female ice hockey athletes (27 elite and 26 casual) and 24 healthy controls were included. The diastolic IVPD of the LV during diastole was measured by vector flow mapping. The peak amplitude of the IVPD during isovolumic relaxation (P0), diastolic rapid filling (P1), and atrial systole (P4); the difference in the peak amplitude between adjacent phases (DiffP01, DiffP14); the time interval between the peak amplitude of adjacent phases (P0P1, P1P4); and the maximum decrease rate in diastolic IVPD were calculated. Differences between groups, as well as correlations between hemodynamic parameters and training time, were analyzed.

Results

Structural parameters of the LV were significantly higher in elite athletes than in casual players and controls. No significant difference in the peak amplitude of the IVPD during the diastolic phase was found among the three groups. The analysis of covariance with heart rate as a covariate showed that P1P4 in the elite athlete and casual player groups was significantly longer than that in the healthy control group (p < 0.001 for all). An increased P1P4 was significantly associated with an increased training year (β = 4.90, p < 0.001).

Conclusions

The diastolic cardiac hemodynamics of the LV in elite female ice hockey athletes could be characterized by a prolonged diastolic IVPD, and P1P4 prolonged with an increase in the training years, reflecting a time-domain adaptation in diastolic hemodynamics after long-term training.

Impact of dobutamine stress on diastolic energetic efficiency of healthy left ventricle: an in vivo kinetic energy analysis

The total kinetic energy (KE) of blood can be decomposed into mean KE (MKE) and turbulent KE (TKE), which are associated with the phase-averaged fluid velocity field and the instantaneous velocity fluctuations, respectively. The aim of this study was to explore the effects of pharmacologically induced stress on MKE and TKE in the left ventricle (LV) in a cohort of healthy volunteers. 4D Flow MRI data were acquired in eleven subjects at rest and after dobutamine infusion, at a heart rate that was ∼60% higher than the one in rest conditions. MKE and TKE were computed as volume integrals over the whole LV and as data mapped to functional LV flow components, i.e., direct flow, retained inflow, delayed ejection flow and residual volume. Diastolic MKE and TKE increased under stress, in particular at peak early filling and peak atrial contraction. Augmented LV inotropy and cardiac frequency also caused an increase in direct flow and retained inflow MKE and TKE. However, the TKE/KE ratio remained comparable between rest and stress conditions, suggesting that LV intracavitary fluid dynamics can adapt to stress conditions without altering the TKE to KE balance of the normal left ventricle at rest.

Evaluation of Left Ventricular Function Using Four-Dimensional Flow Cardiovascular Magnetic Resonance: A Systematic Review

There is increasing recognition of the value of four-dimensional flow cardiovascular magnetic resonance (4D-flow MRI) as a potential means to detect and measure abnormal flow behaviour that occurs during early left ventricular (LV) dysfunction. We performed a systematic review of current literature on the role of 4D-flow MRI-derived flow parameters in quantification of LV function with a focus on potential clinical applicability. A comprehensive literature search was performed in March 2022 on available databases. A total of 1186 articles were identified, and 30 articles were included in the final analysis. All the included studies were ranked as “highly clinically applicable”. There was considerable variability in the reporting of methodologies and analyses. All the studies were small-scale feasibility or pilot studies investigating a diverse range of flow parameters. The most common primary topics of investigation were energy-related flow parameters, flow components and vortex analysis which demonstrated potentials for quantifying early diastolic dysfunction, whilst other parameters including haemodynamic forces, residence time distribution and turbulent kinetic energy remain in need of further evaluation. Systematic quantitative comparison of study findings was not possible due to this heterogeneity, therefore limiting the collective power of the studies in evaluating clinical applicability of the flow parameters. To achieve broader clinical application of 4D-flow MRI, larger scale investigations are required, together with standardisation of methodologies and analytical approach.

Four-dimensional flow cardiac magnetic resonance assessment of left ventricular diastolic function

Left ventricular diastolic dysfunction is a major cause of heart failure and carries a poor prognosis. Assessment of left ventricular diastolic function however remains challenging for both echocardiography and conventional phase contrast cardiac magnetic resonance. Amongst other limitations, both are restricted to measuring velocity in a single direction or plane, thereby compromising their ability to capture complex diastolic hemodynamics in health and disease. Time-resolved three-dimensional phase contrast cardiac magnetic resonance imaging with three-directional velocity encoding known as '4D flow CMR' is an emerging technology which allows retrospective measurement of velocity and by extension flow at any point in the acquired 3D data volume. With 4D flow CMR, complex aspects of blood flow and ventricular function can be studied throughout the cardiac cycle. 4D flow CMR can facilitate the visualization of functional blood flow components and flow vortices as well as the quantification of novel hemodynamic and functional parameters such as kinetic energy, relative pressure, energy loss and vorticity. In this review, we examine key concepts and novel markers of diastolic function obtained by flow pattern analysis using 4D flow CMR. We consolidate the existing evidence base to highlight the strengths and limitations of 4D flow CMR techniques in the surveillance and diagnosis of left ventricular diastolic dysfunction.

Technical Background for 4D Flow MR Imaging

Recently, the hemodynamic assessments with 3D cine phase-contrast (PC) MRI (4D flow MRI) have attracted considerable attention from clinicians. Unlike 2D cine PC MRI, the technique allows for cardiac phase-resolved data acquisitions of flow velocity vectors within the entire FOV during a clinically viable period. Thus, the method has enabled retrospective flowmetry in the spatial and temporal axes, which are essential to derive hemodynamic parameters related to vascular homeostasis and those to the progression of the pathologies. Accelerations in imaging are critical for this technology to be clinically viable; however, a high SNR or velocity-to-noise ratio (VNR) is also vital for accurate flow measurements. In this chapter, the technologies enabling this difficult balance are discussed.

Hemodynamic Parameters for Cardiovascular System in 4D Flow MRI: Mathematical Definition and Clinical Applications

Blood flow imaging becomes an emerging trend in cardiology with the recent progress in computer technology. It not only visualizes colorful flow velocity streamlines but also quantifies the mechanical stress on cardiovascular structures; thus, it can provide the detailed inspections of the pathophysiology of diseases and predict the prognosis of cardiovascular functions. Clinical applications include the comprehensive assessment of hemodynamics and cardiac functions in echocardiography vector flow mapping (VFM), 4D flow MRI, and surgical planning as a simulation medicine in computational fluid dynamics (CFD).For evaluation of the hemodynamics, novel mathematically derived parameters obtained using measured velocity distributions are essential. Among them, the traditional and typical parameters are wall shear stress (WSS) and its related parameters. These parameters indicate the mechanical damages to endothelial cells, resulting in degenerative intimal change in vascular diseases. Apart from WSS, there are abundant parameters that describe the strength of the vortical and/or helical flow patterns. For instance, vorticity, enstrophy, and circulation indicate the rotating flow strength or power of 2D vortical flows. In addition, helicity, which is defined as the cross-linking number of the vortex filaments, indicates the 3D helical flow strength and adequately describes the turbulent flow in the aortic root in cases with complicated anatomies. For the description of turbulence caused by the diseased flow, there exist two types of parameters based on completely different concepts, namely: energy loss (EL) and turbulent kinetic energy (TKE). EL is the dissipated energy with blood viscosity and evaluates the cardiac workload related to the prognosis of heart failure. TKE describes the fluctuation in kinetic energy during turbulence, which describes the severity of the diseases that cause jet flow. These parameters are based on intuitive and clear physiological concepts, and are suitable for in vivo flow measurements using inner velocity profiles.

