Johannes Töger

Quantification of left and right ventricular kinetic energy using four-dimensional intracardiac magnetic resonance imaging flow measurements

We aimed to quantify kinetic energy (KE) during the entire cardiac cycle of the left ventricle (LV) and right ventricle (RV) using four-dimensional phase-contrast magnetic resonance imaging (MRI). KE was quantified in healthy volunteers (n = 9) using an in-house developed software. Mean KE through the cardiac cycle of the LV and the RV were highly correlated (r(2) = 0.96). Mean KE was related to end-diastolic volume (r(2) = 0.66 for LV and r(2) = 0.74 for RV), end-systolic volume (r(2) = 0.59 and 0.68), and stroke volume (r(2) = 0.55 and 0.60), but not to ejection fraction (r(2) < 0.01, P = not significant for both). Three KE peaks were found in both ventricles, in systole, early diastole, and late diastole. In systole, peak KE in the LV was lower (4.9 ± 0.4 mJ, P = 0.004) compared with the RV (7.5 ± 0.8 mJ). In contrast, KE during early diastole was higher in the LV (6.0 ± 0.6 mJ, P = 0.004) compared with the RV (3.6 ± 0.4 mJ). The late diastolic peaks were smaller than the systolic and early diastolic peaks (1.3 ± 0.2 and 1.2 ± 0.2 mJ). Modeling estimated the proportion of KE to total external work, which comprised ∼0.3% of LV external work and 3% of RV energy at rest and 3 vs. 24% during peak exercise. The higher early diastolic KE in the LV indicates that LV filling is more dependent on ventricular suction compared with the RV. RV early diastolic filling, on the other hand, may be caused to a higher degree of the return of the atrioventricular plane toward the base of the heart. The difference in ventricular geometry with a longer outflow tract in the RV compared with the LV explains the higher systolic KE in the RV.

Quantification and visualization of cardiovascular 4D velocity mapping accelerated with parallel imaging or k-t BLAST: head to head comparison and validation at 1.5 T and 3 T.

BackgroundThree-dimensional time-resolved (4D) phase-contrast (PC) CMR can visualize and quantify cardiovascular flow but is hampered by long acquisition times. Acceleration with SENSE or k-t BLAST are two possibilities but results on validation are lacking, especially at 3 T. The aim of this study was therefore to validate quantitative in vivo cardiac 4D-acquisitions accelerated with parallel imaging and k-t BLAST at 1.5 T and 3 T with 2D-flow as the reference and to investigate if field strengths and type of acceleration have major effects on intracardiac flow visualization.MethodsThe local ethical committee approved the study. 13 healthy volunteers were scanned at both 1.5 T and 3 T in random order with 2D-flow of the aorta and main pulmonary artery and two 4D-flow sequences of the heart accelerated with SENSE and k-t BLAST respectively. 2D-image planes were reconstructed at the aortic and pulmonary outflow. Flow curves were calculated and peak flows and stroke volumes (SV) compared to the results from 2D-flow acquisitions. Intra-cardiac flow was visualized using particle tracing and image quality based on the flow patterns of the particles was graded using a four-point scale.ResultsGood accuracy of SV quantification was found using 3 T 4D-SENSE (r2 = 0.86, -0.7 ± 7.6%) and although a larger bias was found on 1.5 T (r2 = 0.71, -3.6 ± 14.8%), the difference was not significant (p = 0.46). Accuracy of 4D k-t BLAST for SV was lower (p < 0.01) on 1.5 T (r2 = 0.65, -15.6 ± 13.7%) compared to 3 T (r2 = 0.64, -4.6 ± 10.0%). Peak flow was lower with 4D-SENSE at both 3 T and 1.5 T compared to 2D-flow (p < 0.01) and even lower with 4D k-t BLAST at both scanners (p < 0.01). Intracardiac flow visualization did not differ between 1.5 T and 3 T (p = 0.09) or between 4D-SENSE or 4D k-t BLAST (p = 0.85).ConclusionsThe present study showed that quantitative 4D flow accelerated with SENSE has good accuracy at 3 T and compares favourably to 1.5 T. 4D flow accelerated with k-t BLAST underestimate flow velocities and thereby yield too high bias for intra-cardiac quantitative in vivo use at the present time. For intra-cardiac 4D-flow visualization, however, 1.5 T and 3 T as well as SENSE or k-t BLAST can be used with similar quality.

