The Impact of Function-Flow Interaction on Left Ventricular Efficiency in Patients with Conduction Abnormalities: A Particle Image Velocimetry and Tissue Doppler Study

Characterization of Vortex Flow in a Mouse Model of Ventricular Dyssynchrony by Plane-Wave Ultrasound Using Hexplex Processing

The rodent heart is frequently used to study human cardiovascular disease (CVD). Although advanced cardiovascular ultrasound imaging methods are available for human clinical practice, application of these techniques to small animals remains limited due to the temporal and spatial-resolution demands. Here, an ultrasound vector-flow workflow is demonstrated that enables visualization and quantification of the complex hemodynamics within the mouse heart. Wild type (WT) and fibroblast growth factor homologous factor 2 (FHF2)-deficient mice (Fhf2 ^KO/Y ), which present with hyperthermia-induced ECG abnormalities highly reminiscent of Brugada syndrome, were used as a mouse model of human CVD. An 18-MHz linear array was used to acquire high-speed (30 kHz), plane-wave data of the left ventricle (LV) while increasing core body temperature up to 41.5 °C. Hexplex (i.e., six output) processing of the raw data sets produced the output of vector-flow estimates (magnitude and phase); B-mode and color-Doppler images; Doppler spectrograms; and local time histories of vorticity and pericardium motion. Fhf2 ^WT/Y mice had repeatable beat-to-beat cardiac function, including vortex formation during diastole, at all temperatures. In contrast, Fhf2 ^KO/Y mice displayed dyssynchronous contractile motion that disrupted normal inflow vortex formation and impaired LV filling as temperature rose. The hexplex processing approach demonstrates the ability to visualize and quantify the interplay between hemodynamic and mechanical function in a mouse model of human CVD.

Noninvasive assessment of intraventricular pressure difference in left ventricular dyssynchrony using vector flow mapping

Diastolic intraventricular pressure difference (IVPD) reflects left ventricular (LV) diastolic function. The relative pressure imaging (RPI) enables the noninvasive quantification of IVPD based on vector flow mapping (VFM) and visualization of regional pressure distribution. LV dyssynchrony causes deterioration of cardiac performance. However, it remains unclear how IVPD is modulated by LV dyssynchrony. LV dyssynchrony was created in ten open-chest dogs by right ventricular (RV) pacing. The other ten dogs undergoing right atrial (RA) pacing set at the similar heart rate with RV pacing were used as controls. Echocardiographic images were acquired at baseline and during pacing simultaneously with LV pressure measurement by a micromanometer. Pressure difference (ΔP) was computed between the apex and the base of the LV inflow tract during a cardiac cycle by RPI and ΔP during isovolumic relaxation time (ΔP_IRT), a parameter of diastolic suction, and that during early filling phase (ΔP_E) were measured. During RV pacing, stroke volume (SV) and ΔP_IRT decreased significantly, while ΔP_E did not change compared to the baseline. During RA pacing, SV, ΔP_IRT and ΔP_E did not change significantly. ΔP_IRT tended to correlate with -dP/dt_min and end-systolic volume, and significantly correlated with ejection fraction. IVPD during isovolumic relaxation time was decreased by LV dyssynchrony, while IVPD during early filling phase was not. A reduction of diastolic suction is observed in LV dyssynchrony and is significantly related to a decrease in SV.

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.

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.

Mechanical dyssynchrony alters left ventricular flow energetics in failing hearts with LBBB: a 4D flow CMR pilot study

The impact of left bundle branch block (LBBB) related mechanical dyssynchrony on left ventricular (LV) diastolic function remains unclear. 4D flow cardiovascular magnetic resonance (CMR) has provided reliable markers of LV dysfunction: reduced volume and kinetic energy (KE) of the portion of LV inflow which passes directly to outflow (Direct Flow) has been demonstrated in failing hearts compared to normal hearts. We sought to investigate the impact of mechanical dyssynchrony on diastolic function by comparing 4D flow in myopathic LVs with and without LBBB. CMR data were acquired at 3 T in 22 heart failure patients; 11 with LBBB and 11 without LBBB matched according to several demographic and clinical parameters. An established 4D flow analysis method was used to separate the LV end-diastolic (ED) volume into functional flow components based on the blood’s timing and route through the heart cavities. While the Direct Flow volume was not different between the groups, the KE possessed at ED was lower in LBBB patients (P = 0.018). Direct Flow entering the LV during early diastolic filling possessed less KE at ED in LBBB patients compared to non-LBBB patients, whereas no intergroup difference was observed during late filling. Pre-systolic KE of LV Direct Flow was reduced in patients with LBBB compared to matched patients with normal conduction. These intriguing findings propose that 4D flow specific measures can serve as markers of LV mechanical dyssynchrony in heart failure patients, and could possibly be investigated as predictors of response to cardiac resynchronization therapy.

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.