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

Evaluation of left ventricular blood flow kinetic energy in patients with acute myocardial infarction by 4D Flow MRI: a preliminary study

PURPOSE

To evaluate the intracavity left ventricular (LV) blood flow kinetic energy (KE) parameters using four-dimensional (4D) flow cardiovascular magnetic resonance (CMR) in patients with acute myocardial infarction (AMI).

METHODS

Thirty AMI patients and twenty controls were examined via CMR, which included cine imaging, late gadolinium enhancement (LGE) and global heart 4D flow imaging. The KE parameters were indexed to LV end-diastolic volume (EDV) to obtain average, systolic and diastolic KE as well as the proportion of LV in-plane KE (%). These parameters were compared between the AMI patients and controls and between the two subgroups.

RESULTS

Analysis of the LV blood flow KE parameters at different levels of the LV cavity and in different segments of the same level showed that the basal level had the highest blood flow KE while the apical level had the lowest in the control group. There were no significant differences in diastolic KE, systolic in-plane KE and diastolic in-plane KE between the anterior wall and posterior wall (p > 0.05), only the systolic KE had a significant difference between them (p < 0.05). Compared with those in the control group, the average (10.7 ± 3.3 µJ/mL vs. 14.7 ± 3.6 µJ/mL, p < 0.001), systolic (14.6 ± 5.1 µJ/mL vs. 18.9 ± 3.9 µJ/mL, p = 0.003) and diastolic KE (7.9 ± 2.5 µJ/mL vs. 10.6 ± 3.8 µJ/mL, p = 0.018) were significantly lower in the AMI group. The average KE in the infarct segment was lower than that in the noninfarct segment in the AMI group (49.5 ± 18.7 µJ/mL vs. 126.3 ± 50.7 µJ/mL, p < 0.001), while the proportion of systolic in-plane KE increased significantly (61.8%±11.5 vs. 42.9%±14.4, p = 0.001).

CONCLUSION

The 4D Flow MRI technique can be used to quantitatively evaluate LV regional hemodynamic parameters. There were differences in the KE parameters of LV blood flow at different levels and in different segments of the same level in healthy people. In AMI patients, the average KE of the infarct segment decreased, while the proportion of systolic in-plane KE significantly increased.

Evaluation of Left Ventricular Flow Kinetic Energy by Four-Dimensional Blood Flow MRI in Nondialysis Chronic Kidney Disease Patients

BACKGROUND

Chronic kidney disease (CKD) is associated with increased, and early cardiovascular disease risk. Changes in hemodynamics within the left ventricle (LV) respond to cardiac remodeling. The LV hemodynamics in nondialysis CKD patients are not clearly understood.

PURPOSE

To use four-dimensional blood flow MRI (4D flow MRI) to explore changes in LV kinetic energy (KE) and the relationship between LV KE and LV remodeling in CKD patients.

STUDY TYPE

Retrospective.

POPULATION

98 predialysis CKD patients (Stage 3: n = 21, stage 4: n = 21, and stage 5: n = 56) and 16 age- and sex-matched healthy controls.

FIELD STRENGTH/SEQUENCE

3.0 T/balanced steady-state free precession (SSFP) cine sequence, 4D flow MRI with a fast field echo sequence, T1 mapping with a modified Look-Locker SSFP sequence, and T2 mapping with a gradient recalled and spin echo sequence.

ASSESSMENT

Demographic characteristics (age, sex, height, weight, blood pressure, heart rate, aortic regurgitation, and mitral regurgitation) and laboratory data (eGFR, Creatinine, hemoglobin, ferritin, transferrin saturation, potassium, and carbon dioxide bonding capacity) were extracted from patient records. Myocardial T1, T2, LV ejection fraction, end diastolic volume (EDV), end systolic volume, LV flow components (direct flow, delayed ejection, retained inflow, and residual volume) and KE parameters (peak systolic, systolic, diastolic, peak E-wave, peak A-wave, E/A ratio, and global) were assessed. The KE parameters were normalized to EDV (KEiEDV). Parameters were compared between disease stage in CKD patients, and between CKD patients and healthy controls.

STATISTICAL TESTS

Differences in clinical and imaging parameters between groups were compared using one-way ANOVA, Kruskal Walls and Mann-Whitney U tests, chi-square test, and Fisher's exact test. Pearson or Spearman's correlation coefficients and multiple linear regression analysis were used to compare the correlation between LV KE and other clinical and functional parameters. A P-value of <0.05 was considered significant.

RESULTS

Compared with healthy controls, peak systolic (24.76 ± 5.40 μJ/mL vs. 31.86 ± 13.18 μJ/mL), systolic (11.62 ± 2.29 μJ/mL vs. 15.27 ± 5.10 μJ/mL), diastolic (7.95 ± 1.92 μJ/mL vs. 13.33 ± 5.15 μJ/mL), peak A-wave (15.95 ± 4.86 μJ/mL vs. 31.98 ± 14.51 μJ/mL), and global KEiEDV (9.40 ± 1.64 μJ/mL vs. 14.02 ± 4.14 μJ/mL) were significantly increased and the KEiEDV E/A ratio (1.16 ± 0.67 vs. 0.69 ± 0.53) was significantly decreased in CKD patients. As the CKD stage progressed, both diastolic KEiEDV (10.45 ± 4.30 μJ/mL vs. 12.28 ± 4.85 μJ/mL vs. 14.80 ± 5.06 μJ/mL) and peak E-wave KEiEDV (15.30 ± 7.06 μJ/mL vs. 14.69 ± 8.20 μJ/mL vs. 19.33 ± 8.29 μJ/mL) increased significantly. In multiple regression analysis, global KEiEDV (β* = 0.505; β* = 0.328), and proportion of direct flow (β* = -0.376; β* = -0.410) demonstrated an independent association with T1 and T2 times.

DATA CONCLUSION

4D flow MRI-derived LV KE parameters show altered LV adaptations in CKD patients and correlate independently with T1 and T2 mapping that may represent myocardial fibrosis and edema.

LEVEL OF EVIDENCE: 4

TECHNICAL EFFICACY

Stage 3.

Robust and fast stochastic 4D flow vector-field signature technique for quantifying composite flow dynamics from 4D flow MRI: Application to left atrial flow in atrial fibrillation

4D flow MRI is an emerging imaging modality that maps voxel-wise blood flow information as velocity vector fields that is acquired in 7-dimensional image volumes (3 spatial dimensions + 3 velocity directions + time). Blood flow in the cardiovascular system is often complex and composite involving multiple flow dynamics and patterns (e.g., vortex flow, jets, stagnating flow) that occur and interact simultaneously. The spectrum of such complex flow dynamics is embedded in the velocity vector field dynamics derived from 4D Flow MRI. However, current flow metrics cannot fully measure high-dimensional vector-field data and embedded complex composite flow data. Instead, these methods need to break down the vector-field data into secondary scalar fields of individual flow components using fluid dynamics operators. These methods are gradient-based and sensitive to data uncertainties, and only focus on individual flow components of the overall composite flow, therefore potentially underestimating the severity of overall flow changes associated with cardiovascular diseases. To address these limitations, in MICCAI 2021, we introduced a novel comprehensive stochastic 4D Flow vector-field signature technique that works directly on the entire spatiotemporal velocity vector field. This technique uses efficient stochastic gradient-free interrogation of multi-million flow vector-pairs per patient to derive the patient's unique flow profile of the complex composite flow alterations and in real-time processing. The signature technique's probabilistic gradient-free formulation should allow for highly robust quantification despite inherent errors in 4D flow MRI acquisitions. Here, we extend the application of the 4D flow vector-field signature technique to the left atrium to analyze complex composite flow changes in patients with atrial fibrillation. In 128 subjects, we performed extensive sensitivity testing and determined that the vector-field signature technique is highly robust to typical sources of data uncertainties in 4D flow MRI: degradation in spatiotemporal resolution, added Gaussian noise, and segmentation errors. We demonstrate the excellent generalizability of the stochastic convergence from the aorta to the left atrium and between different 4D Flow MRI acquisition protocols. We compare the robustness of our technique to existing advanced flow quantification metrics of kinetic energy, vorticity, and energy loss demonstrating a superior performance of up-to 14-fold. Our results show the potential diagnostic and clinical utility of our signature technique in identifying distinctly altered composite flow signatures in atrial fibrillation patients independent of existing flow metrics.

