4-D flow magnetic resonance imaging: blood flow quantification compared to 2-D phase-contrast magnetic resonance imaging and Doppler echocardiography

Abstract

Background: Doppler echocardiography (echo) is the reference standard for blood flow velocity analysis, and two-dimensional (2-D) phase-contrast magnetic resonance imaging (MRI) is considered the reference standard for quantitative blood flow assessment. However, both clinical standard-of-care techniques are limited by 2-D acquisitions and single-direction velocity encoding and may make them inadequate to assess the complex three-dimensional hemodynamics seen in congenital heart disease. Four-dimensional flow MRI (4-D flow) enables qualitative and quantitative analysis of complex blood flow in the heart and great arteries.

Objectives: The objectives of this study are to compare 4-D flow with 2-D phase-contrast MRI for quantification of aortic and pulmonary flow and to evaluate the advantage of 4-D flow-based volumetric flow analysis compared to 2-D phase-contrast MRI and echo for peak velocity assessment in children and young adults.

Materials and methods: Two-dimensional phase-contrast MRI of the aortic root, main pulmonary artery (MPA), and right and left pulmonary arteries (RPA, LPA) and 4-D flow with volumetric coverage of the aorta and pulmonary arteries were performed in 50 patients (mean age: 13.1 ± 6.4 years). Four-dimensional flow analyses included calculation of net flow and regurgitant fraction with 4-D flow analysis planes similarly positioned to 2-D planes. In addition, 4-D flow volumetric assessment of aortic root/ascending aorta and MPA peak velocities was performed and compared to 2-D phase-contrast MRI and echo.

Results: Excellent correlation and agreement were found between 2-D phase-contrast MRI and 4-D flow for net flow (r = 0.97, P < 0.001) and excellent correlation with good agreement was found for regurgitant fraction (r = 0.88, P < 0.001) in all vessels. Two-dimensional phase-contrast MRI significantly underestimated aortic (P = 0.032) and MPA (P < 0.001) peak velocities compared to echo, while volumetric 4-D flow analysis resulted in higher (aortic: P = 0.001) or similar (MPA: P = 0.98) peak velocities relative to echo.

Conclusion: Excellent flow parameter agreement between 2-D phase-contrast MRI and 4-D flow and the improved volumetric 4-D flow velocity analysis relative to echo suggests that 4-D flow has the potential to become a clinical alternative to 2-D phase-contrast MRI.

Fig. 1

Retrospective flow quantification using 4-D flow reconstructed 3-D phase-contrast MR angiography data. a Location of analysis planes in the aorta and pulmonary system used for planar flow quantification. Four 2-D analysis planes were manually positioned in the aortic and pulmonary systems. b Volumetric analysis for the identification of peak blood flow velocities in the aorta and MPA. Ao aortic root (a), Ao aortic root/ascending aorta (b), MPA main pulmonary artery, RPA/LPA right/left pulmonary arteries

Fig. 2

Graphs show comparison of aortic and pulmonary flow parameters between 2-D phase-contrast (PC) MRI and quantification based on 4-D flow. Comparisons are for (a) net flow, (b) peak systolic velocity and (c) regurgitant fraction. Volumetric flow analysis (Fig. 1) in the aortic root/ascending aorta was used for 4-D flow-based peak systolic velocity quantification (b) in the aortic root/ascending aorta and MPA. Ao aorta, LPA left pulmonary artery, MPA main pulmonary artery, RPA right pulmonary artery

Fig. 3

Graphs show correlation of aortic and pulmonary flow parameters between 4-D flow and 2-D phase-contrast (PC) MRI. Correlations are for net flow (a), regurgitant fraction (b) and peak systolic velocities (c). In addition, correlation analysis was performed for peak systolic velocities obtained by echo and 4-D flow (d). c, d Volumetric 4-D flow analysis was based on peak systolic velocities in the aortic root/ascending aorta and MPA. Ao aorta, LPA left pulmonary artery, MPA main pulmonary artery, RPA right pulmonary artery

Fig. 4

Lines of equality and 95% limits of agreement of aortic and pulmonary flow parameters. Bland-Altman plots show the lines of equality between and 95% limits of agreement between 2-D phase-contrast (PC) MRI and 4-D flow for net flow (a) and regurgitant fraction in the aorta, MPA, RPA and LPA (b), 2-D phase-contrast MRI planar and 4-D flow volumetric peak systolic velocities for the aorta and MPA (c), and peak systolic velocities obtained by echo and volumetric 4-D flow in the aorta and MPA (d). LPA left pulmonary artery, MPA main pulmonary artery, RPA right pulmonary artery

Fig. 5

Conservation of mass assessment for 2-D phase-contrast (PC) MRI and 4-D flow MRI. Plots assess conservation of mass (sum of flow through LPA and RPAvs. flow through MPA) for 2-D phase-contrast MRI (a) and 4-D flow (b). LPA left pulmonary artery, MPA main pulmonary artery, RPA right pulmonary artery