Quantitating aortic regurgitation by cardiovascular magnetic resonance: significant variations due to slice location and breath holding

Aortic Regurgitation and Heart Failure: Advances in Diagnosis, Management, and Interventions

This review discusses the contemporary clinical evaluation and management of patients with comorbid aortic regurgitation (AR) and heart failure (HF) (AR-HF). Importantly, as clinical HF exists along the spectrum of AR severity, the present review also details novel strategies to detect early signs of HF before the clinical syndrome ensues. Indeed, there may be a vulnerable cohort of AR patients who benefit from early detection and management of HF. Additionally, while the mainstay of operative management for AR has historically been surgical aortic valve replacement, this review discusses alternate procedures that may be beneficial in high-risk cohorts.

Assessment of Asymptomatic Severe Aortic Regurgitation by Doppler-Derived Echo Indices: Comparison with Magnetic Resonance Quantification

Reliable quantification of aortic regurgitation (AR) severity is essential for clinical management. We aimed to compare quantitative and indirect echo-Doppler indices to quantitative cardiac magnetic resonance (CMR) parameters in asymptomatic chronic severe AR. Methods and Results: We evaluated 104 consecutive patients using echocardiography and CMR. A comprehensive 2D, 3D, and Doppler echocardiography was performed. The CMR was used to quantify regurgitation fraction (RF) and volume (RV) using the phase-contrast velocity mapping technique. Concordant grading of AR severity with both techniques was observed in 77 (74%) patients. Correlation between RV and RF as assessed by echocardiography and CMR was relatively good (rs = 0.50 for RV, rs = 0.40 for RF, p < 0.0001). The best correlation between indirect echo-Doppler and CMR parameters was found for diastolic flow reversal (DFR) velocity in descending aorta (rs = 0.62 for RV, rs = 0.50 for RF, p < 0.0001) and 3D vena contracta area (VCA) (rs = 0.48 for RV, rs = 0.38 for RF, p < 0.0001). Using receiver operating characteristic analysis, the largest area under curve (AUC) to predict severe AR by CMR RV was observed for DFR velocity (AUC = 0.79). DFR velocity of 19.5 cm/s provided 78% sensitivity and 80% specificity. The AUC for 3D VCA to predict severe AR by CMR RV was 0.73, with optimal cut-off of 26 mm2 (sensitivity 80% and specificity 66%). Conclusions: Out of the indirect echo-Doppler indices of AR severity, DFR velocity in descending aorta and 3D vena contracta area showed the best correlation with CMR-derived RV and RF in patients with chronic severe AR.

Peak flow measurements in patients with severe aortic stenosis: a prospective comparative study between cardiovascular magnetic resonance 2D and 4D flow and transthoracic echocardiography

BACKGROUND

Aortic valve stenosis (AS) is the most prevalent valvular disease in the developed countries. Four-dimensional (4D) flow cardiovascular magnetic resonance (CMR) is an emerging imaging technique, which has been suggested to improve the evaluation of AS severity compared to two-dimensional (2D) flow and transthoracic echocardiography (TTE). We investigated the reliability of CMR 2D flow and 4D flow techniques in measuring aortic transvalvular peak systolic flow in patients with severe AS.

METHODS

We prospectively recruited 90 patients referred for aortic valve replacement due to severe AS (73.3 ± 11.3 years, aortic valve area 0.7 ± 0.1 cm2, and 54/36 tricuspid/bicuspid), and 10 non-valvular disease controls. All the patients underwent echocardiography and 2D flow and 4D flow CMR. Peak flow velocity measurements were compared using Wilcoxon signed rank sum test and Bland-Altman analysis.