In-vitro and In-Vivo Assessment of 4D Flow MRI Reynolds Stress Mapping for Pulsatile Blood Flow

Imaging hemodynamics play an important role in the diagnosis of abnormal blood flow due to vascular and valvular diseases as well as in monitoring the recovery of normal blood flow after surgical or interventional treatment. Recently, characterization of turbulent blood flow using 4D flow magnetic resonance imaging (MRI) has been demonstrated by utilizing the changes in signal magnitude depending on intravoxel spin distribution. The imaging sequence was extended with a six-directional icosahedral (ICOSA6) flow-encoding to characterize all elements of the Reynolds stress tensor (RST) in turbulent blood flow. In the present study, we aimed to demonstrate the feasibility of full RST analysis using ICOSA6 4D flow MRI under physiological conditions. First, the turbulence analysis was performed through in vitro experiments with a physiological pulsatile flow condition. Second, a total of 12 normal subjects and one patient with severe aortic stenosis were analyzed using the same sequence. The in-vitro study showed that total turbulent kinetic energy (TKE) was less affected by the signal-to-noise ratio (SNR), however, maximum principal turbulence shear stress (MPTSS) and total turbulence production (TP) had a noise-induced bias. Smaller degree of the bias was observed for TP compared to MPTSS. In-vivo study showed that the subject-variability on turbulence quantification was relatively low for the consistent scan protocol. The in vivo demonstration of the stenosis patient showed that the turbulence analysis could clearly distinguish the difference in all turbulence parameters as they were at least an order of magnitude larger than those from the normal subjects.

Clinical intra-cardiac 4D flow CMR: acquisition, analysis, and clinical applications

Identification of flow patterns within the heart has long been recognized as a potential contribution to the understanding of physiological and pathophysiological processes of cardiovascular diseases. Although the pulsatile flow itself is multi-dimensional and multi-directional, current available non-invasive imaging modalities in clinical practice provide calculation of flow in only 1-direction and lack 3-dimensional volumetric velocity information. Four-dimensional flow cardiovascular magnetic resonance imaging (4D flow CMR) has emerged as a novel tool that enables comprehensive and critical assessment of flow through encoding velocity in all 3 directions in a volume of interest resolved over time. Following technical developments, 4D flow CMR is not only capable of visualization and quantification of conventional flow parameters such as mean/peak velocity and stroke volume but also provides new hemodynamic parameters such as kinetic energy. As a result, 4D flow CMR is being extensively exploited in clinical research aiming to improve understanding of the impact of cardiovascular disease on flow and vice versa. Of note, the analysis of 4D flow data is still complex and accurate analysis tools that deliver comparable quantification of 4D flow values are a necessity for a more widespread adoption in clinic. In this article, the acquisition and analysis processes are summarized and clinical applications of 4D flow CMR on the heart including conventional and novel hemodynamic parameters are discussed. Finally, clinical potential of other emerging intra-cardiac 4D flow imaging modalities is explored and a near-future perspective on 4D flow CMR is provided.

Impact of age, sex and ethnicity on intra-cardiac flow components and left ventricular kinetic energy derived from 4D flow CMR

BACKGROUND

Four-dimensional flow cardiovascular magnetic resonance (4D flow CMR) allows quantification of left ventricular (LV) blood flow. We aimed to 1) establish reference ranges for 4D flow CMR-derived LV relative flow components and kinetic energy parameters indexed to end-diastolic volume (KEi_EDV) among healthy Asian subjects, 2) assess effects of age and sex on these parameters, and 3) compare these parameters between Asian and Caucasian subjects.

METHODS

74 healthy Asian subjects underwent cine and 4D flow CMR. Relative flow components (direct flow, retained inflow, delayed ejection flow, residual volume) and multiple phasic KEi_EDV (LV global, peak systolic, systolic, diastolic, peak E-wave, peak A-wave) were analyzed. Sex- and age-specific reference ranges were reported.

RESULTS

Relative flow components and systolic phase KEi_EDV did not vary with age. Women had higher retained inflow and peak E-wave KEi_EDV, lower residual volume, peak systolic and systolic KEi_EDV than men. Peak A-wave KEi_EDV increased significantly (r = 0.474) whereas peak E-wave KEi_EDV (r = -0.458) and E-wave/A-wave ratio (r = -0.528) decreased with age. A sub-population (n = 44) was compared with 44 sex- and age-matched Caucasian subjects: no significant group differences were observed for all 4D flow CMR parameters.

CONCLUSION

Asian sex- and age-specific 4D flow CMR reference ranges were established. Sex differences in retained inflow, residual volume, peak systolic, systolic KEi_EDV and peak E-wave KEi_EDV were observed. Ageing influenced diastolic KEi_EDV but not systolic phase KEi_EDV or relative flow components. All studied parameters were similar between sex- and age-matched Asian and Caucasian subjects, implying generalizability of the ranges.

Understanding the influence of left ventricular assist device inflow cannula alignment and the risk of intraventricular thrombosis

BACKGROUND

Adverse neurological events associated with left ventricular assist devices (LVADs) have been suspected to be related to thrombosis. This study aimed to understand the risks of thrombosis with variations in the implanted device orientation. A severely dilated pulsatile patient-specific left ventricle, modelled with computational fluid dynamics, was utilised to identify the risk of thrombosis for five cannulation angles. With respect to the inflow cannula axis directed towards the mitral valve, the other angles were 25° and 20° towards the septum and 20° and 30° towards the free wall.

RESULTS

Inflow cannula angulation towards the free wall resulted in longer blood residence time within the ventricle, slower ventricular washout and reduced pulsatility indices along the septal wall. Based on the model, the ideal inflow cannula alignment to reduce the risk of thrombosis was angulation towards the mitral valve and up to parallel to the septum, avoiding the premature clearance of incoming blood.

CONCLUSIONS

This study indicates the potential effects of inflow cannulation angles and may guide optimised implantation configurations; however, the ideal approach will be influenced by other patient factors and is suspected to change over the course of support.

