Evaluation of Bicuspid Aortic Valve and Aortic Coarctation With 4D Flow Magnetic Resonance Imaging
Time-resolved, 3D, phase-contrast magnetic resonance imaging (4D flow) is an effective means of evaluating dynamic multidirectional blood flow in the thoracic aorta.1 We have used the technique for characterization of abnormal flow features in a 14-year-old boy with aortic coarctation and bicuspid aortic valve (BAV) but without evidence of aortic stenosis or regurgitation. In addition to the expected flow disturbance in the region of the juxtaductal coarctation (Figure 1), we show an unusual flow feature in the ascending aorta that has not been previously reported in this clinical setting and that may be unique to BAV: 2 discrete nested helices of midsystolic blood flow in a nonaneurysmal aorta (Figure 2).
Exaggerated helical flow has been described in the context of ascending aortic aneurysm, with the majority of cases seen in patients with aneurysmal dilation >4 cm.2 Our patient, however, had only mild dilation of the ascending aorta (3.2 cm at the level of the main pulmonary artery).
BAV occurs in 1% to 2% of the population and may account for more morbidity and mortality than all other congenital cardiac malformations combined. One theory for the elevated risk of aortic aneurysm in these patients is that an increased hemodynamic load is placed on the proximal aorta, resulting in progressive aortic dilatation.3 Previous work has demonstrated that supraphysiological hemodynamics in the form of elevated shear stress, as seen in the radial artery proximal to an arteriovenous dialysis fistula, lead to an increase in vessel size.4 The pronounced helical flow and eccentric systolic jet in the ascending aorta that we have demonstrated may represent an increased hemodynamic burden and could predispose to aneurysm formation by a similar mechanism. Accurate calculation of wall shear stress in the ascending aorta, which may be possible using 4-dimensional flow data sets, and further characterization of blood flow features in patients with nonstenotic BAV are the next steps for evaluating this hypothesis.
Additional Information About Methods
Magnetic resonance imaging data acquisition consisted of an rf-spoiled gradient echo sequence with velocity encoding in 3 spatial directions. Measurements were respiratory compensated and retrospectively gated to the ECG cycle. Scans were performed with an 8-channel cardiac coil and the following imaging parameters: Velocity encoded (VENC)=160 cm/s, fractional field of view (FOV=(300×270) mm2, slab thickness=78 mm, matrix=(256×192×30), spatial resolution= (1.17×1.56×2.60) mm3, temporal resolution=74.4 ms. Parallel imaging (GRAPPA) with an in-plane acceleration factor of 2 was used, with a scan time of 14 minutes. Data analysis and visualization were performed using a commercially available software package (EnSight, CEI, Apex, NC).
Dr Alley receives grant and research funding from GE Healthcare. The other authors report no conflicts.