In vivo characterization of helical flow in healthy and diseased thoracic aortas from 4D flow MRI data

The bicuspid aortic valve (BAV) is among the most prevalent congenital heart disorders, affecting approximately 1-2% of the population. This condition is often associated with an increased risk of cardiovascular complications, including thoracic aortic aneurysms (TAAs), which are significant risk factors for severe events such as aortic dissection or rupture. The mechanisms behind TAA progression and rupture remain unclear and current guidelines primarily rely on maximum aortic diameter (>5.5 cm) as the main predictor for surgical intervention and monitoring, a parameter which is limited in effectiveness. This limitation underscores the need to investigate further biological and biomechanical factors and their role in these pathologies. Recent studies have focused on the role of intravascular hemodynamics, in particular helical blood flow, given its documented link with arterial pathophysiology. This study aims to evaluate in vivo helicity-based metrics in both healthy and diseased aortas using 4D Flow Magnetic Resonance Imaging (MRI) to deepen our understanding of helical blood flow patterns and their role with the above-mentioned aortic diseases, with the final goal of improving clinical risk assessment. To assess the impact of valve morphology on helical flow, a comparison between a group of healthy subjects, called H, with tricuspid aortic valve and a group of BAV patients without dilation (called BAV) was made. Additionally, to evaluate the impact of aortic dilation, H subjects were compared with TAV patients with aortic dilation (called DIL). To achieve our goals, helicity-based metrics were computed from 4D flow MRI velocity data after a Gaussian interpolation on an unstructured mesh reconstructed from phase-contrast magnetic resonance angiography. All the analyses were performed on the ascending aorta (AAo) segment. Local normalized helicity (LNH) and its absolute value, along with the h-indices (h1, h2, h3, h4), were calculated for the three groups (H, BAV and DIL) and analyzed through correlations with geometrical, demographic, and conventional hemodynamic variables. The main results indicated higher values for |LNH| (p<0.001) and h2 (p<0.001) and lower values for h4 (p<0.05) in H compared to BAV, suggesting that healthy TAV subjects exhibit more pronounced and intense helical flow, as well as more balanced rotational structures, respectively. The comparison between DIL and H groups confirmed the higher values in both |LNH| and h2 for the healthy nondilatated group. In conclusion, the findings of this study contribute to a better understanding of aortic helical flow patterns and, if further supported by longitudinal data, of their role in pathology progression, potentially equipping clinicians with more effective diagnostic tools and decision-making support.