RESULTS

4D flow underestimated peak flow velocity in the AS group when compared with TTE (bias - 1.1 m/s, limits of agreement ± 1.4 m/s) and 2D flow (bias - 1.2 m/s, limits of agreement ± 1.6 m/s). The differences between values obtained by TTE (median 4.3 m/s, range 2.7-6.1 m/s) and 2D flow (median 4.5 m/s, range 2.9-6.5 m/s) compared to 4D flow (median 3.1 m/s, range 1.7-5.1 m/s) were significant (p < 0.001). The difference between 2D flow and TTE were insignificant (bias 0.07 m/s, limits of agreement ± 1.5 m/s). In non-valvular disease controls, peak flow velocity was measured higher by 4D flow than 2D flow (1.4 m/s, 1.1-1.7 m/s and 1.3 m/s, 1.1-1.5 m/s, respectively; bias 0.2 m/s, limits of agreement ± 0.16 m/s).

CONCLUSIONS

CMR 4D flow significantly underestimates systolic peak flow velocity in patients with severe AS. 2D flow, in turn, estimated the AS velocity accurately, with measured peak flow velocities comparable to TTE.

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.

Quantitative assessment of aortic regurgitation following transcatheter aortic valve replacement

Introduction: Transcatheter aortic valve replacement (TAVR) is expanding to lower risk and younger patients with severe symptomatic aortic valve disease. Despite clinical and technological improvements, post-procedural aortic regurgitation (AR) remains a limitation of TAVR, particularly when compared to surgical aortic valve replacement. Although several methods for AR quantification after TAVR are currently available, its exact graduation in everyday clinical practice remains challenging.Areas covered: This review describes the currently available evaluation methods of AR after TAVR, with a special emphasis on the quantitative assessment using videodensitometric angiography, echocardiography and cardiac magnetic resonance imaging.Expert opinion: In the majority of clinical scenarios, satisfactory evaluation of post-TAVR AR can be achieved with a combination of post-procedural angiography, hemodynamic indices and transthoracic echocardiography. Nevertheless, some TAVR patients show 'intermediate' forms of post-procedural AR, in which quantitative evaluation is mandatory for prognostic purposes and further decision-making. Notably, interpretation of quantitative measures early post-TAVR is challenging because of the lack of left ventricular enlargement. Video-densitometric angiography is an emerging method that appears to be clinically attractive for immediate post-TAVR assessment, but requires further validation in everyday clinical practice.

Cardiac magnetic resonance systematically overestimates mitral regurgitations by the indirect method

Objective

Cardiac MRI is quickly emerging as the gold standard for assessment of mitral regurgitation, most commonly with the indirect method subtracting forward flow in aorta from volumetric segmentation of the left ventricle. We aimed to investigate how aortic flow measurements with increasing distance from the aortic valve affect calculated mitral regurgitations and whether measurements were influenced by breath-hold regimen.

Methods

Free-breathing and breath-hold phase contrast flows were measured in aorta at valve level, sinotubular (ST) junction, mid-ascending aorta and in the pulmonary trunk. Flow measurements were pairwise compared, and subsequently, after exclusion of patients with visible mitral and tricuspid regurgitations for left-sided and right-sided comparisons, respectively, flow-measured stroke volumes were compared with ventricular volumetric segmentations.

Results

Thirty-nine participants without arrhythmias or structural abnormalities of the large vessels were included. Stroke volumes measured with free-breathing and breath-hold flow decreased equally with increasing distance to the aortic valves (breath-hold flow: aortic valve 105.6±20.8 mL, ST junction 101.5±20.7 mL, mid-ascending aorta 98.1±21.5 mL). After exclusion of atrioventricular regurgitations, stroke volumes determined by volumetric measurements were higher compared with values determined by flow measurements, corresponding to ‘false’ atrioventricular regurgitations of 8.0%±5.8% with flow measured at valve level, 11.6%±5.2% at the ST junction and 15.3%±5.0% at the mid-ascending aorta.

Conclusions

Stroke volumes determined by flow decrease throughout the proximal aorta and are systematically lower than volumetrically measured stroke volumes. The indirect method systematically overestimates mitral regurgitations, especially with increasing distance from the aortic valves.

