Measurement of cardiac valve and aortic blood flow velocities in stroke patients: a comparison of 4D flow MRI and echocardiography

Four-Dimensional Flow Magnetic Resonance Imaging in the Assessment of Blood Flow in the Heart and Great Vessels: A Systematic Review

Four-dimensional (4D) flow magnetic resonance imaging (MRI) allows multidirectional quantification of blood flow in the heart and great vessels. Comparability of the technique to the current reference standards of flow assessment-two-dimensional (2D) flow MRI and Doppler echocardiography-varies in the literature. Image acquisition parameters likely impact upon the accuracy and reproducibility of 4D flow MRI. We therefore sought to review the current literature on 4D flow MRI in the heart and great vessels, in comparison to 2D flow MRI, Doppler echocardiography, and invasive catheterization. Using a predefined search strategy and inclusion and exclusion criteria, the databases EMBASE and Medline were searched in January 2021 for peer-reviewed research articles comparing cardiac 4D flow MRI to 2D flow MRI, Doppler echocardiography and/or invasive catheterization. The data from all relevant articles were assimilated and analyzed using Mann-Whitney U and chi χ² test. Forty-four manuscripts met the eligibility criteria and were included in the review. The review showed agreement of 4D flow MRI to the reference standard methods of flow assessment, particular in the measurement of peak velocity and stroke volume in 55% of manuscripts. The use of valve tracking significantly improves agreement between 4D flow MRI and the reference modalities (79% matching with the use of valve tracking vs. 50% without, P = 0.04). This review highlights that the impact of acquisition parameters on 4D flow MRI accuracy is multifactorial. It is therefore important that each center conducts its own quality assurance prior to using 4D flow MRI for clinical decision-making. LEVEL OF EVIDENCE: 2 TECHNICAL EFFICACY: Stage 2.

Quantification of peak blood flow velocity at the cardiac valve and great thoracic vessels by four-dimensional flow and two-dimensional phase-contrast MRI compared with echocardiography: a systematic review and meta-analysis

AIM

To objectively examine the agreement and correlation between four-dimensional (4D) flow magnetic resonance imaging (MRI) and traditional two-dimensional (2D) phase-contrast (PC) MRI with the reference standard of Doppler echocardiography for measuring peak blood velocity at the cardiac valve and great arteries, and to assess if 4D flow MRI offers an advantage over the traditional 2D method.

MATERIALS AND METHODS

The literature was searched systematically for studies that evaluate the degree of correlation and agreement between 4D flow MRI or 2D PC MRI and Doppler retrieved from PubMed, EMBASE, and the Cochrane Library. A meta-analysis was conducted to determine the peak velocity pooled bias with 95% limits of agreement (LoA) and correlation coefficient (r) for 4D flow MRI and 2D PC MRI compared with Doppler.

RESULTS

Ten studies that compared 4D flow MRI with Doppler and 12 studies that compared 2D PC MRI with Doppler were included. 4D flow MRI showed an underestimation with bias and 95% LoA of -0.09 (-0.41, 0.24) m/s (p=0.079) while 2D PC MRI showed a poorer agreement with a bias and 95% LoA of -0.25 (-0.53, 0.03), p=0.596. 4D flow MRI and 2D PC MRI showed a strong correlation with R=0.80 (95% CI 0.75, 0.84; p<0.001) and R=0.83 (95% CI 0.79, 0.87; p<0.001), respectively.

CONCLUSION

In this meta-analysis, 4D flow MRI provides improved assessment of peak velocity when compared with traditional 2D PC MRI. 4D flow MRI can be considered an important complement or substitute to Doppler echocardiography for peak velocity assessment.

Clinical Applications of 4D Flow MR Imaging in Aortic Valvular and Congenital Heart Disease

4D flow MRI allows time-resolved 3D velocity-encoded phase-contrast imaging for 3D visualization and quantification of aortic and intracardiac flow. Radiologists should be familiar with the principles of 4D flow MRI and methods for evaluating blood flow qualitatively and quantitatively. The most substantial benefits of 4D flow MRI are that it enables the simultaneous comprehensive assessment of different vessels, and that retrospective analysis can be achieved in all vessels in any direction in the field of view, which is especially beneficial for patients with complicated congenital heart disease (CHD). For aortic valvular diseases, new parameters such as wall shear stress and energy loss may provide new prognostic values for 4D flow MRI. In this review, we introduce the clinical applications of 4D flow MRI for the visualization of blood flow and quantification of hemodynamic metrics in the setting of aortic valvular disease and CHD, including intracardiac shunt and coronary artery anomaly.

On-line parameter identification of the lumped arterial system model: A simulation study

A lumped model of the arterial system has been used in constructing a hybrid mock loop due to its real-time response. However, the parameters of the model are always from a general case and not adapted to a specific patient. In this study, we focused on on-line parameter identification of the lumped model of the arterial system that could be used for a specific patient. A five-element lumped arterial model is adopted in this study, in which the five parameters are to be determined. The aortic flow rate and the venous pressure are chosen as the inputs of the model, and aortic pressure as the output. An iterative optimization based on the established state space equations of the five-element model is used to seek the best parameter values by minimizing the difference between the model prediction and the previously obtained aortic pressure. The method is validated using simulated data from a complete numerical cardiovascular model. Results show that the method can track the dynamic variation of the parameters very well. Finally, a sensitivity analysis of the model parameters is conducted to interpret the effect of parameter changes. The good performance of the identification demonstrates the potential application of this method to customize a hybrid mock loop for a specific patient or clinically monitor the arterial vessel characteristics in real time.