Left Atrial Remodeling Mechanisms Associated with Atrial Fibrillation

Heart disease has always been one of the important diseases that endanger health and cause death. Therefore, it is particularly important to understand left atrium reconstruction and atrial fibrillation before heart image processing. The purpose of this paper is to provide an important review of the mechanisms of left atrial remodeling (LAR) associated with atrial fibrillation (AF). LAR refers to the spectrum of pathophysiological changes in (i) atrial structure and physiological function, and (ii) electric, ionic, and molecular milieu of the LA, in response to stresses imposed by conditions such as hypertension, myocardial ischemia, autonomic denervation and congestive heart failure. The main mechanisms of LAR include electrical remodeling, structural remodeling, metabolic remodeling, autonomic remodeling, neurohormones and inflammation, and other influencing factors. LAR is not only the basic mechanism of AF and heart failure, but also the pathophysiological basis of its progression. In clinical practice, AF is the most common persistent arrhythmia, and is believed to be the result of a combination of mechanisms that have triggers and maintenance mechanisms, including spontaneous ectopic pacing and multiple wavelet reentry. While LA electrophysiological, structural, and ultra-structural changes trigger AF, in turn, AF alters the LA electrical and structural properties that promote its maintenance and recurrence. Chronic AF leads to extensive changes in atrial cellular substructures, including loss of myofibrils, accumulation of glycogen, changes in mitochondrial shape and size, fragmentation of sarcoplasmic reticulum, and dispersion of nuclear chromatin. Electrical remodeling and structural remodeling of the atria during AF, involving structural changes and functional impairment of the left atrium, can lead to serious decline in left ventricular function and severe heart failure. Therefore, LAR and AF are inter-activating phenomena, and the resulting complications can cause serious disabling and fatal events. In this paper, we present (i) the mechanisms of LAR, in the form of structural, electrical, metabolic, and neurohormonal changes, and (ii) their interactive roles in initiating and maintaining AF. These in-depth understanding of the atrial remodeling mechanisms can in turn provide useful insights into the treatment of AF and heart failure.

Intracardiac and Vascular Hemodynamics with Cardiovascular Magnetic Resonance in Heart Failure

In heart failure (HF), the impaired heart loses its ability to competently eject blood during systole or fill with blood during diastole, manifesting in multifaceted abnormal intracardiac or intravascular flow dynamics. Conventional imaging techniques are limited in their ability to evaluate multidirectional multidimensional flow alterations in HF. Four-dimensional (4-D) flow magnetic resonance imaging (MRI) has emerged as a promising technique to comprehensively visualize and quantify changes in 3-dimensional blood flow dynamics in complex cardiovascular diseases. This article reviews emerging applications of 4-D flow MRI hemodynamic markers in HF and etiologies at risk of progressing to HF.

Non-Invasive Assessment of Pulmonary Vasculopathy

Right heart catheterization remains necessary for the diagnosis of pulmonary hypertension and, therefore, for the prognostic evaluation of patients with chronic heart failure. The non-invaSive Assessment of Pulmonary vasculoPathy in Heart failure (SAPPHIRE) study was designed to assess the feasibility and prognostic relevance of a non-invasive evaluation of the pulmonary artery vasculature in patients with heart failure and pulmonary hypertension. Patients will undergo a right heart catheterization, cardiac resonance imaging, and a pulmonary function test in order to identify structural and functional parameters allowing the identification of combined pre- and postcapillary pulmonary hypertension, and correlate these findings with the hemodynamic data.

Impact of prosthetic mitral valve orientation on the ventricular flow field: Comparison using patient-specific computational fluid dynamics

Significant mitral valve regurgitation creates progressive adverse remodeling of the left ventricle (LV). Replacement of the failing valve with a prosthesis generally improves patient outcomes but leaves the patient with non-physiological intracardiac flow patterns that might contribute to their future risk of thrombus formation and embolism. It has been suggested that the angular orientation of the implanted valve might modify the postoperative distortion of the intraventricular flow field. In this study, we investigated the effect of prosthetic valve orientation on LV flow patterns by using heart geometry from a patient with LV dysfunction and a competent native mitral valve to calculate intracardiac flow fields with computational fluid dynamics (CFD). Results were validated using in vivo 4D Flow MRI. The computed flow fields were compared to calculations following virtual implantation of a mechanical heart valve oriented in four different angles to assess the effect of leaflet position. Flow patterns were visualized in long- and short-axes and quantified with flow component analysis. In comparison to a native valve, valve implantation increased the proportion of the mitral inflow remaining in the basal region and further increased the residual volume in the apical area. Only slight changes due to valve orientation were observed. Using our numerical framework, we demonstrated quantitative changes in left ventricular blood flow due to prosthetic mitral replacement. This framework may be used to improve design of prosthetic heart valves and implantation procedures to minimize the potential for apical flow stasis, and potentially assist personalized treatment planning.

Non-invasive estimation of relative pressure for intracardiac flows using virtual work-energy

Intracardiac blood flow is driven by differences in relative pressure, and assessing these is critical in understanding cardiac disease. Non-invasive image-based methods exist to assess relative pressure, however, the complex flow and dynamically moving fluid domain of the intracardiac space limits assessment. Recently, we proposed a method, νWERP, utilizing an auxiliary virtual field to probe relative pressure through complex, and previously inaccessible flow domains. Here we present an extension of νWERP for intracardiac flow assessments, solving the virtual field over sub-domains to effectively handle the dynamically shifting flow domain. The extended νWERP is validated in an in-silico benchmark problem, as well as in a patient-specific simulation model of the left heart, proving accurate over ranges of realistic image resolutions and noise levels, as well as superior to alternative approaches. Lastly, the extended νWERP is applied on clinically acquired 4D Flow MRI data, exhibiting realistic ventricular relative pressure patterns, as well as indicating signs of diastolic dysfunction in an exemplifying patient case. Summarized, the extended νWERP approach represents a directly applicable implementation for intracardiac flow assessments.

4D flow imaging of the thoracic aorta: is there an added clinical value?

Four-dimensional (4D) flow MRI has emerged as a powerful non-invasive technique in cardiovascular imaging, enabling to analyse in vivo complex flow dynamics models by quantifying flow parameters and derived features. Deep knowledge of aortic flow dynamics is fundamental to better understand how abnormal flow patterns may promote or worsen vascular diseases. In the perspective of an increasingly personalized and preventive medicine, growing interest is focused on identifying those quantitative functional features which are early predictive markers of pathological evolution. The thoracic aorta and its spectrum of diseases, as the first area of application and development of 4D flow MRI and supported by an extensive experimental validation, represents the ideal model to introduce this technique into daily clinical practice. The purpose of this review is to describe the impact of 4D flow MRI in the assessment of the thoracic aorta and its most common affecting diseases, providing an overview of the actual clinical applications and describing the potential role of derived advanced hemodynamic measures in tailoring follow-up and treatment.