Fast Phase-Contrast Cine MRI for Assessing Intracranial Hemodynamics and Cerebrospinal Fluid Dynamics

We propose fast phase-contrast cine magnetic resonance imaging (PC-cine MRI) to allow breath-hold acquisition, and we compared intracranial hemo- and hydrodynamic parameters obtained during breath holding between full inspiration and end expiration. On a 3.0 T MRI, using electrocardiogram (ECG)-synchronized fast PC-cine MRI with parallel imaging, rectangular field of view, and segmented k-space, we obtained velocity-mapped phase images at the mid-C2 level with different velocity encoding for transcranial blood flow and cerebrospinal-fluid (CSF) flow. Next, we calculated the peak-to-peak amplitudes of cerebral blood flow (ΔCBF), cerebral venous outflow, intracranial volume change, CSF pressure gradient (ΔPG), and intracranial compliance index. These parameters were compared between the proposed and conventional methods. Moreover, we compared these parameters between different utilized breath-hold maneuvers (inspiration, expiration, and free breathing). All parameters derived from the fast PC method agreed with those from the conventional method. The ΔPG was significantly higher during full inspiration breath holding than at the end of expiration and during free breathing. The proposed fast PC-cine MRI reduced scan time (within 30 s) with good agreement with conventional methods. The use of this method also makes it possible to assess the effects of respiration on intracranial hemo- and hydrodynamics.

Stationary tissue background correction increases the precision of clinical evaluation of intra-cardiac shunts by cardiovascular magnetic resonance

We aimed to evaluate the clinical utility of stationary tissue background phase correction for affecting precision in the measurement of Qp/Qs by cardiovascular magnetic resonance (CMR). We enrolled consecutive patients (n = 91) referred for CMR at 1.5T without suspicion of cardiac shunt, and patients (n = 10) with verified cardiac shunts in this retrospective study. All patients underwent phase contrast flow quantification in the ascending aorta and pulmonary trunk. Flow was quantified using two semi-automatic software platforms (SyngoVia VA30, Vendor 1; Segment 2.0R4534, Vendor 2). Measurements were performed both uncorrected and corrected for linear (Vendor 1 and Vendor 2) or quadratic (Vendor 2) background phase. The proportion of patients outside the normal range of Qp/Qs was compared using the McNemar’s test. Compared to uncorrected measurements, there were fewer patients with a Qp/Qs outside the normal range following linear correction using Vendor 1 (10% vs 18%, p < 0.001), and Vendor 2 (10% vs 18%, p < 0.001), and following quadratic correction using Vendor 2 (7% vs 18%, p < 0.001). No patient with known shunt was reclassified as normal following stationary background correction. Therefore, we conclude that stationary tissue background correction reduces the number of patients with a Qp/Qs ratio outside the normal range in a consecutive clinical population, while simultaneously not reclassifying any patient with known cardiac shunts as having a normal Qp/Qs. Stationary tissue background correction may be used in clinical patients to increase diagnostic precision.

Investigation of phase-contrast magnetic resonance imaging underestimation of turbulent flow through the aortic valve phantom: experimental and computational study using lattice Boltzmann method

OBJECTIVE

The accuracy of phase-contrast magnetic resonance imaging (PC-MRI) measurement is investigated using a computational fluid dynamics (CFD) model with the objective to determine the magnitude of the flow underestimation due to turbulence behind a narrowed valve in a phantom experiment.

MATERIALS AND METHODS

An acrylic stationary flow phantom is used with three insertable plates mimicking aortic valvular stenoses of varying degrees. Positive and negative horizontal fluxes are measured at equidistant slices using standard PC-MRI sequences by 1.5T and 3T systems. The CFD model is based on the 3D lattice Boltzmann method (LBM). The experimental and simulated data are compared using the Bland-Altman-derived limits of agreement. Based on the LBM results, the turbulence is quantified and confronted with the level of flow underestimation.

RESULTS

LBM gives comparable results to PC-MRI for valves up to moderate stenosis on both field strengths. The flow magnitude through a severely stenotic valve was underestimated due to signal void in the regions of turbulent flow behind the valve, consistently with the level of quantified turbulence intensity.

