Four-Dimensional Flow MRI for the Evaluation of Aortic Endovascular Graft: A Pilot Study

Abstract

We aimed to explore the feasibility of 4D flow magnetic resonance imaging (MRI) for patients undergoing thoracic aorta endovascular repair (TEVAR). We retrospectively evaluated ten patients (two female), with a mean (±standard deviation) age of 61 ± 20 years, undergoing MRI for a follow-up after TEVAR. All 4D flow examinations were performed using a 1.5-T system (MAGNETOM Aera, Siemens Healthcare, Erlangen, Germany). In addition to the standard examination protocol, a 4D flow-sensitive 3D spatial-encoding, time-resolved, phase-contrast prototype sequence was acquired. Among our cases, flow evaluation was feasible in all patients, although we observed some artifacts in 3 out of 10 patients. Three individuals displayed a reduced signal within the vessel lumen where the endograft was placed, while others presented with turbulent or increased flow. An aortic endograft did not necessarily hinder the visualization of blood flow through 4D flow sequences, although the graft could generate flow artifacts in some cases. A 4D Flow MRI may represent the ideal tool to follow up on both healthy subjects deemed to be at an increased risk based on their anatomical characteristics or patients submitted to TEVAR for whom a surveillance protocol with computed tomography angiography would be cumbersome and unjustified.

Keywords: aortic coarctation; computational fluid dynamic (CFD); endoleak; four-dimensional (4D) flow magnetic resonance; helical flow; thoracic aortic dissection; thoracic aortic endovascular repair (TEVAR); vortical flow.

Figure 1

TEVAR used to treat sub-acute type B aortic dissection in a 60-year-old male. Maximum intensity projection image of flow speed, 3D streamlines visualization. (Left) Peak systolic phase. (Middle) Mid-systolic phase showing a vortex forming in the distal portion of the ascending aorta (black arrowhead). (Right) Maximum intensity projection image from computed tomography scan.

Figure 2

A 70-year-old male patient treated with off-pump arch debranching followed by TEVAR in zone 0 for aortic aneurysm post chronic type B dissection. From the left, a maximum intensity projection image of flow speed with 3D streamlined visualization is shown. An increased velocity in the distal part in the overlapping zone of the two endografts and shape of the aorta causing a slight narrowing of the lumen is shown. On the right, a computed tomography scan demonstrated the position of the endograft.

Figure 3

A 51-year-old male patient with TEVAR. (Left) Maximum intensity projection image of flow speed. (Right) Computed tomography scan showing the position of the endograft.

Figure 4

A 10-year-old female patient treated for aortic coarctation at isthmus level. (Left) Maximum intensity projection image of flow speed, 3D streamlines visualization. (Right) Maximum intensity projection image from computed tomography scan is shown.

Figure 5

A patient treated with endograft for traumatic aortic rupture. (Left and middle) Maximum intensity projection image of flow speed with 3D streamlined visualization. Note the higher velocity in the ascending aorta and flow velocity is also valuable in an endovascular graft. (Right) Computed tomography scan showing the position of the endograft.

Figure 6

Bird-beak configuration specific features: the angle between the undersurface of the endograft and the aortic wall (α angle), and the length of the protruding longitudinal segment (PLS) of the unopposed stent-graft.