Left Ventricular Blood Flow Kinetic Energy Assessment by 4D Flow Cardiovascular Magnetic Resonance: A Systematic Review of the Clinical Relevance

Background: There is an emerging body of evidence that supports the potential clinical value of left ventricular (LV) intracavity blood flow kinetic energy (KE) assessment using four-dimensional flow cardiovascular magnetic resonance imaging (4D flow CMR). The aim of this systematic review is to summarize studies evaluating LV intracavity blood flow KE quantification methods and its potential clinical significance. Methods: A systematic review search was carried out on Medline, Pubmed, EMBASE and CINAHL. Results: Of the 677 articles screened, 16 studies met eligibility. These included six (37%) studies on LV diastolic function, another six (37%) studies on heart failure or cardiomyopathies, three (19%) studies on ischemic heart disease or myocardial infarction and finally, one (6%) study on valvular heart disease, namely, mitral regurgitation. One of the main strengths identified by these studies is high reproducibility of LV blood flow KE hemodynamic assessment (mean coefficient of variability = 6 ±  2%) for the evaluation of LV diastolic function. Conclusions: The evidence gathered in this systematic review suggests that LV blood flow KE has great promise for LV hemodynamic assessment. Studies showed increased diagnostic confidence at no cost of additional time. Results were highly reproducible with low intraobserver variability.

Non-invasive estimation of relative pressure in turbulent flow using virtual work-energy

Vascular pressure differences are established risk markers for a number of cardiovascular diseases. Relative pressures are, however, often driven by turbulence-induced flow fluctuations, where conventional non-invasive methods may yield inaccurate results. Recently, we proposed a novel method for non-turbulent flows, νWERP, utilizing the concept of virtual work-energy to accurately probe relative pressure through complex branching vasculature. Here, we present an extension of this approach for turbulent flows: νWERP-t. We present a theoretical method derivation based on flow covariance, quantifying the impact of flow fluctuations on relative pressure. νWERP-t is tested on a set of in-vitro stenotic flow phantoms with data acquired by 4D flow MRI with six-directional flow encoding, as well as on a patient-specific in-silico model of an acute aortic dissection. Over all tests νWERP-t shows improved accuracy over alternative energy-based approaches, with excellent recovery of estimated relative pressures. In particular, the use of a guaranteed divergence-free virtual field improves accuracy in cases where turbulent flows skew the apparent divergence of the acquired field. With the original νWERP allowing for assessment of relative pressure into previously inaccessible vasculatures, the extended νWERP-t further enlarges the method's clinical scope, underlining its potential as a novel tool for assessing relative pressure in-vivo.

Quantitative analysis of intraventricular flow-energetics and vortex in ischaemic hearts

OBJECTIVE

This study investigated the intraventricular flow dynamics in ischaemic heart disease patients.

PATIENTS AND METHODS

Fourteen patients with normal ejection fraction and 16 patients with reduced ejection fraction were compared with 20 healthy individuals. Phase-contrast MRI was used to assess intraventricular flow variables and speckle-tracking echocardiography to assess myocardial strain and left ventricular (LV) dyssynchrony. Infarct size was acquired using delayed-enhancement MRI.

RESULTS

The results obtained showed no significant differences in intraventricular flow variables between the healthy group and the patients with normal ejection fraction group, whereas considerable reductions in kinetic energy (KE) fluctuation index, E' (P<0.001) and vortex KE (P=0.003) were found in the patients with reduced ejection fraction group. In multivariate analysis, only vortex KE and infarct size were significantly related to LV ejection fraction (P<0.001); furthermore, vortex KE was correlated negatively with energy dissipation, energy dissipation index (r=-0.44, P=0.021).

CONCLUSION

This study highlights that flow energetic indices have limited applicability as early predictors of LV progressive dysfunction, whereas vortex KE could be an alternative to LV performance.

Impact of myocardial infarction on intraventricular vortex and flow energetics assessed using computational simulations

Flow energetics have been proposed as early indicators of progressive left ventricular (LV) functional impairment in patients with myocardial infarction (MI), but its correlation with individual MI parameters has not been fully explored. Using electro-fluid-structure interaction LV models, this study investigated the correlation between four MI parameters: infarct size, infarct multiplicity, regional enhancement of contractility at the viable myocardium area (RECVM), and LV mechanical dyssynchrony (LVMD) with intraventricular vortex and flow energetics. In LV with small infarcts, our results showed that infarct appearance amplified the energy dissipation index (DI), where substantial viscous energy loss was observed in areas with high flow velocity and near the infarct-vortex interface. The LV with small multiple infarcts and RECVM showed remarkable DI increment during systole and diastole. In correlation analysis, the systolic kinetic energy fluctuation index (E') was positively related to ejection fraction (EF) (R²  = 0.982) but negatively correlated with diastolic E' (R²  = 0.970). Diastolic E' was inversely correlated with vortex kinetic energy (R²  = 0.960) and vortex depth (R²  = 0.876). We showed an excessive systolic DI could differentiate infarcted LV with normal EF from healthy LV. Strong flow acceleration, LVMD, and vortex-infarct interactions were predominant factors that induced excessive DI in infarcted LVs. Instead of causing undesired flow turbulence, high systolic E' suggested the existence of energetic flow acceleration, while high diastolic E' implied an inefficient diastolic filling. Thus, systolic E' is not a suitable early indicator for progressive LV dysfunction in MI patients, while diastolic E' may be a useful index to indicate diastolic impairment in these patients.

Characterization and estimation of turbulence-related wall shear stress in patient-specific pulsatile blood flow

Disturbed, turbulent-like blood flow promotes chaotic wall shear stress (WSS) environments, impairing essential endothelial functions and increasing the susceptibility and progression of vascular diseases. These flow characteristics are today frequently detected at various anatomical, lesion and intervention-related sites, while their role as a pathological determinant is less understood. To present-day, numerous WSS-based descriptors have been proposed to characterize the spatiotemporal nature of the WSS disturbances, however, without differentiation between physiological laminar oscillations and turbulence-related WSS (tWSS) fluctuations. Also, much attention has been focused on magnetic resonance (MR) WSS estimations, so far with limited success; promoting the need of a near-wall surrogate marker. In this study, a new approach is explored to characterize the tWSS, by taking advantage of the tensor characteristics of the fluctuating WSS correlations, providing both a magnitude and an anisotropy measure of the disturbances. These parameters were studied in two patient-specific coarctation models (sever and mild), using large eddy simulations, and correlated against near-wall reciprocal Reynolds stress parameters. Collectively, results showed distinct regions of differing tWSS characteristics, features which were sensitive to changes in flow conditions. Generally, the post-stenotic tWSS was governed by near axisymmetric fluctuations, findings that where not consistent with conventional WSS disturbance predictors. At the 2-3 mm wall-offset range, a strong linear correlation was found between tWSS magnitude and near-wall turbulence kinetic energy (TKE), in contrast to the anisotropy indices, suggesting that MR-measured TKE can be used to assess elevated tWSS regions while tWSS anisotropy estimates request well-resolved simulation methods.