Deep learning based automated left ventricle segmentation and flow quantification in 4D flow cardiac MRI

BACKGROUND

4D flow MRI enables assessment of cardiac function and intra-cardiac blood flow dynamics from a single acquisition. However, due to the poor contrast between the chambers and surrounding tissue, quantitative analysis relies on the segmentation derived from a registered cine MRI acquisition. This requires an additional acquisition and is prone to imperfect spatial and temporal inter-scan alignment. Therefore, in this work we developed and evaluated deep learning-based methods to segment the left ventricle (LV) from 4D flow MRI directly.

METHODS

We compared five deep learning-based approaches with different network structures, data pre-processing and feature fusion methods. For the data pre-processing, the 4D flow MRI data was reformatted into a stack of short-axis view slices. Two feature fusion approaches were proposed to integrate the features from magnitude and velocity images. The networks were trained and evaluated on an in-house dataset of 101 subjects with 67,567 2D images and 3030 3D volumes. The performance was evaluated using various metrics including Dice, average surface distance (ASD), end-diastolic volume (EDV), end-systolic volume (ESV), LV ejection fraction (LVEF), LV blood flow kinetic energy (KE) and LV flow components. The Monte Carlo dropout method was used to assess the confidence and to describe the uncertainty area in the segmentation results.

RESULTS

Among the five models, the model combining 2D U-Net with late fusion method operating on short-axis reformatted 4D flow volumes achieved the best results with Dice of 84.52% and ASD of 3.14 mm. The best averaged absolute and relative error between manual and automated segmentation for EDV, ESV, LVEF and KE was 19.93 ml (10.39%), 17.38 ml (22.22%), 7.37% (13.93%) and 0.07 mJ (5.61%), respectively. Flow component results derived from automated segmentation showed high correlation and small average error compared to results derived from manual segmentation.

CONCLUSIONS

Deep learning-based methods can achieve accurate automated LV segmentation and subsequent quantification of volumetric and hemodynamic LV parameters from 4D flow MRI without requiring an additional cine MRI acquisition.

Enhancement of intra-cardiac flow-field data using adaptive Kernel filtering

A method of determining the optimal kernel size for filtering noise in vortex dominated flow-fields, as found in the cardiac chambers is presented in this paper. Using synthetic flow fields generated using harmonic functions and perturbed using Gaussian noises of different amplitudes and spreads, the effect of kernel size on noise removal using the Median filter is tested systematically. It is shown that there exists an optimal kernel size at which the Median filter works best. The size of the optimal kernel is shown to be related to the vortex size. When applied to MRI generated cardiac flow-fields, the approach is seen to reveal underlying vortex patterns thereby aiding as an effective tool in the diagnosis and prognosis of cardiac diseases based on vortices as clinical biomarkers. The behavior of the restored cardiac flow fields which are filtered with the optimal kernel size and also with some values preceding and succeeding it are similar to that observed in studies with synthetic flow fields. This confirms that the optimal size of the kernel is related to the cardiac vortex size as is observed in the case of synthetic flow fields.

Benchtop Models of Patient-Specific Intraventricular Flow During Heart Failure and LVAD Support

The characterization of intraventricular flow is critical to evaluate the efficiency of fluid transport and potential thromboembolic risk but challenging to measure directly in advanced heart failure (HF) patients with left ventricular assist device (LVAD) support. The study aims to validate an in-house mock loop (ML) by simulating specific conditions of HF patients with normal and prosthetic mitral valves (MV) and LVAD patients with small and dilated left ventricle volumes, then comparing the flow-related indices result of vortex parameters, residence time (RT), and shear-activation potential (SAP). Patient-specific inputs for the ML studies included heart rate, end-diastolic and end-systolic volumes, ejection fraction, aortic pressure, E/A ratio, and LVAD speed. The ML effectively replicated vortex development and circulation patterns, as well as RT, particularly for HF patient cases. The LVAD velocity fields reflected altered flow paths, in which all or most incoming blood formed a dominant stream directing flow straight from the mitral valve to the apex. RT estimation of patient and ML compared well for all conditions, but SAP was substantially higher in the LVAD cases of the ML. The benchtop system generated comparable and reproducible hemodynamics and fluid dynamics for patient-specific conditions, validating its reliability and clinical relevance. This study demonstrated that ML is a suitable platform to investigate the fluid dynamics of HF and LVAD patients and can be utilized to investigate heart-implant interactions.

Four-Dimensional Flow MR Imaging: Technique and Advances

4D Flow MRI is an advanced imaging technique for comprehensive non-invasive assessment of the cardiovascular system. The capture of the blood velocity vector field throughout the cardiac cycle enables measures of flow, pulse wave velocity, kinetic energy, wall shear stress, and more. Advances in hardware, MRI data acquisition and reconstruction methodology allow for clinically feasible scan times. The availability of 4D Flow analysis packages allows for more widespread use in research and the clinic and will facilitate much needed multi-center, multi-vendor studies in order to establish consistency across scanner platforms and to enable larger scale studies to demonstrate clinical value.

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.

Fragmented Vortex in Heart Failure With Reduced Ejection Fraction: A Prospective Vector Flow Mapping Study

OBJECTIVE

Heart failure with reduced ejection fraction (HFrEF) is associated with structural and functional left ventricular changes. We compared intracardiac vortices between patients with HFrEF and normal participants using echocardiographic vector flow mapping, a novel intracardiac vortex analysis technology.

METHODS

Transthoracic echocardiography was performed on 20 patients with HFrEF (age: 61 ± 15 y, 15 men) and 20 normal participants (age: 59 ± 12 y, 12 men) age- and sex-balanced at the cohort level. Systolic and diastolic energy loss, area (indexed by left ventricular end-diastolic diameter), circulation (reflects vortex strength) and relative positions of the largest vortex during systole (S-vortex), early (E-vortex) and late (A-vortex) diastole and maximal number of vortices in a single frame (MNV) were assessed.

DISCUSSION

Patients with HFrEF had disproportionately sized vortices with smaller indexed vortex areas (p < 0.0001), and more fragmented vortices with higher MNV during both systole (p = 0.030) and diastole (p < 0.0001). These accompanied higher diastolic energy loss (p = 0.001). Additionally, the E-vortex (p = 0.002) and A-vortex (p < 0.0001) were more apically positioned, and the S-vortex was weaker (p = 0.033) in patients with HFrEF. More severe fragmentation (higher MNV) correlated with worse energy efficiency (higher energy loss).

CONCLUSION

Patients with HFrEF had more fragmented intracardiac vortices and lower energy efficiency predominantly during diastole.

Evaluation of left ventricular blood flow kinetic energy in patients with hypertension by four-dimensional flow cardiovascular magnetic resonance imaging: a preliminary study

OBJECTIVES

To evaluate the intra-cavity left ventricular (LV) blood flow kinetic energy (KE) parameters using four-dimensional (4D) flow cardiovascular magnetic resonance (CMR) in patients with hypertension (HTN).

METHODS

Forty-two HTN patients and twenty age-/gender-matched healthy controls who underwent CMR including cines, pre-/post-T1 mapping, and whole-heart 4D flow imaging were retrospectively evaluated. HTN patients were further divided into two subgroups: with preserved ejection fraction (HTN-pEF) and with reduced ejection fraction (HTN-rEF). KE parameters were indexed to LV end-diastolic volume (EDV) to obtain averaged LV, minimal, systolic, diastolic, peak E-wave, peak A-wave, E-wave, and A-wave KEi_EDV, as well as the proportion of in-plane LV KE (%), the time difference (TD). These parameters were compared between the HTN group and healthy controls, also between two subgroups. The correlation of LV blood flow KE parameters with LV function and extracellular volume fraction (ECV) were analyzed in the HTN group using multivariate regression analysis.

RESULTS

Peak E-wave KEi_EDV in the HTN group was significantly lower (p = 0.01), while in-plane KE and TD were significantly higher (all p < 0.01) than those in healthy controls. Compared to the HTN-pEF subgroup, the proportion of in-plane KE and TD was significantly increased in the HTN-rEF subgroup (all p < 0.01). Only the proportion of in-plane KE demonstrated an independent correlation with ECV (β* = 0.59, p < 0.01).

CONCLUSIONS

The decreased peak E-wave KEi_EDV and the increased proportion of in-plane KE, TD reflected the alterations of LV blood flow in HTN patients, and the proportion of in-plane KE was independently associated with ECV.

