Four-dimensional flow magnetic resonance imaging-based characterization of aortic morphometry and haemodynamics: impact of age, aortic diameter, and valve morphology

Abstract

Aims: Four-dimensional (4D) flow magnetic resonance imaging (MRI) was employed for the simultaneous assessment of morphometry and flow parameters along the thoracic aorta to investigate associations between flow, age, aorta diameter, and aortic valve morphology.

Methods and results: One hundred and sixty-five subjects, 65 controls, 50 patients with bicuspid aortic valve (BAV), and 50 patients with a dilated aorta, and a tricuspid aortic valve (TAV) underwent 4D flow MRI. Following 3D segmentation of the aorta, a vessel centreline was calculated and used to extract aorta diameter, peak systolic velocity, and normalized systolic flow displacement. Validation of 4D flow MRI-based morphometric measurements compared with manual diameter measurements from standard contrast-enhanced MR angiography in 20 controls showed good agreement (mean difference = 0.4 mm, limits of agreement = ±1.31 mm) except at the sinus of valsalva. BAV showed significant differences in average peak velocity (PV; P < 0.016) compared with TAV and controls between the left ventricle outflow tract to sino-tubular junction (BAV: 1.3 ± 0.3 m/s; TAV: 1.2 ± 0.2 m/s; controls: 1.0 ± 0.1 m/s) and the ascending aorta for average normalized flow displacement (BAV: 0.11 ± 0.02; TAV: 0.09 ± 0.02; controls: 0.06 ± 0.01, P < 0.016) despite similar average aortic dimensions for BAV (37 ± 1 mm) and TAV (39 ± 1 mm). Multivariate linear regression showed a significant correlation of maximal aortic diameter to age, PV, and normalized flow displacement (R(2) = 0.413, P < 0.001).

Conclusion: A single acquisition of 4D flow MRI characterized local morphological and haemodynamic differences between groups along the aorta. BAV showed altered haemodynamics when compared with TAV in spite of having similar aorta dimensions. Maximal aorta diameter was associated with age, PV, and normalized flow displacement.

Keywords: Aortic diseases; Haemodynamics; Magnetic resonance imaging.

Figure 1

Workflow for aortic diameter and flow parameters measurements. (A) 3D PC-MRA, which was used for aorta segmentation. (B) Centreline calculation from the segmented volume and the anatomic landmarks definition along the centreline. (C) Examples of plane generation along the centreline. The generated planes were used to calculate aorta diameter, PV, and normalized flow displacement as it is shown in (D). All measurements were normalized to standard landmarks distance.

Figure 2

Centreline plane-based measurements. (A) Analysis workflow, multiple analysis planes were created along the centreline to measure vessel diameter and flow parameters, anatomic landmark planes are displayed for distance reference. (B) Example of manual measurement of diameter using standard CE-MRA, this measurement was performed only in 20 cases for diameter validation and the larger diameter was used for assessment. (C) Automatic measurement of diameters obtained from a plane based on the 3D CE-MRA segmentation (red line corresponds to segmented vessel area), note that a circular cross section was assumed to back calculate diameter (d). (D) Examples of flow displacement measurements for symmetric and asymmetric flow profiles. The black dashed line corresponds to the vessel centreline. The normalized flow displacement was calculated as the distance from the vessel centreline to the positive velocity-weighted centre of mass (black circle) normalized to the vessel diameter. The corresponding equation is also shown in (D). MPA, main pulmonary artery.

Figure 3

Validation of aorta diameter measurement based on centreline planes. Aorta diameter determined by three different methods: (i) manual measurements based on standard multi-planar CE-MRA (black dots and bars representing mean and standard deviation), (ii) centreline-based automatic diameter detection using PC-MRA (yellow dots and bars representing mean and standard deviation), and (iii) centreline-based automatic diameter detection using CE-MRA (grey dots and bars representing mean and standard deviation) for 20 subjects were used to validate centreline-based diameters. Red dots indicate significant differences (P < 0.05, independent-sample t-test) between centreline-based CE-MRA and PC-MRA diameters.

Figure 4

Morphometric and haemodynamic measurements in the entire aorta. (A) Aorta diameter for healthy controls (grey dots and bars for mean and standard deviation), patients with BAV (orange dots and bars for mean and standard deviation) and TAV patients (blue dots and bars for mean and standard deviation). (B) PV for controls, patients with BAV and TAV. (C) Normalized flow displacement for controls, patients with BAV and TAV. Orange dots indicate significant differences between controls and BAV patients. Blue dots indicate significant differences between controls and TAV patients. Yellow dots indicate significant differences between BAV and TAV patients. P < 0.016, after Bonferroni correction.