Volume Tracking: A new method for quantitative assessment and visualization of intracardiac blood flow from three-dimensional, time-resolved, three-component magnetic resonance velocity mapping

BackgroundFunctional and morphological changes of the heart influence blood flow patterns. Therefore, flow patterns may carry diagnostic and prognostic information. Three-dimensional, time-resolved, three-directional phase contrast cardiovascular magnetic resonance (4D PC-CMR) can image flow patterns with unique detail, and using new flow visualization methods may lead to new insights. The aim of this study is to present and validate a novel visualization method with a quantitative potential for blood flow from 4D PC-CMR, called Volume Tracking, and investigate if Volume Tracking complements particle tracing, the most common visualization method used today.MethodsEight healthy volunteers and one patient with a large apical left ventricular aneurysm underwent 4D PC-CMR flow imaging of the whole heart. Volume Tracking and particle tracing visualizations were compared visually side-by-side in a visualization software package. To validate Volume Tracking, the number of particle traces that agreed with the Volume Tracking visualizations was counted and expressed as a percentage of total released particles in mid-diastole and end-diastole respectively. Two independent observers described blood flow patterns in the left ventricle using Volume Tracking visualizations.ResultsVolume Tracking was feasible in all eight healthy volunteers and in the patient. Visually, Volume Tracking and particle tracing are complementary methods, showing different aspects of the flow. When validated against particle tracing, on average 90.5% and 87.8% of the particles agreed with the Volume Tracking surface in mid-diastole and end-diastole respectively. Inflow patterns in the left ventricle varied between the subjects, with excellent agreement between observers. The left ventricular inflow pattern in the patient differed from the healthy subjects.ConclusionVolume Tracking is a new visualization method for blood flow measured by 4D PC-CMR. Volume Tracking complements and provides incremental information compared to particle tracing that may lead to a better understanding of blood flow and may improve diagnosis and prognosis of cardiovascular diseases.

Independent validation of four-dimensional flow MR velocities and vortex ring volume using particle imaging velocimetry and planar laser-Induced fluorescence

This study aimed to: (i) present and characterize a phantom setup for validation of four‐dimensional (4D) flow using particle imaging velocimetry (PIV) and planar laser‐induced fluorescence (PLIF); (ii) validate 4D flow velocity measurements using PIV; and (iii) validate 4D flow vortex ring volume (VV) using PLIF.

Vortex ring behavior provides the epigenetic blueprint for the human heart.

The laws of fluid dynamics govern vortex ring formation and precede cardiac development by billions of years, suggesting that diastolic vortex ring formation is instrumental in defining the shape of the heart. Using novel and validated magnetic resonance imaging measurements, we show that the healthy left ventricle moves in tandem with the expanding vortex ring, indicating that cardiac form and function is epigenetically optimized to accommodate vortex ring formation for volume pumping. Healthy hearts demonstrate a strong coupling between vortex and cardiac volumes (R2 = 0.83), but this optimized phenotype is lost in heart failure, suggesting restoration of normal vortex ring dynamics as a new, and possibly important consideration for individualized heart failure treatment. Vortex ring volume was unrelated to early rapid filling (E-wave) velocity in patients and controls. Characteristics of vortex-wall interaction provide unique physiologic and mechanistic information about cardiac diastolic function that may be applied to guide the design and implantation of prosthetic valves, and have potential clinical utility as therapeutic targets for tailored medicine or measures of cardiac health.

Accuracy of four-dimensional phase-contrast velocity mapping for blood flow visualizations: a phantom study

Background Time-resolved three-dimensional, three-directional phase-contrast magnetic resonance velocity mapping (4D PC-MRI) is a powerful technique to depict dynamic blood flow patterns in the human body. However, the impact of phase background effects on flow visualizations has not been thoroughly studied previously, and it has not yet been experimentally demonstrated to what degree phase offsets affect flow visualizations and create errors such as inaccurate particle traces. Purpose To quantify background phase offsets and their subsequent impact on particle trace visualizations in a 4D PC-MRI sequence. Additionally, we sought to investigate to what degree visualization errors are reduced by background phase correction. Material and Methods A rotating phantom with a known velocity field was used to quantify background phase of 4D PC-MRI sequences accelerated with SENSE as well as different k-t BLAST speed-up factors. The deviation in end positions between particle traces in the measured velocity fields were compared before and after the application of two different phase correction methods. Results Phantom measurements revealed background velocity offsets up to 7 cm/s (7% of velocity encoding sensitivity) in the central slice, increasing with distance from the center. Background offsets remained constant with increasing k-t BLAST speed-up factors. End deviations of up to 5.3 mm (1.8 voxels) in the direction perpendicular to the rotating disc were found between particle traces and the seeding plane of the traces. Phase correction by subtraction of the data from the stationary phantom reduced the average deviation by up to 56%, while correcting the data-set with a first-order polynomial fit to stationary regions decreased average deviation up to 78%. Conclusion Pathline visualizations can be significantly affected by background phase errors, highlighting the importance of dedicated and robust phase correction methods. Our results show that pathline deviation can be substantial if adequate phase background errors are not minimized.

Vortex-ring mixing as a measure of diastolic function of the human heart: Phantom validation and initial observations in healthy volunteers and patients with heart failure.