KEY POINTS

• 4D flow CMR demonstrated that the peak E-wave KEi_EDV was decreased, while the in-plane KE and time difference (TD) were increased in hypertensive (HTN) patients. • The proportion of in-plane KE and TD was further increased in HTN patients with reduced ejection fraction than in HTN patients with preserved ejection fraction, and the proportion of in-plane KE was independently associated with extracellular volume fraction in HTN patients. • 4D flow CMR intra-cavity blood flow KE parameters might reveal the LV hemodynamic status in preclinical HTN patients.

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.

Association of left ventricular flow energetics with remodeling after myocardial infarction: New hemodynamic insights for left ventricular remodeling

BACKGROUND

Myocardial infarction leads to complex changes in left ventricular (LV) hemodynamics. It remains unknown how four-dimensional acute changes in LV-cavity blood flow kinetic energy affects LV-remodeling.

METHODS AND RESULTS

In total, 69 revascularised ST-segment elevation myocardial infarction (STEMI) patients were enrolled. All patients underwent cardiovascular magnetic resonance (CMR) examination within 2 days of the index event and at 3-month. CMR examination included cine, late gadolinium enhancement, and whole-heart four-dimensional flow acquisitions. LV volume-function, infarct size (indexed to body surface area), microvascular obstruction, mitral inflow, and blood flow KEi (kinetic energy indexed to end-diastolic volume) characteristics were obtained. Adverse LV-remodeling was defined and categorized according to increase in LV end-diastolic volume of at least 10%, 15%, and 20%. Twenty-four patients (35%) developed at least 10%, 17 patients (25%) at least 15%, 11 patients (16%) at least 20% LV-remodeling. Demographics and clinical history were comparable between patients with/without LV-remodeling. In univariable regression-analysis, A-wave KEi was associated with at least 10%, 15%, and 20% LV-remodeling (p = 0.03, p = 0.02, p = 0.02, respectively), whereas infarct size only with at least 10% LV-remodeling (p = 0.02). In multivariable regression-analysis, A-wave KEi was identified as an independent marker for at least 10%, 15%, and 20% LV-remodeling (p = 0.09, p < 0.01, p < 0.01, respectively), yet infarct size only for at least 10% LV-remodeling (p = 0.03).

CONCLUSION

In patients with STEMI, LV hemodynamic assessment by LV blood flow kinetic energetics demonstrates a significant inverse association with adverse LV-remodeling. Late-diastolic LV blood flow kinetic energetics early after acute MI was independently associated with adverse LV-remodeling.

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.

Longitudinal Changes in Left Ventricular Blood Flow Kinetic Energy After Myocardial Infarction: Predictive Relevance for Cardiac Remodeling

BACKGROUND

Four-dimensional (4D) flow cardiac magnetic resonance (cardiac MR) imaging provides quantification of intracavity left ventricular (LV) flow kinetic energy (KE) parameters in three dimensions. ST-elevation myocardial infarction (STEMI) patients have been shown to have altered intracardiac blood flow compared to controls; however, how 4D flow parameters change over time has not been explored previously.

PURPOSE

Measure longitudinal changes in intraventricular flow post-STEMI and ascertain its predictive relevance of long-term cardiac remodeling.

STUDY TYPE

Prospective.

POPULATION

Thirty-five STEMI patients (M:F = 26:9, aged 56 ± 9 years).

FIELD STRENGTH/SEQUENCE

A 3 T/3D EPI-based, fast field echo (FFE) free-breathing 4D-flow sequence with retrospective cardiac gating.

ASSESSMENT

Serial imaging at 3-7 days (V1), 3-months (V2), and 12-months (V3) post-STEMI, including the following protocol: functional imaging for measuring volumes and 4D-flow for calculating parameters including systolic and peakE-wave LVKE, normalized to end-diastolic volume (iEDV) and stroke volume (iSV). Data were analyzed by H.B. (3 years experience). Patients were categorized into two groups: preserved ejection fraction (pEF, if EF > 50%) and reduced EF (rEF, if EF < 50%).

STATISTICAL TESTS

Independent sample t-tests were used to detect the statistical significance between any two cohorts. P < 0.05 was considered statistically significant.

RESULTS

Across the cohort, systolic KEisv was highest at V1 (28.0 ± 4.4 μJ/mL). Patients with rEF retained significantly higher systolic KEisv than patients with pEF at V2 (18.2 ± 3.4 μJ/mL vs. 6.9 ± 0.6 μJ/mL, P < 0.001) and V3 (21.6 ± 5.1 μJ/mL vs. 7.4 ± 0.9 μJ/mL, P < 0.001). Patients with pEF had significantly higher peakE-wave KEiEDV than rEF patients throughout the study (V1: 25.4 ± 11.6 μJ/mL vs. 18.1 ± 9.9 μJ/mL, P < 0.03, V2: 24.0 ± 10.2 μJ/mL vs. 17.2 ± 12.2 μJ/mL, P < 0.05, V3: 27.7 ± 14.8 μJ/mL vs. 15.8 ± 7.6 μJ/mL, P < 0.04).

DATA CONCLUSION

Systolic KE increased acutely following MI; in patients with pEF, this decreased over 12 months, while patients with rEF, this remained raised. Compared to patients with pEF, persistently lower peakE-wave KE in rEF patients is suggestive of early and fixed impairment in diastolic function.

EVIDENCE LEVEL

1 TECHNICAL EFFICACY: Stage 3.

Kinetic energy of left ventricular blood flow across heart failure phenotypes and in subclinical diastolic dysfunction

BACKGROUND

Kinetic energy (KE) of intracardiac blood flow reflects myocardial work spent on accelerating blood and provides a mechanistic window into diastolic filling dynamics. Diastolic dysfunction may represent an early stage in the development of heart failure (HF). Here we evaluated the hemodynamic effects of impaired diastolic function in subjects with and without HF, testing the hypothesis that left ventricular KE differs between controls, subjects with subclinical diastolic dysfunction (SDD), and HF patients.

METHODS

We studied 77 subjects (16 controls, 20 subjects with SDD, 16 HFpEF, 9 HFmrEF, and 16 HFrEF patients, age- and sex-matched at the group level). Cardiac magnetic resonance at 1.5T included intracardiac 4D flow and cine imaging. Left ventricular KE was calculated as 0.5*m*v².

RESULTS

Systolic KE was similar between groups (p>0.4), also after indexing to stroke volume (p=0.25), and was primarily driven by ventricular emptying rate (p<0.0001, R²=0.52). Diastolic KE was higher in heart failure patients than controls (p<0.05) but similar between SDD and HFpEF (p>0.18), correlating with inflow conditions (E-wave velocity, p<0.0001, R²=0.24) and end-diastolic volume (p=0.0003, R²=0.17) but not with average e' (p=0.07).

CONCLUSIONS

Diastolic KE differs between controls and heart failure, suggesting more work is spent filling the failing ventricle, while systolic KE does not differentiate between well-matched groups with normal ejection fraction even in the presence of relaxation abnormalities and heart failure. Mechanistically, KE reflects the acceleration imparted on the blood and is driven by variations in ventricular emptying and filling rates, volumes, and heart rate, regardless of underlying pathology.

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.

Surrogate models provide new insights on metrics based on blood flow for the assessment of left ventricular function

Recent developments on the grading of cardiac pathologies suggest flow-related metrics for a deeper evaluation of cardiac function. Blood flow evaluation employs space-time resolved cardiovascular imaging tools, possibly integrated with direct numerical simulation (DNS) of intraventricular fluid dynamics in individual patients. If a patient-specific analysis is a promising method to reproduce flow details or to assist virtual therapeutic solutions, it becomes impracticable in nearly-real-time during a routine clinical activity. At the same time, the need to determine the existence of relationships between advanced flow-related quantities of interest (QoIs) and the diagnostic metrics used in the standard clinical practice requires the adoption of techniques able to generalize evidences emerging from a finite number of single cases. In this study, we focus on the left ventricular function and use a class of reduced-order models, relying on the Polynomial Chaos Expansion (PCE) technique to learn the dynamics of selected QoIs based on a set of synthetic cases analyzed with a high-fidelity model (DNS). The selected QoIs describe the left ventricle blood transit and the kinetic energy and vorticity at the peak of diastolic filling. The PCE-based surrogate models provide straightforward approximations of these QoIs in the space of widely used diagnostic metrics embedding relevant information on left ventricle geometry and function. These surrogates are directly employable in the clinical analysis as we demonstrate by assessing their robustness against independent patient-specific cases ranging from healthy to diseased conditions. The surrogate models are used to perform global sensitivity analysis at a negligible computational cost and provide insights on the impact of each diagnostic metric on the QoIs. Results also suggest how common flow transit parameters are principally dictated by ejection fraction.