Hemodynamic Changes in the Right Ventricle Induced by Variations of Cardiac Output: A Possible Mechanism for Arrhythmia Occurrence in the Outflow Tract

The rationale of this paper is to investigate right ventricular (RV) hemodynamics in relation to changes in cardiac output, and in particular to study exercise-induced stresses at the RV outflow tract (RVOT), which is a common site of ventricular arrhythmias in the athlete’s heart. We hypothesize that the thin-walled RVOT is exposed to high wall shear stresses (WSS) during physiological states associated with high cardiac output such as exercise, and therefore, may be particularly prone to substrate formation leading to ventricular tachyarrhythmias. 3D Particle Tracking Velocimetry (3D-PTV), an optical imaging method, has been performed in a novel anatomically accurate compliant silicone right heart model derived from a high resolution MRI heart scan of a healthy male proband. RV and RVOT flow patterns at resting conditions were obtained from two healthy athletic male proband’s hearts and two patients with arrhythmogenic right ventricular cardiomyopathy/dysplasia (ARVC/D) via phase contrast magnetic resonance imaging (PC-MRI). The healthy case was used as a reference for validating the in vitro flow patterns of the silicone model, while the diseased cases were used to generalize our findings and investigate possible changes in hemodynamic stresses with RV morphological remodelling. Our results showed that both healthy and diseased geometries consistently displayed an increased WSS in the RVOT relative to the rest of the RV. We found that increases in cardiac output may lead to increases of mean kinetic energy (MKE), laminar viscous dissipation and WSS at the RVOT. Furthermore, higher peak WSS magnitudes were found for the diseased cases. The identified high WSS regions may correlate with the common site of RVOT ventricular tachycardia in athletes and patients with ARVC/D. Our results imply that exercise, as well as anatomical and functional remodeling might alter RV wall shear stress both in magnitude and spatial distribution, leading to increased hemodynamic stresses in the RVOT.

Ventricular flow dynamics with varying LVAD inflow cannula lengths: In-silico evaluation in a multiscale model

Left ventricular assist devices are associated with thromboembolic events, which are potentially caused by altered intraventricular flow. Due to patient variability, differences in apical wall thickness affects cannula insertion lengths, potentially promoting unfavourable intraventricular flow patterns which are thought to be correlated to the risk of thrombosis. This study aimed to present a 3D multiscale computational fluid dynamic model of the left ventricle (LV) developed using a commercial software, Ansys, and evaluate the risk of thrombosis with varying inflow cannula insertion lengths in a severely dilated LV. Based on a HeartWare HVAD inflow cannula, insertion lengths of 5, 19, 24 and 50 mm represented cases of apical hypertrophy, typical ranges of apical thicknesses and an experimental length, respectively. The risk of thrombosis was evaluated based on blood washout, residence time, instantaneous blood stagnation and a pulsatility index. By introducing fresh blood to displace pre-existing blood in the LV, after 5 cardiac cycles, 46.7%, 45.7%, 45.1% and 41.8% of pre-existing blood remained for insertion lengths of 5, 19, 24 and 50 mm, respectively. Compared to the 50 mm insertion, blood residence time was at least 9%, 7% and 6% higher with the 5, 19 and 24 mm insertion lengths, respectively. No instantaneous stagnation at the apex was observed directly after the E-wave. Pulsatility indices adjacent to the cannula increased with shorter insertion lengths. For the specific scenario studied, a longer insertion length, relative to LV size, may be advantageous to minimise thrombosis by increasing LV washout and reducing blood residence time.

Exploring kinetic energy as a new marker of cardiac function in the single ventricle circulation

Ventricular volumetric ejection fraction (VV EF) is often normal in patients with single ventricle circulations despite them experiencing symptoms related to circulatory failure. We sought to determine if kinetic energy (KE) could be a better marker of ventricular performance. KE was prospectively quantified using four-dimensional flow MRI in 41 patients with a single ventricle circulation (aged 0.5-28 yr) and compared with 43 healthy volunteers (aged 1.5-62 yr) and 14 patients with left ventricular (LV) dysfunction (aged 28-79 yr). Intraventricular end-diastolic blood was tracked through systole and divided into ejected and residual blood components. Two ejection fraction (EF) metrics were devised based on the KE of the ejected component over the total of both the ejected and residual components using 1) instantaneous peak KE to assess KE EF or 2) summating individual peak particle energy (PE) to assess PE EF. KE EF and PE EF had a smaller range than VV EF in healthy subjects (97.9 ± 0.8 vs. 97.3 ± 0.8 vs. 60.1 ± 5.2%). LV dysfunction caused a fall in KE EF ( P = 0.01) and PE EF ( P = 0.0001). VV EF in healthy LVs and single ventricle hearts was equivalent; however, KE EF and PE EF were lower ( P < 0.001) with a wider range indicating a spectrum of severity. Those reporting the greatest symptomatic impairment (New York Heart Association II) had lower PE EF than asymptomatic subjects ( P = 0.0067). KE metrics are markers of healthy cardiac function. PE EF may be useful in grading dysfunction. NEW & NOTEWORTHY Kinetic energy (KE) represents the useful work of the heart in ejecting blood. This article details the utilization of KE indexes to assess cardiac function in health and a variety of pathophysiological conditions. KE ejection fraction and particle energy ejection fraction (PE EF) showed a narrow range in health and a lower wider range in disease representing a spectrum of severity. PE EF was altered by functional status potentially offering the opportunity to grade dysfunction.

Clinical applications of intra-cardiac four-dimensional flow cardiovascular magnetic resonance: A systematic review

BACKGROUND

Four-dimensional flow cardiovascular magnetic resonance (4D flow CMR) is an emerging non-invasive imaging technology used to visualise and quantify intra-cardiac blood flow. The aim of this systematic review is to assess the literature on the current clinical applications of intra-cardiac 4D flow CMR.

METHODS

A systematic review was conducted to evaluate the literature on the intra-cardiac clinical applications of 4D flow CMR. Structured searches were carried out on Medline, EMBASE and the Cochrane Library in October 2016. A modified Critical Skills Appraisal Programme (CASP) tool was used to objectively assess and score the included studies. Studies were categorised as 'highly clinically applicable' for scores of 67-100%, 'potentially clinically applicable' for 34-66% and 'less clinically applicable' for 0-33%.

RESULTS

Of the 1608 articles screened, 44 studies met eligibility for systematic review. The included literature consisted of 22 (50%) mechanistic studies, 18 (40.9%) pilot studies and 4 (9.1%) diagnostic studies. Based on the modified CASP tool, 27 (62%) studies were 'highly clinically applicable', 9 (20%) were 'potentially clinically applicable' and 8 (18%) were 'less clinically applicable'.

CONCLUSIONS

There are many proposed methods for using 4D flow CMR to quantify intra-cardiac flow. The evidence base is mainly mechanistic, featuring single-centred designs. Larger, multi-centre studies are required to validate the proposed techniques and investigate the clinical advantages that 4D flow CMR offers over standard practices. PROSPERO=CRD42016051438.