PURPOSE To present and validate a new method for 4D flow quantification of vortex-ring mixing during early, rapid filling of the left ventricle (LV) as a potential index of diastolic dysfunction and heart failure. MATERIALS AND METHODS 4D flow mixing measurements were validated using planar laser-induced fluorescence (PLIF) in a phantom setup. Controls (n = 23) and heart failure patients (n = 23) were studied using 4D flow at 1.5T (26 subjects) or 3T (20 subjects) to determine vortex volume (VV) and inflowing volume (VVinflow ). The volume mixed into the vortex-ring was quantified as VVmix-in = VV-VVinflow . The mixing ratio was defined as MXR = VVmix-in /VV. Furthermore, we quantified the fraction of the end-systolic volume (ESV) mixed into the vortex-ring (VVmix-in /ESV) and the fraction of the LV volume at diastasis (DV) occupied by the vortex-ring (VV/DV). RESULTS PLIF validation of MXR showed fair agreement (R(2) = 0.45, mean ± SD 1 ± 6%). MXR was higher in patients compared to controls (28 ± 11% vs. 16 ± 10%, P < 0.001), while VVmix-in /ESV and VV/DV were lower in patients (10 ± 6% vs. 18 ± 12%, P < 0.01 and 25 ± 8% vs. 50 ± 6%, P < 0.0001). CONCLUSION Vortex-ring mixing can be quantified using 4D flow. The differences in mixing parameters observed between controls and patients motivate further investigation as indices of diastolic dysfunction. J. Magn. Reson. Imaging 2016;43:1386-1397.

On estimating intraventricular hemodynamic forces from endocardial dynamics: A comparative study with 4D flow MRI

Intraventricular pressure gradients or hemodynamic forces, which are their global measure integrated over the left ventricular volume, have a fundamental importance in ventricular function. They may help revealing a sub-optimal cardiac function that is not evident in terms of tissue motion, which is naturally heterogeneous and variable, and can influence cardiac adaptation. However, hemodynamic forces are not utilized in clinical cardiology due to the unavailability of simple non-invasive measurement tools. Hemodynamic forces depend on the intraventricular flow; nevertheless, most of them are imputable to the dynamics of the endocardial flow boundary and to the exchange of momentum across the mitral and aortic orifices. In this study, we introduce a simplified model based on first principles of fluid dynamics that allows estimating hemodynamic forces without knowing the velocity field inside the LV. The model is validated with 3D phase-contrast MRI (known as 4D flow MRI) in 15 subjects, (5 healthy and 10 patients) using the endocardial surface reconstructed from the three standard long-axis projections. Results demonstrate that the model provides consistent estimates for the base-apex component (mean correlation coefficient r=0.77 for instantaneous values and r=0.88 for root mean square) and good estimates of the inferolateral-anteroseptal component (r=0.50 and 0.84, respectively). The present method represents a potential integration to the existing ones quantifying endocardial deformation in MRI and echocardiography to add a physics-based estimation of the corresponding hemodynamic forces. These could help the clinician to early detect sub-clinical diseases and differentiate between different cardiac dysfunctional states.

Vortex ring mixing in the left ventricle of the human heart.

Background During rapid filling of the left ventricle, a vortex ring forms downstream from the mitral valve. Previous experiments in water tanks have shown that vortex ring formation is an optimized method for fluid transport. The rotation of the vortex ring leads to mixing of the inflowing blood and blood that was already in the ventricle (Figure 1). In water tanks, the amount of mixing decreases with increasing vortex formation ratio (VFR), a dimensionless parameter relating inflowing volume to the annulus diameter (Figure 2). However, the flow and anatomy of the left ventricle is more complex which may affect this established relationship. Therefore, we aimed to investigate if the relationship between VFR and mixing ratio demonstrated in water tank experiments holds in the human heart.

Diastolic vortex ring formation in the human left ventricle: quantitative analysis using Lagrangian coherent structures and 4D cardiovascular magnetic resonance velocity mapping

Summary We show that 4D magnetic resonance velocity mapping and Lagrangian Coherent Structures can be used to quantify vortex ring volume in the human left ventricle. Background The vortex ring formed in the left ventricle (LV) of the human heart during early diastolic inflow (corresponding to the Doppler E-wave) contains information about the normal and pathophysiologic aspects of diastole. Previous studies suggest that the volume of the vortex ring is an important characteristic of vortex ring formation. However, due to lack of quantitative methods, vortex ring volume has not previously been studied in the human left ventricle. The study of Lagrangian Coherent Structures (LCS) is a new flow analysis method, which for the first time enables a description of vortex ring shape and the quantification of vortex ring volume. LCS have not previously been used to quantify diastolic vortex volume in humans. Therefore, the purpose of this study was to investigate if LCS and three-dimensional, time-resolved, three-directional phase contrast magnetic resonance velocity mapping (4D PC-MR) can be used to describe vortex ring shape and quantify vortex ring volume in the human LV. Methods