Aortic arch hypoplasia has been linked to aneurysm formation after coarctation repair, with abnormal blood flow proposed as a mechanism (1). Time-resolved, 3-dimensional phase-contrast magnetic resonance imaging (4-dimensional flow) allowed dynamic visualization of flow and computation of vectorial wall shear stress in 5 patients after coarctation repair, 4 with arch hypoplasia (2). Symmetrical flow and wall shear stress were demonstrated in the case with normal arch geometry, whereas asymmetrically elevated systolic blood flow and wall shear stress were shown in the cases with arch hypoplasia. These findings suggest that aberrant blood flow may contribute to aneurysm formation in this context.

Four-dimensional flow was used to visualize systolic blood flow and calculate wall shear stress in 5 patients after coarctation repair, 4 with arch hypoplasia. Maximum intensity projection magnetic resonance angiography is provided to demonstrate aortic arch geometry in all cases. Peak systolic velocity–coded streamlines show localized flow velocity throughout the aortic arch (Figure 1), and cross sections show velocity and wall sheer stress at the coarctation repair site; green bars represent the magnitude of shear stress vectors. Normal systolic streamlines and symmetrical wall shear stress were demonstrated in a patient with normal arch geometry (Figure 1A). However, with arch hypoplasia (Figure 1B), accelerated flow through the narrowed arch resulted in elevated and asymmetrical shear stress in the distal arch (Figure 1B).

Asymmetrically elevated wall shear stress on the posterior wall of the aorta downstream of a hypoplastic arch after coarctation repair is further demonstrated in 3 patients, 2 with an aneurysm (Figure 2). Time-resolved evaluation of the case depicted in (Figure 2)A reveals that accelerated flow through the hypoplastic arch impacts the posterior wall and then spins in a large helix within a repair site aneurysm (Online Video 1).