Comparison of Four-Dimensional Magnetic Resonance Imaging Analysis of Left Ventricular Fluid Dynamics and Energetics in Ischemic and Restrictive Cardiomyopathies

Background

Time‐resolved three‐directional velocity‐encoded (4D flow) magnetic resonance imaging (MRI) enables the quantification of left ventricular (LV) intracavitary fluid dynamics and energetics, providing mechanistic insight into LV dysfunctions. Before becoming a support to diagnosis and patient stratification, this analysis should prove capable of discriminating between clearly different LV derangements.

Purpose

To investigate the potential of 4D flow in identifying fluid dynamic and energetics derangements in ischemic and restrictive LV cardiomyopathies.

Study Type

Prospective observational study.

Population

Ten patients with post‐ischemic cardiomyopathy (ICM), 10 patients with cardiac light‐chain cardiac amyloidosis (AL‐CA), and 10 healthy controls were included.

Field Strength/Sequence

1.5 T/balanced steady‐state free precession cine and 4D flow sequences.

Assessment

Flow was divided into four components: direct flow (DF), retained inflow, delayed ejection flow, and residual volume (RV). Demographics, LV morphology, flow components, global and regional energetics (volume‐normalized kinetic energy [KE_V] and viscous energy loss [EL_V]), and pressure‐derived hemodynamic force (HDF) were compared between the three groups.

Statistical Tests

Intergroup differences in flow components were tested by one‐way analysis of variance (ANOVA); differences in energetic variables and peak HDF were tested by two‐way ANOVA. A P‐value of <0.05 was considered significant.

Results

ICM patients exhibited the following statistically significant alterations vs. controls: reduced KE_V, mostly in the basal region, in systole (−44%) and in diastole (−37%); altered flow components, with reduced DF (−33%) and increased RV (+26%); and reduced basal–apical HDF component on average by 63% at peak systole. AL‐CA patients exhibited the following alterations vs. controls: significantly reduced KE_V at the E‐wave peak in the basal segment (−34%); albeit nonstatistically significant, increased peaks and altered time‐course of the HDF basal–apical component in diastole and slightly reduced HDF components in systole.

Data Conclusion

The analysis of multiple 4D flow‐derived parameters highlighted fluid dynamic alterations associated with systolic and diastolic dysfunctions in ICM and AL‐CA patients, respectively.

Level of Evidence

2

Technical Efficacy Stage

3

Intraventricular Vector Flow Imaging with Blood Speckle Tracking in Adults: Feasibility, Normal Physiology and Mechanisms in Healthy Volunteers

This study examines the feasibility of blood speckle tracking for vector flow imaging in healthy adults and describes the physiologic flow pattern and vortex formation in relation to the wall motion in the left ventricle. The study included 21 healthy volunteers and quantified and visualized flow patterns with high temporal resolution down to a depth of 10-12 cm without the use of contrast agents. Intraventricular flow seems to originate during the isovolumetric relaxation with a propagation of blood from base to apex. With the E-wave, rapid inflow and vortex formation occurred on both sides of the valve basally. During diastasis the flow gathers in a large vortex before the pattern from the E-wave repeats during the A-wave. In isovolumetric contraction, the flow again gathers in a large vortex that seems to facilitate the flow out in the aorta during systole. No signs of a persistent systolic vortex were visualized. The geometry of the left ventricle and the movement of the AV-plane is important in creating vortices that are favorable for the blood flow and facilitate outflow. The quantitative measurements are in concordance with these findings, but the clinical interpretation must be evaluated in future clinical studies.

Computational fluid dynamics of blood flow in an idealized left human heart

We construct an idealized computational model of the left human heart for the study of the blood flow dynamics in the left atrium and ventricle. We solve the Navier-Stokes equations in the ALE formulation and we prescribe the left heart wall displacement based on physiological data; moreover, we consider the presence of both the mitral and aortic valves through the resistive method. We simulate the left heart hemodynamics by means of the finite element method and we consider the variational multiscale large eddy simulation (LES) formulation to account for the transitional and nearly turbulent regimes of the blood flow in physiological conditions. The main contribution of this paper is the characterization of the blood flow in an idealized configuration of the left heart aiming at reproducing function in normal conditions. Our assessment is based on the analysis of instantaneous and phase averaged velocity fields, blood pressure, and other clinically meaningful fluid dynamics indicators. Finally, we show that our idealized computational model can be suitably used to study and critically discuss pathological scenarios like that of a regurgitant mitral valve.

The kinocardiograph for assessment of changes in haemodynamic load in patients with chronic heart failure with reduced ejection fraction

Aims

The kinocardiograph (KCG) is an unobtrusive device, consisting of a chest sensor, which records local thoracic vibrations produced in result of cardiac contraction and ejection of blood into the great vessels [seismocardiography (SCG)], and a lower back sensor, which records micromovements of the body in reaction to blood flowing through the vasculature [ballistocardiography (BCG)]. SCG and BCG signals are translated to the integral of cardiac kinetic energy (iK) and cardiac maximum power (Pmax), which might be promising metrics for future telemonitoring purposes in heart failure (HF). As a first step of validation, this study aimed to determine whether iK and Pmax are responsive to exercise‐induced changes in the haemodynamic load of the heart in HF patients.

Methods and results

Fifteen patients with stable HF with reduced ejection fraction performed a submaximal exercise protocol. KCG and cardiac ultrasound measurements were obtained both at rest and at submaximal exercise. BCG iK over the cardiac cycle (CC) increased significantly (0.0026 ± 0.0017 to 0.0052 ± 0.0061 mJ.s.; P = 0.01) during exercise, in contrast to a non‐significant increase in SCG iK CC. BCG Pmax CC increased significantly (0.92 ± 0.89 to 2.03 ± 1.95 mJ/s; P = 0.02), in contrast to a non‐significant increase in SCG Pmax CC. When analysing the systolic phase of the CC, similar patterns were found. Cardiac output (CO) ratio (i.e. CO exercise/CO rest) showed a moderate, significant correlation with BCG Pmax CC ratio (r = +0.65; P = 0.008) and with SCG Pmax CC ratio (r = +0.54; P = 0.04).

Conclusions

iK and Pmax measured with the KCG, preferentially using BCG, are responsive to changes in the haemodynamic load of the heart in HF patients. The combination of the BCG and SCG sensor might be of added value to fully understand changes in haemodynamics and to discriminate between an HF patient and a healthy individual.

Cardiac Kinetic Energy and Viscous Dissipation Rate From Radial Flow Data

Recent studies have correlated kinetic energy (KE) and viscous dissipation rate (VDR) in the left ventricle (LV) with heart health. These studies have relied on 4D-flow imaging or computational fluid dynamics modeling, which are able to measure, or compute, all 3 components (3C) of the blood flow velocity in 3 dimensional (3D) space. This richness of data is difficult to acquire clinically. Alternatively, color Doppler echocardiography (CDE) is more widespread clinically, but only measures a single radial component of velocity and typically only over a planar section. Because of this limitation, prior CDE-based studies have first reconstructed a second component of velocity in the measurement plane prior to evaluating VDR or KE. Herein, we propose 1C-based surrogates of KE and VDR that can be derived directly from the radial component of the flow velocity in the LV. Our results demonstrate that the proposed 1C-based surrogates of KE and VDR are generally as well-correlated with the true KE and VDR values as surrogates that use reconstructed 2C flow data. Moreover, the correlation of these 1C-based surrogates with the true values indicate that CDE (3D in particular) may be useful in evaluating these metrics in practice.

The role of 4D flow MRI for clinical applications in cardiovascular disease: current status and future perspectives

Magnetic resonance imaging (MRI) four-dimensional (4D) flow is a type of phase-contrast (PC) MRI that uses blood flow encoded in 3 directions, which is resolved relative to 3 spatial and temporal dimensions of cardiac circulation. It can be used to simultaneously quantify and visualize hemodynamics or morphology disorders. 4D flow MRI is more comprehensive and accurate than two-dimensional (2D) PC MRI and echocardiography. 4D flow MRI provides numerous hemodynamic parameters that are not limited to the basic 2D parameters, including wall shear stress (WSS), pulse wave velocity (PWV), kinetic energy, turbulent kinetic energy (TKE), pressure gradient, and flow component analysis. 4D flow MRI is widely used to image many parts of the body, such as the neck, brain, and liver, and has a wide application spectrum to cardiac diseases and large vessels. This present review aims to summarize the hemodynamic parameters of 4D flow MRI technology and generalize their usefulness in clinical practice in relation to the cardiovascular system. In addition, we note the improvements that have been made to 4D flow MRI with the application of new technologies. The application of new technologies can improve the speed of 4D flow, which would benefit clinical applications.