DISCUSSION

Flow measured by PC-MRI is affected by noise and turbulence. LBM can simulate turbulent flow efficiently and accurately, it has therefore the potential to improve clinical interpretation of PC-MRI.

Clinical use of 4D flow MRI for quantification of aortic regurgitation

Objective

The main objective of the present study was to compare the use of four-dimensional (4D) flow MRI with the habitual sequence (two-dimensional phase-contrast (2DPC) MRI) for the assessment of aortic regurgitation (AR) in the clinical routine.

Methods

This was a retrospective, observational cohort study of patients with varying grades of AR. For the purposes of the present study, we selected all the cases with a regurgitant fraction (RF)>5% as determined by 2DPC MRI (n=34). In all cases, both sequences (2DPC and 4D flow MRI) were acquired in a single session to ensure comparability. We compared the results of the two techniques by evaluating forward flow, regurgitant flow and regurgitation fraction. Then, the patients were divided into subgroups to determine if these factors had any influence on the measurements: aortic diameter (≤ vs >38 mm), valve anatomy (tricuspid vs bicuspid/quadricuspid), stenosis (gradient ≥15 vs <15) and region of interest location (aortic valve vs sinotubular junction).

Results

No statistically significant differences were observed between the two techniques with Pearson’s correlation coefficients (r) of forward flow (r=0.826/p value<0001), regurgitant flow (r=0.866/p value<0001) and RF (r=0.761/p value<0001).

Conclusions

The findings of this study confirm the value of 4D flow MRI for grading AR in clinical practice with an excellent correlation with the standard technique (2DPC MRI).

Phase-Contrast Magnetic Resonance Quantification of Aortic Regurgitation in Patients With Turbulent Aortic Flow

OBJECTIVE

This study aimed to assess variability in measurements and accurately quantify aortic regurgitation in patients with coexisting turbulent aortic flow using phase-contrast magnetic resonance.

METHODS

All patients (n = 21) underwent phase-contrast magnetic resonance at 2 or more sites: ascending aorta, sinuses of Valsalva, and left ventricular outflow tract. The net flow/minute (NF), forward flow/minute (FF), regurgitant flow/minute (RF), and regurgitant fraction (RF%) were compared with the sum of superior vena cava and descending aortic flow/minute, left ventricular cardiac output, difference between the 2, and percentage difference, respectively.

RESULTS

The NF, FF, and RF were significantly different between each site. The combination of FF in the left ventricular outflow tract and NF from the superior vena cava + descending aorta provided the best reliability of RF and regurgitant fraction (intraclass correlation coefficients, 0.881 [95% confidence interval, 0.882-0.878] and 0.838 [95% confidence interval, 0.837-0.838]).

CONCLUSION

Combining flow measurements from more than 1 site provides the most accurate quantification of aortic regurgitation in patients with turbulent aortic flow.

Analysis of chronic aortic regurgitation by 2D and 3D echocardiography and cardiac MRI

Purpose The study compares the feasibility of the quantitative volumetric and semi-quantitative approach for quantification of chronic aortic regurgitation (AR) using different imaging modalities. Methods Left ventricular (LV) volumes, regurgitant volumes (RVol) and regurgitant fractions (RF) were assessed retrospectively by 2D, 3D echocardiography and cMRI in 55 chronic AR patients. Semi-quantitative parameters were assessed by 2D echocardiography. Results 22 (40%) patients had mild, 25 (46%) moderate and 8 (14%) severe AR. The quantitative volumetric approach was feasible using 2D, 3D echocardiography and cMRI, whereas the feasibility of semi-quantitative parameters varied considerably. LV volume (LVEDV, LVESV, SVtot) analyses showed good correlations between the different imaging modalities, although significantly increased LV volumes were assessed by cMRI. RVol was significantly different between 2D/3D echocardiography and 2D echocardiography/cMRI but was not significantly different between 3D echocardiography/cMRI. RF was not statistically different between 2D echocardiography/cMRI and 3D echocardiography/cMRI showing poor correlations (r < 0.5) between the different imaging modalities. For AR grading by RF, moderate agreement was observed between 2D/3D echocardiography and 2D echocardiography/cMRI and good agreement was observed between 3D echocardiography/cMRI. Conclusion Semi-quantitative parameters are difficult to determine by 2D echocardiography in clinical routine. The quantitative volumetric RF assessment seems to be feasible and can be discussed as an alternative approach in chronic AR. However, RVol and RF did not correlate well between the different imaging modalities. The best agreement for grading of AR severity by RF was observed between 3D echocardiography and cMRI. LV volumes can be verified by different approaches and different imaging modalities.

