4D flow imaging of the thoracic aorta: is there an added clinical value?

Abstract

Four-dimensional (4D) flow MRI has emerged as a powerful non-invasive technique in cardiovascular imaging, enabling to analyse in vivo complex flow dynamics models by quantifying flow parameters and derived features. Deep knowledge of aortic flow dynamics is fundamental to better understand how abnormal flow patterns may promote or worsen vascular diseases. In the perspective of an increasingly personalized and preventive medicine, growing interest is focused on identifying those quantitative functional features which are early predictive markers of pathological evolution. The thoracic aorta and its spectrum of diseases, as the first area of application and development of 4D flow MRI and supported by an extensive experimental validation, represents the ideal model to introduce this technique into daily clinical practice. The purpose of this review is to describe the impact of 4D flow MRI in the assessment of the thoracic aorta and its most common affecting diseases, providing an overview of the actual clinical applications and describing the potential role of derived advanced hemodynamic measures in tailoring follow-up and treatment.

Keywords: Four-dimensional flow imaging (4D flow imaging); cardiovascular magnetic resonance; fluid dynamics; phase contrast sequence; thoracic aorta.

Figure 1

DATA acquisition and processing. (A) In order to reconstruct 4D volumetric data sets, volumetric magnitude data and three phase-difference volumes, encoded in the three directions of space (x, y, z), are acquired. (B) Raw data are preprocessed. 4D flow data are affected by systematic velocity encoding errors caused by magnetic field inhomogeneity, concomitant magnetic fields (Maxwell terms), and eddy currents. Data pre-processing is a crucial moment during data analysis workflow: background phase-offset errors or aliasing artefacts may compromise the flow visualization and quantification, so is fundamental to correct them before the analysis step. (C) Finally, it’s time of visual and quantitative analysis of the images.

Figure 2

Colour-encoded thoracic aorta flow visualization modalities. Time-resolved path-lines (A) show the trajectories of fluid particles during a systolic peak in a dilated aorta, drawing a big vortex into the aneurism. Eccentric systolic jet in red, represented by the 3D streamlines (B) and velocity vectors (C), principally arranged along the right-anterior wall of the ascending aorta. The flow subsequently turns clockwise to the left and upwards, showing high degree of vorticity, until it reaches the aortic arch. In the transversal view, from the bottom to the top of the ascending aorta (C, on the bottom), the vectors clearly display the clockwise rotation of the flow into the aneurysm.

Figure 3

Aortic stenosis. A 45-year-old man with type 0 BAV and aortic valve moderate stenosis. The 4D flow velocity map (A) shows an increased flow velocity at measurement plane (sinus), as specified in the graph above. (B) Three chambers Cine SSFP shows the signal loss of stenotic jet (B2); on the systolic phase contrast images (B1) positioned at the valvular plane (corresponding to the dotted white line on B2) the planimetry of the effective orifice area was calculated as 1.53 cm². (C) WSS map and (D) viscous energy loss map describe increased WSS and viscous energy loss along the posterior AscAo wall with similar distribution patterns at systolic peak. The related graph represents the trend of the energy loss at the sinus level, with the maximum value recorded at the systolic peak. BAV, bicuspid aortic valve; WSS, wall shear stress.

Figure 4

Aortic valve regurgitation. MRI images from a 25-year-old woman with BAV and moderate aortic valve insufficiency. Three chambers cine SSFP (A) shows the aortic regurgitation during diastolic phase (on the top) and with the overlapping bi-dimensional flow image reconstructed from 4D flow dataset (on the bottom). Three-dimensional flow velocity map in diastolic phase (B) and four different axial planes for flow measurements. 4D flow MRI enables the post-hoc hemodynamic analysis at any level along the course of the vessel, as shown in the axial sectional images acquired during peak systolic and diastolic phase (C) acquired above (yellow) and below (red) the valve plane. Graph (D) represents flow-to-time curves measured through the reconstructed axial planes (according to the different colors of image B); the red curve, acquired below the valve plane, better estimates the regurgitant flow. BAV, bicuspid aortic valve.

Figure 5

Bicuspid aortic valve. A 33-year-old man with RL-type bicuspid aortic valve, mild aortic valve steno-insufficiency and AscAo dilatation (AscAo maximum diameter =44 mm). Streamlines map (A) shows an increased helicoidal pattern into the AscAo during systole (arrow) due to the flow jet angulation (α), as shown on the three chamber CineMR image (B, left), and increased flow eccentricity, depicted on the axial images of the AscAo (B, right; CineMR in white square and velocity-encoded images in the red squares). WSS Map (C) shows increased systolic WSS on the anterolateral right wall of the dilated portion of AscAo, which reflects the impaired flow pattern caused by the redirection of the outflow jet. WSS, wall shear stress.

Figure 6

Aortic aneurysm. 4D flow MRI images from a 20-year-old male with AscAo aneurysm and moderate aortic valve insufficiency (AscAo maximum diameter =50 mm, STJ maximum diameter =45 mm). Flow velocity map (A) shows systolic peak flow velocity increased at the center of the aortic root with deceleration through the AscAo. 3D streamlines visualization of systolic blood flow (B) in the thoracic aorta draw a turbulent helicoidal flow pattern along the dilated AscAo during systole; in the black square the rotational streamlines of the turbulent flow are zoomed and visualized with a point of view form the bottom up of the aortic root. Viscous energy loss (C) and WSS (D) maps reconstructed at systolic peak show increased viscous energy loss in the proximal AscAo (red color) and the associated increase of WSS at the downstream aneurysmatic portion of the AscAo (asterix). Pressure gradient is decreased in the aneurysmatic part of the Ao (blue color) during the early systolic phase on the pressure map (E). WSS, wall shear stress.

Figure 7

Coarctation. Maximum intensity projection reconstruction of MR Angiography (A) from a 13-year-old female patient with aortic coarctation (arrow). Systolic streamlines (B) and WSS (C) maps describe an increased flow velocity and increased WSS at coarctation site and at the immediately downstream portion of the vessel, respectively. Increased turbulent kinetic energy loss located pre- and post-coarctation site is shown on Viscous Energy Loss map (D), whereas the pressure map during systolic phase (E) highlights pressure drop after the stenosis. WSS, wall shear stress.

Figure 8

Post-VSRR imaging. 4D flow MRI images from a 37-year-old male patient operated with VSRR and neo-sinus reconstruction. Color encoded streamlines maps shows the flow at systolic peak (A) with increased flow velocity in the aortic root and formation of small organized vortex in the neo-sinus. The operation allowed the restoration of a central laminar flow. The images on the right (B, C and D) demonstrate the progressive deceleration of flow during the following phases and the persistence of the vortices in neo-sinus in diastole.