Septaly Oriented Mild Aortic Regurgitant Jets Negatively Influence Left Ventricular Blood Flow-Insights From 4D Flow MRI Animal Study

Objectives: Paravalvular leakage (PVL) and eccentric aortic regurgitation remain a major clinical concern in patients receiving transcatheter aortic valve replacement (TAVR), and regurgitant volume remains the main readout parameter in clinical assessment. In this work we investigate the effect of jet origin and trajectory of mild aortic regurgitation on left ventricular hemodynamics in a porcine model.

Methods: A pig model of mild aortic regurgitation/PVL was established by transcatheter piercing and dilating the non-coronary (NCC) or right coronary cusp (RCC) of the aortic valve close to the valve annulus. The interaction between regurgitant blood and LV hemodynamics was assessed by 4D flow cardiovascular MRI.

Results: Six RCC, six NCC, and two control animals were included in the study and with one dropout in the NCC group, the success rate of model creation was 93%. Regurgitant jets originating from NCC were directed along the ventricular side of the anterior mitral leaflet and integrated well into the diastolic vortex forming in the left ventricular outflow tract. However, jets from the RCC were orientated along the septum colliding with flow within the vortex, and progressing down to the apex. As a consequence, the presence as well as the area of the vortex was reduced at the site of impact compared to the NCC group. Impairment of vortex formation was localized to the area of impact and not the entire vortex ring. Blood from the NCC jet was largely ejected during the following systole, whereas ejection of large portion of RCC blood was protracted.

Conclusions: Even for mild regurgitation, origin and trajectory of the regurgitant jet does cause a different effect on LV hemodynamics. Septaly oriented jets originating from RCC collide with the diastolic vortex, reduce its size, and reach the apical region of the left ventricle where blood resides extendedly. Hence, RCC jets display hemodynamic features which may have a potential negative impact on the long-term burden to the heart.

Altered Right Ventricular Filling at Four-dimensional Flow MRI in Young Adults Born Prematurely

Purpose

To use four-dimensional (4D) flow MRI to measure intraventricular flow in young adults who were born prematurely to investigate mechanisms that may account for increased heart failure risk in this population.

Materials and Methods

In this secondary analysis of a prospective study, a total of 56 young adults participated in an observational cardiac 4D flow MRI study from 2016 to 2020. There were 35 participants who had been born moderately to extremely prematurely (birth weight <1500 g or gestational age ≤32 weeks; 23 women; mean age, 26 years ± 4) and 21 term-born participants (11 women; mean age, 25 years ± 3). Participants underwent cardiac MRI, including cine cardiac structure and function assessment, as well as 4D flow MRI. In each ventricle, normalized kinetic energy (KE/end diastolic volume) and flow through the atrioventricular valve were computed and compared between term-born and preterm participants at systolic and diastolic (early diastolic filling rate [E wave] and late diastolic filling [atrial contraction] rate [A wave]) time points by using Wilcoxon rank-sum tests.

Results

Preterm-born participants had lower right ventricular (RV) E wave/A wave (E/A) KE ratios (2.4 ± 1.7 vs 3.5 ± 1.4; P <.01) and lower E/A peak filling rate ratios (computed from RV volume-time curves; 2.3 ± 1.3 vs 3.5 ± 2.5; P = .03). Additionally, viscous energy dissipation was increased during systole (5.7 µW/mL ± 3.0 vs 4.2 µW/mL ± 1.6; P = .03), increased during late diastole (3.9 µW/mL ± 4.0 vs 2.2 µW/mL ± 1.6; P = .03), and summed over the cardiac cycle (2.4 µJ/mL ± 1.0 vs 1.9 µJ/mL ± 0.6; P = .02) in preterm relative to term participants.

Conclusion

These results suggest that RV diastolic filling is altered in young adults who were born moderately to severely prematurely.Supplemental material is available for this article. Keywords: Adults, Cardiac, Comparative Studies, MR-Imaging, Right Ventricle © RSNA, 2021.

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.

4D flow MRI left atrial kinetic energy in hypertrophic cardiomyopathy is associated with mitral regurgitation and left ventricular outflow tract obstruction

To noninvasively assess left atrial (LA) kinetic energy (KE) in hypertrophic cardiomyopathy (HCM) patients using 4D flow MRI and evaluate coupling associations with mitral regurgitation (MR) and left ventricular outflow tract (LVOT) obstruction. Twenty-nine retrospectively identified patients with HCM underwent 4D flow MRI. MRI-estimated peak LVOT pressure gradient (∆P_MRI) was used to classify patients into non-obstructive and obstructive HCM. Time-resolved volumetric LA kinetic energy (KE_LA) was computed throughout systole. Average systolic (KE_LA-avg) and peak systolic (KE_LA-peak) KE_LA were compared between non-obstructive and obstructive HCM groups, and associations to MR severity and LVOT ∆P_MRI were tested.The study included 15 patients with non-obstructive HCM (58.6 [45.9, 65.2] years, 7 females) and 14 patients with obstructive HCM (51.9 [47.6, 62.6] years, 6 females). Obstructive HCM patients demonstrated significantly elevated instantaneous KE_LA over all systolic time-points compared to non-obstructive HCM (P < 0.05). Obstructive HCM patients also demonstrated higher KE_LA-avg (14.8 [10.6, 20.4] J/m³ vs. 33.4 [23.9, 61.3] J/m³, P < 0.001) and KE_LA-peak (22.1 [15.9, 28.7] J/m³ vs. 57.2 [44.5, 121.4] J/m³, P < 0.001) than non-obstructive HCM. MR severity was significantly correlated with KE_LA-avg (rho = 0.81, P < 0.001) and KE_LA-peak (rho = 0.79, P < 0.001). LVOT ∆P_MRI was strongly correlated with KE_LA metrics in obstructive HCM (KE_LA-avg: rho = 0.86, P < 0.001; KE_LA-peak: rho = 0.85, P < 0.001).In HCM patients, left atrial kinetic energy, by 4D flow MRI, is associated with MR severity and the degree of LVOT obstruction.

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.

Cardiac Magnetic Resonance Quantification of Structure-Function Relationships in Heart Failure

Classification of heart failure is based on the left ventricular ejection fraction (EF): preserved EF, midrange EF, and reduced EF. There remains an unmet need for further heart failure phenotyping of ventricular structure-function relationships. Because of high spatiotemporal resolution, cardiac magnetic resonance (CMR) remains the reference modality for quantification of ventricular contractile function. The authors aim to highlight novel frameworks, including theranostic use of ferumoxytol, to enable more efficient evaluation of ventricular function in heart failure patients who are also frequently anemic, and to discuss emerging quantitative CMR approaches for evaluation of ventricular structure-function relationships in heart failure.

Automated mitral valve vortex ring extraction from 4D-flow MRI

Purpose

To present and validate a method for automated extraction and analysis of the temporal evolution of the mitral valve (MV) vortex ring from MR 4D‐flow data.

Methods

The proposed algorithm uses the divergence‐free part of the velocity vector field for Q criterion‐based identification and tracking of MV vortex ring core and region within the left ventricle (LV). The 4D‐flow data of 20 subjects (10 healthy controls, 10 patients with ischemic heart disease) were used to validate the algorithm against visual analysis as well as to assess the method’s sensitivity to manual LV segmentation. Quantitative MV vortex ring parameters were analyzed with respect to both their differences between healthy subjects and patients and their correlation with transmitral peak velocities.

Results

The algorithm successfully extracted MV vortex rings throughout the entire cardiac cycle, which agreed substantially with visual analysis (Cohen’s kappa = 0.77). Furthermore, vortex cores and regions were robustly detected even if a static end‐diastolic LV segmentation mask was applied to all frames (Dice coefficients 0.82 ± 0.08 and 0.94 ± 0.02 for core and region, respectively). Early diastolic MV vortex ring vorticity, kinetic energy and circularity index differed significantly between healthy controls and patients. In contrast to vortex shape parameters, vorticity and kinetic energy correlated strongly with transmitral peak velocities.