Quantitative assessment of aortic regurgitation by Doppler echocardiography: Usefulness of the comparison of aortic and pulmonary flows

AIMS

Patients with significant (3+/4+) aortic regurgitation (AR) require careful monitoring or valve surgery. We sought to evaluate the diagnostic performance of aortic and pulmonary flow comparison in identifying patients with significant AR, by echocardiography.

METHOD

Two hundred forty-six patients with more than trivial AR were prospectively enrolled from three centers. Aortic regurgitation (AR) severity was assessed by an expert using the currently recommended integrative approach. Aortic and pulmonary flows were independently assessed by another investigator to calculate the regurgitant fraction (RF), the aortic to pulmonary flow ratio (Qao/Qp) and the aortic to pulmonary velocity-time integral (VTIao/VTIp) ratio. The control group consisted of 195 patients without AR.

RESULTS

A significant correlation was observed between AR grading and RF (r = .82, P < .0001) and Qao/Qp (r = .81, P < .0001), but the correlation was modest for VTIao/VTIp ratio (r = .63; P < .0001). The accuracy of RF and Qao/Qp ratio to identify patients with significant AR was excellent (0.96 and 0.95, respectively), but was significantly lower for VTIao/VTIp ratio at 0.82. A RF > 40% indicated grade 3 or 4 AR with a sensitivity of 83% and a specificity of 93%. A Qao/Qp ratio > 1.6 indicated grade 3 or 4 AR with a sensitivity of 88% and a specificity of 89%. The VTIao/VTIp ratio was not helpful in identifying patients with significant AR, as a wide overlap was found between 1+/2+ and 3+/4+ patients.

CONCLUSION

Regurgitant fraction (RF) and Qao/Qp are helpful in identifying significant AR. The assessment of Doppler aortic/pulmonary flow should be incorporated in the comprehensive evaluation of AR.

Comparison of effects of losartan and metoprolol on left ventricular and aortic function at rest and during exercise in chronic aortic regurgitation

Aortic regurgitation (AR) increases the hemodynamic load on both the left ventricle (LV) and the aorta. Vasodilators and beta-blockers both reduce systemic blood pressure, but their relative effects on the LV and aortic function and aortic regurgitant fraction in chronic AR are uncertain. We aimed to compare short-term effects of losartan and metoprolol on LV and aortic function in asymptomatic patients with chronic moderate to severe AR, both at rest and during exercise, using cardiac magnetic resonance (CMR) imaging. 17 chronic AR patients were randomized to 4-6 weeks losartan followed by metoprolol, or vice versa, in a cross-over design. Aortic regurgitant fraction, aortic distensibility, pulse wave velocity and LV function were assessed at rest and after moderate exercise stress (29 ± 7 W, heart rate increase 25 ± 6 bpm) using CMR. Chronic AR patients on metoprolol had a significantly lower mean heart rate, cardiac power index and rate-pressure product, than on losartan (all p < 0.01). However, aortic regurgitant fraction was greater on metoprolol compared to losartan (by 7 ± 11%, p = 0.02). Metoprolol was also associated with a greater reduction in aortic distensibility during exercise than losartan (- 2.4 ± 1.5 × 10^-3 vs - 1.7 ± 2.1 × 10^-3 mmHg^-1 respectively, p = 0.04). End-diastolic volume index was higher on metoprolol than losartan at exercise (difference 6.6 ± 7.8 ml/m², p < 0.01), as was end-systolic volume index (difference 4.0 ± 5.2 ml/m², p < 0.01). Losartan and metoprolol have significantly different short-term effects on aortic regurgitation and LV and aortic function in chronic AR. Further research is required to determine the long-term clinical significance of these changes.