Estimating the irreversible pressure drop across a stenosis by quantifying turbulence production using 4D Flow MRI

The pressure drop across a stenotic vessel is an important parameter in medicine, providing a commonly used and intuitive metric for evaluating the severity of the stenosis. However, non-invasive estimation of the pressure drop under pathological conditions has remained difficult. This study demonstrates a novel method to quantify the irreversible pressure drop across a stenosis using 4D Flow MRI by calculating the total turbulence production of the flow. Simulation MRI acquisitions showed that the energy lost to turbulence production can be accurately quantified with 4D Flow MRI within a range of practical spatial resolutions (1-3 mm; regression slope = 0.91, R2 = 0.96). The quantification of the turbulence production was not substantially influenced by the signal-to-noise ratio (SNR), resulting in less than 2% mean bias at SNR > 10. Pressure drop estimation based on turbulence production robustly predicted the irreversible pressure drop, regardless of the stenosis severity and post-stenosis dilatation (regression slope = 0.956, R2 = 0.96). In vitro validation of the technique in a 75% stenosis channel confirmed that pressure drop prediction based on the turbulence production agreed with the measured pressure drop (regression slope = 1.15, R2 = 0.999, Bland-Altman agreement = 0.75 ± 3.93 mmHg).

Creating hemodynamic atlases of cardiac 4D flow MRI

Purpose

Hemodynamic atlases can add to the pathophysiological understanding of cardiac diseases. This study proposes a method to create hemodynamic atlases using 4D Flow magnetic resonance imaging (MRI). The method is demonstrated for kinetic energy (KE) and helicity density (H_d).

Materials and Methods

Thirteen healthy subjects underwent 4D Flow MRI at 3T. Phase‐contrast magnetic resonance cardioangiographies (PC‐MRCAs) and an average heart were created and segmented. The PC‐MRCAs, KE, and H_d were nonrigidly registered to the average heart to create atlases. The method was compared with 1) rigid, 2) affine registration of the PC‐MRCAs, and 3) affine registration of segmentations. The peak and mean KE and H_d before and after registration were calculated to evaluate interpolation error due to nonrigid registration.

Results

The segmentations deformed using nonrigid registration overlapped (median: 92.3%) more than rigid (23.1%, P < 0.001), and affine registration of PC‐MRCAs (38.5%, P < 0.001) and affine registration of segmentations (61.5%, P < 0.001). The peak KE was 4.9 mJ using the proposed method and affine registration of segmentations (P = 0.91), 3.5 mJ using rigid registration (P < 0.001), and 4.2 mJ using affine registration of the PC‐MRCAs (P < 0.001). The mean KE was 1.1 mJ using the proposed method, 0.8 mJ using rigid registration (P < 0.001), 0.9 mJ using affine registration of the PC‐MRCAs (P < 0.001), and 1.0 mJ using affine registration of segmentations (P = 0.028). The interpolation error was 5.2 ± 2.6% at mid‐systole, 2.8 ± 3.8% at early diastole for peak KE; 9.6 ± 9.3% at mid‐systole, 4.0 ± 4.6% at early diastole, and 4.9 ± 4.6% at late diastole for peak H_d. The mean KE and H_d were not affected by interpolation.

Conclusion

Hemodynamic atlases can be obtained with minimal user interaction using nonrigid registration of 4D Flow MRI.

Level of Evidence: 2

Technical Efficacy: Stage 1

J. Magn. Reson. Imaging 2017;46:1389–1399.

Decreased Diastolic Ventricular Kinetic Energy in Young Patients with Fontan Circulation Demonstrated by Four-Dimensional Cardiac Magnetic Resonance Imaging

Four-dimensional (4D) flow magnetic resonance imaging (MRI) enables quantification of kinetic energy (KE) in intraventricular blood flow. This provides a novel way to understand the cardiovascular physiology of the Fontan circulation. In this study, we aimed to quantify the KE in functional single ventricles. 4D flow MRI was acquired in eleven patients with Fontan circulation (median age 12 years, range 3-29) and eight healthy volunteers (median age 26 years, range 23-36). Follow-up MRI after surgical or percutaneous intervention was performed in 3 patients. Intraventricular KE was calculated throughout the cardiac cycle and indexed to stroke volume (SV). The systolic/diastolic ratio of KE in Fontan patients was similar to the ratio of the controls' left ventricle (LV) or right ventricle (RV) depending on the patients' ventricular morphology (Cohen´s κ = 1.0). Peak systolic KE/SV did not differ in patients compared to the LV in controls (0.016 ± 0.006 mJ/ml vs 0.020 ± 0.004 mJ/ml, p = 0.09). Peak diastolic KE/SV in Fontan patients was lower than in the LV of the control group (0.028 ± 0.010 mJ/ml vs 0.057 ± 0.011 mJ/ml, p < 0.0001). The KE during diastole showed a plateau in patients with aortopulmonary collaterals. This is to our knowledge the first study that quantifies the intraventricular KE of Fontan patients. KE is dependent on the morphology of the ventricle, and diastolic KE indexed to SV in patients is decreased compared to controls. The lower KE in Fontan patients may be a result of impaired ventricular filling.

On the quantification and visualization of transient periodic instabilities in pulsatile flows

Turbulent-like flows without cycle-to-cycle variations are more frequently being reported in studies of cardiovascular flows. The associated stimuli might be of mechanobiological relevance, but how to quantify them objectively is not obvious. Classical Reynolds decomposition, where the flow is separated into mean and fluctuating velocity components, is not applicable as the phase-average is zero. We therefore expanded on established techniques and present the idea, analogous to Reynolds decomposition, to decompose a flow with transient instabilities into low- versus high frequency components, respectively, to discriminate flow instabilities from the underlying cardiac pulsatility. Transient wall shear stress and velocity signals derived from computational fluid dynamic simulations were transferred to the frequency domain. A high-pass filter was applied to subtract the 99% most-energy-containing frequencies, which gave a cut-off frequency of 25Hz. We introduce here the spectral power index, and compute the fluctuating kinetic energy, based on the high-pass filtered velocity components, both being frequency-based operators. The efficacy was evaluated in an aneurysm model for multiple flow rates demonstrating transition to turbulent-like flows. The frequency-based operators were found to better correlate with the qualitatively observed flow instabilities compared to conventional descriptors, like time-averaged wall shear stress or oscillatory shear index. We demonstrate how the high frequencies beyond the physiological range could be analyzed and/or transferred back to the time domain for quantification and visualization purposes. We have introduced general frequency-based operators, easily extendable to other cardiovascular territories based on a posteriori heuristic filtering that allows for separation, isolation, and quantification of cycle-invariant turbulent-like flows.