Conclusion

An automated method for temporal MV vortex ring extraction demonstrating robustness with respect to LV segmentation strategies is introduced. Quantitative vortex parameter analysis indicates importance of the MV vortex ring for LV diastolic (dys)function.

Feasibility of Cardiovascular Four-dimensional Flow MRI during Exercise in Healthy Participants

Purpose

To explore the feasibility of using four-dimensional (4D) flow MRI to quantify blood flow and kinetic energy (KE) in the heart during strenuous exercise.

Materials and Methods

For this prospective study, cardiac 4D flow MRI was performed in 11 healthy young adult participants (eight men, three women; mean age, 26 years ± 1 [standard deviation]) at rest and during exercise with an MRI-compatible exercise stepper between March 2016 and July 2017. Flow was measured in the ascending aorta (AAo) and main pulmonary artery (MPA). KE was quantified in the left and right ventricle. Significant changes in flow and KE during exercise were identified by using t tests. Repeatability was assessed with inter- and intraobserver comparisons and an analysis of internal flow consistency.

Results

Nine participants successfully completed both rest and exercise imaging. Internal flow consistency analysis in systemic and pulmonary circulation showed average relative differences of 10% at rest and 16% during exercise. For flow measurements in the AAo and MPA, relative differences between observers never exceeded 6% in any vessel and showed excellent correlation, even during exercise. Relative differences were increased for KE, typically on the order of 30%, with poor interobserver correlation between measurements.

Conclusion

Four-dimensional flow MRI can quantify increases in flow in the AAo and MPA during strenuous exercise and is highly repeatable. KE had reduced repeatability because of suboptimal segmentation methods and requires further development before clinical implementation. Supplemental material is available for this article. © RSNA, 2020See also the commentary by Markl and Lee in this issue.

Left Ventricular Flow Analysis

BACKGROUND

Cardiac remodeling, after a myocardial insult, often causes progression to heart failure. The relationship between alterations in left ventricular blood flow, including kinetic energy (KE), and remodeling is uncertain. We hypothesized that increasing derangements in left ventricular blood flow would relate to (1) conventional cardiac remodeling markers, (2) increased levels of biochemical remodeling markers, (3) altered cardiac energetics, and (4) worsening patient symptoms and functional capacity. Methods Thirty-four dilated cardiomyopathy patients, 30 ischemic cardiomyopathy patients, and 36 controls underwent magnetic resonance including 4-dimensional flow, BNP (brain-type natriuretic peptide) measurement, functional capacity assessment (6-minute walk test), and symptom quantification. A subgroup of dilated cardiomyopathy and control subjects underwent cardiac energetic assessment. Left ventricular flow was separated into 4 components: direct flow, retained inflow, delayed ejection flow, and residual volume. Average KE throughout the cardiac cycle was calculated.

RESULTS

Patients had reduced direct flow proportion and direct-flow average KE compared with controls ( P<0.0001). The residual volume proportion and residual volume average KE were increased in patients ( P<0.0001). Importantly, in a multiple linear regression model to predict the patient's 6-minute walk test, the independent predictors were age (β=-0.3015; P=0.019) and direct-flow average KE (β=0.280, P=0.035; R² model, 0.466, P=0.002). In contrast, neither ejection fraction nor left ventricular volumes were independently predictive.

CONCLUSIONS

This study demonstrates an independent predictive relationship between the direct-flow average KE and a prognostic measure of functional capacity. Intracardiac 4-dimensional flow parameters are novel biomarkers in heart failure and may provide additive value in monitoring new therapies and predicting prognosis.

Sex Differences in Cardiac Flow Dynamics of Healthy Volunteers

Purpose

The purpose of this study was to further understand the relationship between cardiac function and flow, on the basis of sex, by quantifying cardiac flow characteristics and relating them to cardiac muscle performance in young adults.

Materials and Methods

In this cross-sectional study, cardiac four-dimensional flow (4D flow) magnetic resonance imaging (MRI) and two-dimensional cine MRI were performed on 20 male and 19 female volunteers aged 20-35. Velocity-based metrics of flow, kinetic energy, vorticity, and efficiency indices were quantified, as well as cardiac strain metrics.

Results*

Peak systolic blood kinetic energy (male: 4.76 ± 2.66 mJ; female: 3.36 ± 1.43 mJ; p=0.047) was significantly higher in the male left ventricle (LV) than in the female LV. Peak systolic vorticity index (male: 0.008 ± 0.005 rad-m²/ml-s; female: 0.014 ± 0.007 rad-m²/ml-s; p=0.007), peak diastolic vorticity index (male: 0.007 ± 0.006 rad-m²/ml-s; female: 0.014 ± 0.010 rad-m²/ml-s; p=0.015), and cycle-average vorticity (male: 0.006 ± 0.001 rad-m²/ml-s; female: 0.011 ± 0.002 rad/s; p=0.001) were all significantly higher in the LV of women than they were in the LV of men. Radial, circumferential, and long-axis strain metrics were significantly higher in the female LV than in the male LV (p<0.05). Circumferential systolic and diastolic strain rates displayed moderate correlation to peak systolic (r=-0.38, p=0.022) and diastolic vorticity (r=0.40, p=0.015) values, respectively. *Results are reported as mean ± standard deviation.

Conclusion

Left ventricular vorticity metrics were observed to be higher in women than in men and displayed moderate correlation to cardiac strain metrics. The methods and results of this study may be used to further understand the sex-based cardiac efficiency relationship between cardiac function and flow.

Reduced regional flow in the left ventricle after anterior acute myocardial infarction: a case control study using 4D flow MRI

BACKGROUND

Acute myocardial infarction (AMI) alters left ventricular (LV) hemodynamics, resulting in decreased global LV ejection fraction and global LV kinetic energy. We hypothesize that anterior AMI effects localized alterations in LV flow and developed a regional approach to analyze these local changes with 4D flow MRI.

METHODS

4D flow cardiac magnetic resonance (CMR) data was compared between 12 anterior AMI patients (11 males; 66 ± 12yo; prospectively acquired in 2016-2017) and 19 healthy volunteers (10 males; 40 ± 16yo; retrospective from 2010 to 2011 study). The LV cavity was contoured on short axis cine steady-state free procession CMR and partitioned into three regions: base, mid-ventricle, and apex. 4D flow data was registered to the short axis segmentation. Peak systolic and diastolic through-plane flows were compared region-by-region between groups using linear models of flow with age, sex, and heart rate as covariates.

RESULTS

Peak systolic flow was reduced in anterior AMI subjects compared to controls in the LV mid-ventricle (fitted reduction = 3.9 L/min; P = 0.01) and apex (fitted reduction = 1.4 L/min; P = 0.02). Peak diastolic flow was also lower in anterior AMI subjects compared to controls in the apex (fitted reduction = 2.4 L/min; P = 0.01).

CONCLUSIONS

A regional method to analyze 4D LV flow data was applied in anterior AMI patients and controls. Anterior AMI patients had reduced regional flow relative to controls.

Ventricular vortex loss analysis due to various tricuspid valve repair techniques: an ex vivo study

The deteriorating nature of severe functional tricuspid regurgitation (FTR) has led to the heightened interest in this pathology. However, therapies are heterogeneous and an ideal technique is uncertain. The hemodynamic impact on the cardiac chamber following therapeutic repairs has not been well studied, while its analysis could be used to predict the treatment success. In this study, the hemodynamics of the right ventricle (RV) after 1) clover edge-to-edge tricuspid repair, and 2) double orifice tricuspid repair was evaluated in three right heart models using an ex vivo pulsatile platform emulating severe FTR with the aid of stereoscopic particle image velocimetry. Although all repairs substantially reduced tricuspid regurgitant area, they resulted in a more than 50% reduction in diastolic tricuspid valve (TV) opening area. Splitting the TV orifice into multiple smaller orifices by both repairs eliminated the ring-shaped vortical structure inside the RV observed in FTR cases. Postrepair RV domain was mostly occupied with irregular vortical features and isolated vortex residuals. Moreover, vortical features varied among repair samples, indicating enhanced sensitivity of RV flow to postrepair TV morphology. Compared with clover repair, double orifice subjected the RV to enhanced swirling motions and exposed more regions to vortical motions, potentially indicating better rinsing and lower risk of mural thrombus formation. Double orifice repair increased the levels of RV mean kinetic energy and viscous energy loss than those observed in clover repair, although the impact of these on the cardiac efficiency remains unclear. These preliminary insights could be used to improve future treatment design and planning.NEW & NOTEWORTHY While clover and double orifice tricuspid repairs markedly improved leaflet coaptation, they substantially reduced diastolic tricuspid opening area. Postrepair right ventricle (RV) exhibited specific hemodynamic traits, including the loss of ring-shaped vortical structure and the enhanced sensitivity of RV flow to postrepair tricuspid valve morphology. Compared with clover technique, double orifice repair led to higher swirling motions in the RV domain, which could indicate lower risk of mural thrombus formation.