Evaluation of aortic regurgitation with cardiac magnetic resonance imaging: a systematic review

This review summaries the utility, application and data supporting use of cardiac magnetic resonance imaging (CMR) to evaluate and quantitate aortic regurgitation. We systematically searched Medline and PubMed for original research articles published since 2000 that provided data on the quantitation of aortic regurgitation by CMR and identified 11 articles for review. Direct aortic measurements using phase contrast allow quantitation of volumetric flow across the aortic valve and are highly reproducible and accurate compared with echocardiography. However, this technique requires diligence in prescribing the correct imaging planes in the aorta. Volumetric analytic techniques using differences in ventricular volumes are also highly accurate but less than phase contrast techniques and only accurate when concomitant valvular disease is absent. Comparison of both aortic and ventricular data for internal data verification ensures fidelity of aortic regurgitant data. CMR data can be applied to many types of aortic valve regurgitation including combined aortic stenosis with regurgitation, congenital valve diseases and post-transcatheter valve placement. CMR also predicts those patients who progress to surgery with high overall sensitivity and specificity. Future studies of CMR in patients with aortic regurgitation to quantify the incremental benefit over echocardiography as well as prediction of cardiovascular events are warranted.

Current Management of Patients with Severe Aortic Regurgitation

OPINION STATEMENT

Chronic aortic regurgitation can result from various congenital and acquired anomalies and can be associated with proximal aortic disease. As the number of aortic valve procedures is growing, the incidence of post-procedural regurgitation also increases with associated morbidity. Typical evolution is characterized by a clinically silent phase of variable duration followed by a rather rapid decline with high incidence of adverse events. A challenge remains to find the optimal timing for an intervention: Patients are exposed to unnecessary surgical risks if treated prematurely, but peri- and post-operative prognosis is worse when the intervention is performed too late. Clinical evaluation and serial imaging tests can optimize the timing for intervention. Clinical follow-up should try to elucidate associated symptoms, with quantitative measurement of functional capacity as needed. Serial imaging examinations are required to identify sub-clinical left ventricular dysfunction or severe dilatation that should prompt a surgery. At least in selected cases, newer imaging modalities (MRI, 3D echocardiography) and/or biomarkers can help for the management of these patients, and more research is needed to determine if their systematic use can be beneficial. Medical treatment with vasodilators and anti-remodeling drugs can be helpful in some patients but should not replace or delay aortic valve surgery when indicated. Most patients will eventually be treated with surgical aortic valve replacement. Although possible in selected cases, transcatheter aortic valve replacement is not commonly used for patients with pure aortic regurgitation. For patients with prior aortic valve replacement and aortic regurgitation (paravalvular or intravalvular), emerging percutaneous approaches can be considered when available, especially for those at high surgical risk.

The Role of Imaging in Aortic Valve Disease

PURPOSE OF REVIEW

Aortic valve disease is the most common form of heart valve disease in developed countries. Imaging remains central to the diagnosis and risk stratification of patients with both aortic stenosis and regurgitation and has traditionally been performed with echocardiography. Indeed, echocardiography remains the cornerstone of aortic valve imaging as it is cheap, widely available and provides critical information concerning valve hemodynamics and ventricular function.

RECENT FINDINGS

Whilst diagnostic in the vast majority of patients, echocardiography has certain limitations including operator variability, potential for measurement errors and internal inconsistencies in severity grading. In particular, low-gradient severe aortic stenosis is common and challenging to diagnose. Aortic valve imaging may therefore be improved with alternative and complimentary multimodality approaches.

SUMMARY

This review investigates established and novel techniques for imaging both the aortic valve and the myocardial remodelling response including echocardiography, computed tomography, cardiovascular magnetic resonance and positron emission tomography. Moreover, we examine how the complementary information provided by each modality may be used in both future clinical practice and the research arena.