Clinical assessment of aortic valve stenosis: Comparison between 4D flow MRI and transthoracic echocardiography

Background

The prevalence of valvular aortic stenosis (AS) increases as the population ages. Echocardiographic measurements of peak jet velocity (V_peak), mean pressure gradient (P_mean), and aortic valve area (AVA) determine AS severity and play a pivotal role in the stratification towards valvular replacement. A multimodality imaging approach might be needed in cases of uncertainty about the actual severity of the stenosis.

Purpose

To compare four‐dimensional phase‐contrast magnetic resonance (4D PC‐MR), two‐dimensional (2D) PC‐MR, and transthoracic echocardiography (TTE) for quantification of AS.

Study Type

Prospective.

Population

Twenty patients with various degrees of AS (69.3 ± 5.0 years).

Field Strength/Sequences

4D PC‐MR and 2D PC‐MR at 3T.

Assessment

We compared V_peak, P_mean, and AVA between TTE, 4D PC‐MR, and 2D PC‐MR. Flow eccentricity was quantified by means of normalized flow displacement, and its influence on the accuracy of TTE measurements was investigated.

Statistical Tests

Pearson's correlation, Bland–Altman analysis, paired t‐test, and intraclass correlation coefficient.

Results

4D PC‐MR measured higher V_peak (r = 0.95, mean difference + 16.4 ± 10.7%, P <0.001), and P_mean (r = 0.92, mean difference + 14.9 ± 16.0%, P = 0.013), but a less critical AVA (r = 0.80, mean difference + 19.9 ± 20.6%, P = 0.002) than TTE. In contrast, unidirectional 2D PC‐MR substantially underestimated AS severity when compared with TTE. Differences in V_peak between 4D PC‐MR and TTE showed to be strongly correlated with the eccentricity of the flow jet (r = 0.89, P <0.001). Use of 4D PC‐MR improved the concordance between V_peak and AVA (from 0.68 to 0.87), and between PG_mean and AVA (from 0.68 to 0.86).

Data Conclusion

4D PC‐MR improves the concordance between the different AS parameters and could serve as an additional imaging technique next to TTE. Future studies should address the potential value of 4D PC‐MR in patients with discordant echocardiographic parameters.

Level of Evidence: 2

Technical Efficacy: Stage 2

J. Magn. Reson. Imaging 2020;51:472–480.

Is transesophageal echocardiography needed for evaluating tissue-based transient ischemic attack?

Transient ischemic attack (TIA) is a warning signal for stroke. A comprehensive evaluation of TIA may reduce the risk for subsequent stroke. Data on the findings of cardiac evaluation with transesophageal echocardiography (TEE) in patients with TIA are sparse. Our aims were to determine the frequency of TEE performance and to investigate the findings of TEE in patients with TIA based on the new definition of TIA (i.e., transient neurological symptoms without evidence of infarction). During a 4-year period (2011-2014), 1071 patients (mean age, 70 ± 13 years; female, 49.7%) with TIA were included in a prospective study and evaluated. Of 1071 consecutive patients suffering from TIA, 288 patients (27%) underwent TEE. The median time between admission and TEE was 6 days. Patients with TIA who were evaluated by TEE were younger (67 vs. 71 years, P < 0.001) than those who were not evaluated by TEE. They had a higher rate of sensibility disturbance as a TIA symptom (39% vs. 31%, P = 0.012) but a lower rate of previous stroke (15% vs. 25%, P = 0.001) and atrial fibrillation (2% vs. 21%, P < 0.001) than those who did not. Foramen ovale was detected in 71 patients (25.7%), atrial septal aneurysm in 13 patients (4.6%), and severe atherosclerotic plaques (grade 4 and 5) in the aortic arch in 25 patients (8.7%). One patient (0.3%) had a fibroma detected by TEE. In 17 of the 288 patients (6%) who underwent TEE, the indication for anticoagulation therapy was based on the TEE results, and 1 patient with fibroma underwent heart surgery. During hospitalization, 7 patients experienced a subsequent stroke, and 27 patients had a recurrent TIA. At 3 months following discharge, the rates of readmission, stroke, recurrent TIA, and death were 19%, 2.7%, 4.2%, and 1.6%, respectively. The rates of mortality (0.9% vs. 1.8%, P = 0.7), stroke risk (1.9% vs. 3.0%, P = 0.8), and recurrent TIA (5.0% vs. 3.9%, P = 0.8) were similar in patients who underwent TEE and in those who did not. Performing TEE in patients with tissue-based TIA is helpful in detecting cardiac sources for embolism and may indicate for anticoagulation.