Ex vivo assessment of bicuspidization repair in treating severe functional tricuspid regurgitation: a stereo-scopic PIV study

There has been a resurgence of interest in the treatment of severe functional tricuspid regurgitation (FTR) due to the awareness of its poor outcomes and potential percutaneous therapies. Kay bicuspidization has been adapted in percutaneous therapies but its clinical outcome remains uncertain. The present study evaluates the efficacy of Kay repair in a novel ex vivo pulsatile system. Porcine tricuspid valve (TV) (n = 3) was extracted and incorporated into a patient-specific silicon right ventricle (RV) emulating severe FTR, on which Kay repair was subsequently performed. TV area metrics and RV hemodynamic assessment by means of stereo-scopic particle image velocimetry were quantified in both FTR and post-repair conditions. Bicuspidization led to significant increase in cardiac output although the overall increment due to this approach alone was generally small, possibly due to existence of residual TR and the large reduction in TV opening area. Kinetic energy and viscous loss levels were increased post-repair, especially during diastolic filling. Main vortex structures generally maintained post-procedural. However, there was enhanced swirling motion in larger RV domain. Although this might reduce mural-thrombus risk, the relatively more complex vortex phenomenon likely resulted in elevated viscous loss observed and may potentially impact long-term adaptation. The RV hemodynamic alteration after tricuspid repair could be used to predict the success of these future transcatheter solutions.

Intraventricular flow patterns during right ventricular apical pacing

Objectives

To assess differences in blood flow momentum (BFM) and kinetic energy (KE) dissipation in a model of cardiac dyssynchrony induced by electrical right ventricular apical (RVA) stimulation compared with spontaneous sinus rhythm.

Methods

We cross-sectionally enrolled 12 consecutive patients (mean age 74±8 years, 60% male, mean left ventricular ejection fraction 58%±6 %), within 48 hours from pacemaker (PMK) implantation. Inclusion criteria were: age>18 years, no PMK-dependency, sinus rhythm with a spontaneous narrow QRS at the ECG, preserved ejection fraction (>50%) and a low percentage of PMK-stimulation (<20%). All the participants underwent a complete echocardiographic evaluation, including left ventricular strain analysis and particle image velocimetry.

Results

Compared with sinus rhythm, BFM shifted from 27±3.3 to 34±7.6° (p=0.016), while RVA-pacing was characterised by a 35% of increment in KE dissipation, during diastole (p=0.043) and 32% during systole (p=0.016). In the same conditions, left ventricle global longitudinal strain (LV GLS) significantly decreased from 17±3.3 to 11%±2.8% (p=0.004) during RVA-stimulation. At the multivariable analysis, BFM and diastolic KE dissipation were significantly associated with LV GLS deterioration (Beta Coeff.=0.54, 95% CI 0.07 to 1.00, p=0.034 and Beta Coeff.=0.29, 95% CI 0.02 to 0.57, p=0.049, respectively).

Conclusions

In RVA-stimulation, BFM impairment and KE dissipation were found to be significantly associated with LV GLS deterioration, when controlling for potential confounders. Such changes may favour the onset of cardiac remodelling and sustain heart failure.

Validation and reproducibility of cardiovascular 4D-flow MRI from two vendors using 2 × 2 parallel imaging acceleration in pulsatile flow phantom and in vivo with and without respiratory gating

Background

4D-flow magnetic resonance imaging (MRI) is increasingly used.

Purpose

To validate 4D-flow sequences in phantom and in vivo, comparing volume flow and kinetic energy (KE) head-to-head, with and without respiratory gating.

Material and Methods

Achieva dStream (Philips Healthcare) and MAGNETOM Aera (Siemens Healthcare) 1.5-T scanners were used. Phantom validation measured pulsatile, three-dimensional flow with 4D-flow MRI and laser particle imaging velocimetry (PIV) as reference standard. Ten healthy participants underwent three cardiac MRI examinations each, consisting of cine-imaging, 2D-flow (aorta, pulmonary artery), and 2 × 2 accelerated 4D-flow with (Resp+) and without (Resp−) respiratory gating. Examinations were acquired consecutively on both scanners and one examination repeated within two weeks. Volume flow in the great vessels was compared between 2D- and 4D-flow. KE were calculated for all time phases and voxels in the left ventricle.

Results

Phantom results showed high accuracy and precision for both scanners. In vivo, higher accuracy and precision (P < 0.001) was found for volume flow for the Aera prototype with Resp+ (–3.7 ± 10.4 mL, r = 0.89) compared to the Achieva product sequence (–17.8 ± 18.6 mL, r = 0.56). 4D-flow Resp− on Aera had somewhat larger bias (–9.3 ± 9.6 mL, r = 0.90) compared to Resp+ (P = 0.005). KE measurements showed larger differences between scanners on the same day compared to the same scanner at different days.

Conclusion

Sequence-specific in vivo validation of 4D-flow is needed before clinical use. 4D-flow with the Aera prototype sequence with a clinically acceptable acquisition time (<10 min) showed acceptable bias in healthy controls to be considered for clinical use. Intra-individual KE comparisons should use the same sequence.

Analysis of cavopulmonary and cardiac flow characteristics in fontan Patients: Comparison with healthy volunteers

BACKGROUND

Characterizing the flow of the Fontan circuit, and correlating flow characteristics with the development of complications, is an important clinical challenge. Past work has analyzed the flow characteristics of Fontan circulation on a component-by-component basis. 4D flow MRI with radial projections allows for large volumetric coverage, and therefore can be used to analyze the flow through many codependent cardiovascular components in a single imaging session.

PURPOSE

To describe flow characteristics across the entire Fontan circuit and to compare these with the flow characteristics in healthy volunteers.

STUDY TYPE

Prospective.

SUBJECTS

Eleven single ventricle patients with a Fontan connection and 15 healthy controls.

SEQUENCE

Phase contrast with vastly undersampled isotropic projection reconstruction (PC-VIPR) at a field strength of 3 T.

ASSESSMENT

Cavopulmonary and ventricular flow distributions, blood flow kinetic energy, vorticities, efficiency indices, and other flow parameters were analyzed using Ensight and MatLab.

STATISTICAL TESTS

The results were compared across Fontan subjects, between respiratory phases, and between Fontan subjects and healthy volunteers using a Student's t-test for unequal sample sizes and linear regression.

RESULTS

Cava-specific pulmonary flow distributions of Fontan patients varied significantly between respiratory phases (P < 0.05). Ventricular kinetic energy (KE) was significantly higher in Fontan patients than it was in healthy controls, leading to a lower cardiac efficiency metric in the Fontan group. A significant diastolic KE time-shift was also observed in the Fontan patient group. Peak diastolic KE was significantly higher in the single ventricle of patients with right ventricle morphology than it was in left ventricle morphology patients.

DATA CONCLUSION

Radial 4D flow MRI can be used for comprehensive analysis of single ventricle Fontan flow characteristics.

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019.

Left ventricular thrombus formation in myocardial infarction is associated with altered left ventricular blood flow energetics

Aims

The main aim of this study was to characterize changes in the left ventricular (LV) blood flow kinetic energy (KE) using four-dimensional (4D) flow cardiovascular magnetic resonance imaging (CMR) in patients with myocardial infarction (MI) with/without LV thrombus (LVT).

Methods and results

This is a prospective cohort study of 108 subjects [controls = 40, MI patients without LVT (LVT- = 36), and MI patients with LVT (LVT+ = 32)]. All underwent CMR including whole-heart 4D flow. LV blood flow KE wall calculated using the formula: KE=12 ρblood . Vvoxel . v2, where ρ = density, V = volume, v = velocity, and was indexed to LV end-diastolic volume. Patient with MI had significantly lower LV KE components than controls (P < 0.05). LVT+ and LVT- patients had comparable infarct size and apical regional wall motion score (P > 0.05). The relative drop in A-wave KE from mid-ventricle to apex and the proportion of in-plane KE were higher in patients with LVT+ compared with LVT- (87 ± 9% vs. 78 ± 14%, P = 0.02; 40 ± 5% vs. 36 ± 7%, P = 0.04, respectively). The time difference of peak E-wave KE demonstrated a significant rise between the two groups (LVT-: 38 ± 38 ms vs. LVT+: 62 ± 56 ms, P = 0.04). In logistic-regression, the relative drop in A-wave KE (beta = 11.5, P = 0.002) demonstrated the strongest association with LVT.