Assessment of turbulent viscous stress using ICOSA 4D Flow MRI for prediction of hemodynamic blood damage

Flow-induced blood damage plays an important role in determining the hemodynamic impact of abnormal blood flow, but quantifying of these effects, which are dominated by shear stresses in highly fluctuating turbulent flow, has not been feasible. This study evaluated the novel application of turbulence tensor measurements using simulated 4D Flow MRI data with six-directional velocity encoding for assessing hemodynamic stresses and corresponding blood damage index (BDI) in stenotic turbulent blood flow. The results showed that 4D Flow MRI underestimates the maximum principal shear stress of laminar viscous stress (PLVS), and overestimates the maximum principal shear stress of Reynolds stress (PRSS) with increasing voxel size. PLVS and PRSS were also overestimated by about 1.2 and 4.6 times at medium signal to noise ratio (SNR) = 20. In contrast, the square sum of the turbulent viscous shear stress (TVSS), which is used for blood damage index (BDI) estimation, was not severely affected by SNR and voxel size. The square sum of TVSS and the BDI at SNR >20 were underestimated by less than 1% and 10%, respectively. In conclusion, this study demonstrated the feasibility of 4D Flow MRI based quantification of TVSS and BDI which are closely linked to blood damage.

Patient-Specific Simulation of Cardiac Blood Flow From High-Resolution Computed Tomography

Cardiac hemodynamics can be computed from medical imaging data, and results could potentially aid in cardiac diagnosis and treatment optimization. However, simulations are often based on simplified geometries, ignoring features such as papillary muscles and trabeculae due to their complex shape, limitations in image acquisitions, and challenges in computational modeling. This severely hampers the use of computational fluid dynamics in clinical practice. The overall aim of this study was to develop a novel numerical framework that incorporated these geometrical features. The model included the left atrium, ventricle, ascending aorta, and heart valves. The framework used image registration to obtain patient-specific wall motion, automatic remeshing to handle topological changes due to the complex trabeculae motion, and a fast interpolation routine to obtain intermediate meshes during the simulations. Velocity fields and residence time were evaluated, and they indicated that papillary muscles and trabeculae strongly interacted with the blood, which could not be observed in a simplified model. The framework resulted in a model with outstanding geometrical detail, demonstrating the feasibility as well as the importance of a framework that is capable of simulating blood flow in physiologically realistic hearts.

Assessment of left ventricular hemodynamic forces in healthy subjects and patients with dilated cardiomyopathy using 4D flow MRI

We hypothesized that the direction of global left ventricular (LV) hemodynamic forces during diastolic filling are concordant with the main flow axes in normal LVs, but that this pattern would be altered in dilated and dysfunctional LVs. Therefore, we aimed to assess the LV hemodynamic filling forces in a group of healthy subjects and compare them to the results from a group of patients with dilated cardiomyopathy (DCM). Ten healthy subjects and 10 DCM patients were enrolled. Morphological short‐ (SAx) and long‐axis (LAx) images and 4D flow MRI data were acquired at 1.5T. The LV pressure gradients were computed from the 4D flow data using the Navier–Stokes equations. By integrating the pressure gradients over the LV volume at each time frame, the magnitude and direction of the global hemodynamic force was calculated over the cardiac cycle. The hemodynamic forces acting in the SAx‐ and LAx‐directions were used to calculate the “SAx‐max/LAx‐max”‐ratio for the early (E‐wave) and late (A‐wave) diastolic filling. In the LAx‐plane, the temporal progression of the hemodynamic force followed a consistent pattern in the healthy subjects. The “SAx‐max/LAx‐max”‐ratio was significantly larger at both E‐wave (0.53 ± 0.15 vs. 0.23 ± 0.12, P < 0.0001) and A‐wave (0.44 ± 0.21 vs. 0.26 ± 0.09, P < 0.03) in the DCM patients compared to the healthy subjects. 4D flow MRI data allow quantification of LV hemodynamic forces acting on the LV myocardial wall. The LV hemodynamic filling forces showed a similar temporal progression among healthy subjects, whereas DCM patients had forces that were more heterogeneous in their direction and magnitude during diastole.

Assessment of turbulent flow effects on the vessel wall using four-dimensional flow MRI

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 R²  = 0.84). Near-wall TKE estimates from MR simulations have good agreement to CFD-derived ground truth (mean R²  = 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. Magn Reson Med 77:2310-2319, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Hemodynamic Measurement Using Four-Dimensional Phase-Contrast MRI: Quantification of Hemodynamic Parameters and Clinical Applications

Recent improvements have been made to the use of time-resolved, three-dimensional phase-contrast (PC) magnetic resonance imaging (MRI), which is also named four-dimensional (4D) PC-MRI or 4D flow MRI, in the investigation of spatial and temporal variations in hemodynamic features in cardiovascular blood flow. The present article reviews the principle and analytical procedures of 4D PC-MRI. Various fluid dynamic biomarkers for possible clinical usage are also described, including wall shear stress, turbulent kinetic energy, and relative pressure. Lastly, this article provides an overview of the clinical applications of 4D PC-MRI in various cardiovascular regions.

Determinants of kinetic energy of blood flow in the four-chambered heart in athletes and sedentary controls

The kinetic energy (KE) of intracardiac blood may play an important role in cardiac function. The aims of the present study were to 1) quantify and investigate the determinants of KE, 2) compare the KE expenditure of intracardiac blood between athletes and control subjects, and 3) quantify the amount of KE inside and outside the diastolic vortex. Fourteen athletes and fourteen volunteers underwent cardiac MRI, including four-dimensional phase-contrast sequences. KE was quantified in four chambers, and energy expenditure was calculated by determining the mean KE/cardiac index. Left ventricular (LV) mass was an independent predictor of diastolic LVKE ( R2= 0.66, P < 0.001), whereas right ventricular (RV) end-diastolic volume was important for diastolic RVKE ( R2= 0.76, P < 0.001). The mean KE/cardiac index did not differ between groups (control subjects: 0.53 ± 0.14 mJ·l−1·min·m2and athletes: 0.56 ± 0.21 mJ·l−1·min·m2, P = 0.98). Mean LV diastolic vortex KE made up 70 ± 1% and 73 ± 2% of total LV diastolic KE in athletes and control subjects ( P = 0.18). In conclusion, the characteristics of the LV as a pressure pump and the RV as a volume pump are demonstrated as an association between LVKE and LV mass and between RVKE and end-diastolic volume. This also suggests different filling mechanisms where the LV is dependent on diastolic suction, whereas the RV fills with a basal movement of the atrioventricular plane over “stationary” blood. Both groups had similar energy expenditure for intracardiac blood flow, indicating similar pumping efficiency, likely explained by the lower heart rate that cancels the higher KE per heart beat in athletes. The majority of LVKE is found within the LV diastolic vortex, in contrast to earlier findings.

Left ventricular fluid kinetic energy time curves in heart failure from cardiovascular magnetic resonance 4D flow data

BACKGROUND

Measurement of intracardiac kinetic energy (KE) provides new insights into cardiac hemodynamics and may improve assessment and understanding of heart failure. We therefore aimed to investigate left ventricular (LV) KE time curves in patients with heart failure and in controls.

