Four-dimensional flow magnetic resonance imaging with wall shear stress analysis before and after repair of aortopulmonary fistula.

We used 4D flow magnetic resonance imaging1 to evaluate a 34-year-old man with a complicated course after aortic coarctation repair including development of an aortopulmonary fistula. Our initial 4D flow study (Figure 1) reveals abnormal vortical-type flow in the aneurysmal distal aortic arch. Evaluation of wall shear stress calculated from near-wall velocity gradients using proprietary software (flow tool, University of Freiburg, Germany; software is experimental and not FDA approved)2 demonstrates skewing of higher shear values along the posterior and left lateral aortic arch, just upstream of a pseudoaneurysm. Two months after this study, the patient presented with hemoptysis and was shown to have an aortopulmonary fistula between the pseudoaneurysm and left upper lobe. Surgical treatment with a left upper lobectomy and repair of the aneurysm with interposition graft was performed. Follow-up 4D flow imaging 4.5 months after surgery (Figure 2) shows smaller dimensions of the distal arch and proximal descending aorta after repair, with significantly decreased circular systolic flow in this region. The velocity profile is more central, and the distribution of wall shear stress is less skewed, with relatively uniform values involving all but the inner curvature. Our group and others have observed abnormal helical and vortical-type flow after coarctation repair,3 but the clinical significance of these findings is unknown. This case suggests that a consequence of vortical-type flow is marginalization of the systolic flow stream, resulting in an uneven distribution of hemodynamic burden in the form of elevated wall shear stress along the aortic wall, which may promote and/or exacerbate vascular pathology.4 Additionally, this case exhibits how 4D flow can be used to assess postoperative physiology by demonstrating clear changes in blood flow and wall shear stress after surgical repair.