Conclusion

Patients with MI have reduced global LV flow KE. Additionally, MI patients with LVT have significantly reduced and delayed wash-in of the LV. The relative drop of distal intra-ventricular A-wave KE, which represents the distal late-diastolic wash-in of the LV, is most strongly associated with the presence of LVT.

Heart filling exceeds emptying during late ventricular systole in patients with systolic heart failure and healthy subjects - a cardiac MRI study

Background

Total heart volume (THV) within the pericardium is not constant throughout the cardiac cycle and THV would intuitively be lowest at end systole. We have, however, observed a phase shift between ventricular outflow and atrial inflow which causes the minimum THV to occur before end systole. The aims were to explain the mechanism of the late‐systolic net inflow to the heart and determine whether this net inflow is affected by increased cardiac output or systolic heart failure.

Methods and Results

Healthy controls (n = 21) and patients with EF<35% (n = 14) underwent magnetic resonance imaging with flow measurements in vessels to and from the heart, and this was repeated in nine controls during 140 μgram kg⁻¹ min⁻¹ adenosine infusion. Minimum THV occurred 78 ± 6 ms before end of systolic ejection (8 ± 1% of the cardiac cycle) in controls. The late‐systolic net inflow was 12·3 ± 1·1 ml or 6·0 ± 0·5% of total stroke volume (TSV). Cardiac output increased 66 ± 8% during adenosine but late‐systolic net inflow to the heart did not change (P = 0·73). In patients with heart failure, late‐systolic net inflow of the heart′s left side was lower (3·4 ± 0·5%) compared to healthy subjects (5·3 ± 0·6%, P = 0·03).

Conclusions

Heart size increases before end systole due to a late‐systolic net inflow which is unaffected by increased cardiac output. This may be explained by inertia of blood that flows into the atria generated by ventricular systole. The lower late‐systolic net inflow in patients with systolic heart failure may be a measure of decreased ventricular filling due to decreased systolic function, thus linking systolic to diastolic dysfunction.

Hemodynamic forces using four-dimensional flow MRI: an independent biomarker of cardiac function in heart failure with left ventricular dyssynchrony?

Patients with heart failure with left ventricular (LV) dyssynchrony often do not respond to cardiac resynchronization therapy (CRT), indicating that the pathophysiology is insufficiently understood. Intracardiac hemodynamic forces computed from four-dimensional (4-D) flow MRI have been proposed as a new measure of cardiac function. We therefore aimed to investigate how hemodynamic forces are altered in LV dyssynchrony. Thirty-one patients with heart failure and LV dyssynchrony and 39 control subjects underwent cardiac MRI with the acquisition of 4-D flow. Hemodynamic forces were computed using Navier-Stokes equations and integrated over the manually delineated LV volume. The ratio between transverse (lateral-septal and inferior-anterior) and longitudinal (apical-basal) forces was calculated for systole and diastole separately and compared with QRS duration, aortic valve opening delay, global longitudinal strain, and ejection fraction (EF). Patients exhibited hemodynamic force patterns that were significantly altered compared with control subjects, including loss of longitudinal forces in diastole (force ratio, control subjects vs. patients: 0.32 vs. 0.90, P < 0.0001) and increased transverse force magnitudes. The systolic force ratio was correlated with global longitudinal strain and EF ( P < 0.01). The diastolic force ratio separated patients from control subjects (area under the curve: 0.98, P < 0.0001) but was not correlated to other dyssynchrony measures ( P > 0.05 for all). Hemodynamic forces by 4-D flow represent a new approach to the quantification of LV dyssynchrony. Diastolic force patterns separate healthy from diseased ventricles. Different force patterns in patients indicate the possible use of force analysis for risk stratification and CRT implantation guidance.

NEW & NOTEWORTHY In this report, we demonstrate that patients with heart failure with left ventricular dyssynchrony exhibit significantly altered hemodynamic forces compared with normal. Force patterns in patients mechanistically reflect left ventricular dysfunction on the organ level, largely independent of traditional dyssynchrony measures. Force analysis may help clinical decision making and could potentially be used to improve therapy outcomes.

Impact of Pulmonary Venous Inflow on Cardiac Flow Simulations: Comparison with In Vivo 4D Flow MRI

Blood flow simulations are making their way into the clinic, and much attention is given to estimation of fractional flow reserve in coronary arteries. Intracardiac blood flow simulations also show promising results, and here the flow field is expected to depend on the pulmonary venous (PV) flow rates. In the absence of in vivo measurements, the distribution of the flow from the individual PVs is often unknown and typically assumed. Here, we performed intracardiac blood flow simulations based on time-resolved computed tomography on three patients, and investigated the effect of the distribution of PV flow rate on the flow field in the left atrium and ventricle. A design-of-experiment approach was used, where PV flow rates were varied in a systematic manner. In total 20 different simulations were performed per patient, and compared to in vivo 4D flow MRI measurements. Results were quantified by kinetic energy, mitral valve velocity profiles and root-mean-square errors of velocity. While large differences in atrial flow were found for varying PV inflow distributions, the effect on ventricular flow was negligible, due to a regularizing effect by mitral valve. Equal flow rate through all PVs most closely resembled in vivo measurements and is recommended in the absence of a priori knowledge.

Intraventricular Vortex Interaction between Transmitral Flow and Paravalvular Leak

Paravalvular leak (PVL) is a complication of transcatheter aortic valve replacement. Despite its marked clinical impact, no previous study has reported how PVL affects the intraventricular fluid dynamics. This study aims to delineate vortex interaction between PVL and transmitral flow and the influence of PVL orifice location on intraventricular fluid dynamics using Echocardiographic Particle Image Velocimetry. Three different conditions of no PVL, anterior PVL and posterior PVL were experimentally studied and clinically compared. Circulation, impulse, kinetic energy (KE) and change in KE (ΔKE) were calculated. As well, vortex formation analyses and streamline description were performed to study vortex interactions. The anterior PVL jet streamed into the LV and interfered with the transmitral flow. Posterior PVL jet formed a large clockwise vortex and collided with transmitral flow, which resulted in flow disturbance. Compared to no PVL condition, average circulation, impulse, KE and ΔKE increased in presence of PVL. In conclusion, we found that PVL jets lead to abnormal vortex formation that interfere with natural advancement of transmitral flow, and negatively affect the LV fluid dynamics parameters. PVL orifice location strongly affects the intraventricular vortex formation, and posterior PVL may have more negative effects compared to anterior PVL.

Impact of Age and Diastolic Function on Novel, 4D flow CMR Biomarkers of Left Ventricular Blood Flow Kinetic Energy

Two-dimensional (2D) methods of assessing mitral inflow velocities are pre-load dependent, limiting their reliability for evaluating diastolic function. Left ventricular (LV) blood flow kinetic energy (KE) derived from four-dimensional flow cardiovascular magnetic resonance imaging (4D flow CMR) may offer improvements. It remains unclear whether 4D LV blood flow KE parameters are associated with physiological factors, such as age when compared to 2D mitral inflow velocities. Fifty-three healthy volunteers underwent standard CMR, plus 4D flow acquisition. LV blood flow KE parameters demonstrated good reproducibility with mean coefficient of variation of 6 ± 2% and an accuracy of 99% with a precision of 97%. The LV blood flow KEi_EDV E/A ratio demonstrated good association to the 2D mitral inflow E/A ratio (r = 0.77, P < 0.01), with both decreasing progressively with advancing age (P < 0.01). Furthermore, peak E-wave KEi_EDV and A-wave KEi_EDV displayed a stronger association to age than the corresponding 2D metrics, peak E-wave and A-wave velocity (r = −0.51 vs −0.17 and r = 0.65 vs 0.46). Peak E-wave KEi_EDV decreases whilst peak A-wave KEi_EDV increases with advancing age. This study presents values for various LV blood flow KE parameters in health, as well as demonstrating that they show stronger and independent correlations to age than standard diastolic metrics.