METHODS

Patients with heart failure (n = 29, NYHA class I-IV) and controls (n = 12) underwent cardiovascular magnetic resonance (CMR) including 4D flow. The vortex-ring boundary was computed using Lagrangian coherent structures. The LV endocardium and vortex-ring were manually delineated and KE was calculated as ½mv(2) of the blood within the whole LV and the vortex ring, respectively.

RESULTS

The systolic average KE was higher in patients compared to controls (2.2 ± 1.4 mJ vs 1.6 ± 0.6 mJ, p = 0.048), but lower when indexing to EDV (6.3 ± 2.2 μJ/ml vs 8.0 ± 2.1 μJ/ml, p = 0.025). No difference was seen in diastolic average KE (3.2 ± 2.3 mJ vs 2.0 ± 0.8 mJ, p = 0.13) even when indexing to EDV (9.0 ± 4.4 μJ/ml vs 10.2 ± 3.3 μJ/ml, p = 0.41). In patients, a smaller fraction of diastolic average KE was observed inside the vortex ring compared to controls (72 ± 6% vs 54 ± 9%, p < 0.0001). Three distinctive KE time curves were seen in patients which were markedly different from findings in controls, and with a moderate agreement between KE time curve patterns and degree of diastolic dysfunction (Cohen's kappa = 0.49), but unrelated to NYHA classification (p = 0.12), or 6-minute walk test (p = 0.72).

CONCLUSION

Patients with heart failure exhibit higher systolic average KE compared to controls, suggesting altered intracardiac blood flow. The different KE time curves seen in patients may represent a conceptually new approach for heart failure classification.

4D flow cardiovascular magnetic resonance consensus statement

Pulsatile blood flow through the cavities of the heart and great vessels is time-varying and multidirectional. Access to all regions, phases and directions of cardiovascular flows has formerly been limited. Four-dimensional (4D) flow cardiovascular magnetic resonance (CMR) has enabled more comprehensive access to such flows, with typical spatial resolution of 1.5×1.5×1.5 - 3×3×3 mm(3), typical temporal resolution of 30-40 ms, and acquisition times in the order of 5 to 25 min. This consensus paper is the work of physicists, physicians and biomedical engineers, active in the development and implementation of 4D Flow CMR, who have repeatedly met to share experience and ideas. The paper aims to assist understanding of acquisition and analysis methods, and their potential clinical applications with a focus on the heart and greater vessels. We describe that 4D Flow CMR can be clinically advantageous because placement of a single acquisition volume is straightforward and enables flow through any plane across it to be calculated retrospectively and with good accuracy. We also specify research and development goals that have yet to be satisfactorily achieved. Derived flow parameters, generally needing further development or validation for clinical use, include measurements of wall shear stress, pressure difference, turbulent kinetic energy, and intracardiac flow components. The dependence of measurement accuracy on acquisition parameters is considered, as are the uses of different visualization strategies for appropriate representation of time-varying multidirectional flow fields. Finally, we offer suggestions for more consistent, user-friendly implementation of 4D Flow CMR acquisition and data handling with a view to multicenter studies and more widespread adoption of the approach in routine clinical investigations.

Whole-heart four-dimensional flow can be acquired with preserved quality without respiratory gating, facilitating clinical use: a head-to-head comparison

Background

Respiratory gating is often used in 4D-flow acquisition to reduce motion artifacts. However, gating increases scan time. The aim of this study was to investigate if respiratory gating can be excluded from 4D flow acquisitions without affecting quantitative intracardiac parameters.

Methods

Eight volunteers underwent CMR at 1.5 T with a 5-channel coil (5ch). Imaging included 2D flow measurements and whole-heart 4D flow with and without respiratory gating (Resp(+), Resp(−)). Stroke volume (SV), particle-trace volumes, kinetic energy, and vortex-ring volume were obtained from 4D flow-data. These parameters were compared between 5ch Resp(+) and 5ch Resp(−). In addition, 20 patients with heart failure were scanned using a 32-channel coil (32ch), and particle-trace volumes were compared to planimetric SV. Paired comparisons were performed using Wilcoxon’s test and correlation analysis using Pearson r. Agreement was assessed as bias ± SD.

Results

Stroke volume from 4D flow was lower compared to 2D flow both with and without respiratory gating (5ch Resp(+) 88 ± 18 vs 97 ± 24.0, p = 0.001; 5ch Resp(−) 86 ± 16 vs 97.1 ± 22.7, p < 0.01). There was a good correlation between Resp(+) and Resp(−) for particle-trace derived volumes (R² = 0.82, 0.2 ± 9.4 ml), mean kinetic energy (R² = 0.86, 0.07 ± 0.21 mJ), peak kinetic energy (R² = 0.88, 0.14 ± 0.77 mJ), and vortex-ring volume (R² = 0.70, −2.5 ± 9.4 ml). Furthermore, good correlation was found between particle-trace volume and planimetric SV in patients for 32ch Resp(−) (R² = 0.62, −4.2 ± 17.6 ml) and in healthy volunteers for 5ch Resp(+) (R² = 0.89, −11 ± 7 ml), and 5ch Resp(−) (R² = 0.93, −7.5 ± 5.4 ml), Average scan duration for Resp(−) was shorter compared to Resp(+) (27 ± 9 min vs 61 ± 19 min, p < 0.05).

Conclusions

Whole-heart 4D flow can be acquired with preserved quantitative results without respiratory gating, facilitating clinical use.

Electronic supplementary material

The online version of this article (doi:10.1186/s12880-015-0061-4) contains supplementary material, which is available to authorized users.

Quantitative Assessment of Turbulence and Flow Eccentricity in an Aortic Coarctation: Impact of Virtual Interventions

Turbulence and flow eccentricity can be measured by magnetic resonance imaging (MRI) and may play an important role in the pathogenesis of numerous cardiovascular diseases. In the present study, we propose quantitative techniques to assess turbulent kinetic energy (TKE) and flow eccentricity that could assist in the evaluation and treatment of stenotic severities. These hemodynamic parameters were studied in a pre-treated aortic coarctation (CoA) and after several virtual interventions using computational fluid dynamics (CFD), to demonstrate the effect of different dilatation options on the flow field. Patient-specific geometry and flow conditions were derived from MRI data. The unsteady pulsatile flow was resolved by large eddy simulation including non-Newtonian blood rheology. Results showed an inverse asymptotic relationship between the total amount of TKE and degree of dilatation of the stenosis, where turbulent flow proximal the constriction limits the possible improvement by treating the CoA alone. Spatiotemporal maps of TKE and flow eccentricity could be linked to the characteristics of the jet, where improved flow conditions were favored by an eccentric dilatation of the CoA. By including these flow markers into a combined MRI-CFD intervention framework, CoA therapy has not only the possibility to produce predictions via simulation, but can also be validated pre- and immediate post treatment, as well as during follow-up studies.