Cardiovascular systems are thought to be affected to Marfan syndrome (MFS) patients. Previously, helical flow has been considered to be a normal feature in healthy subjects [1]. However, helical flow has also shown to be increased locally or globally due to altered aortic outflow resulted from aortic valve diseases [2]. In this study, we aim to quantitatively investigate the helical flow of MFS patients by measuring relative helicity as well as the related indices.The study population recruited 15 normal subjects (age = 25.0 ± 7.1 years; female/male = 7/8) and 18 MFS patients (age = 29.0 ± 10.6 years; female/male = 6/12). All measurements were performed on a 3 Tesla system (Tim Trio, Siemens, Erlangen, Germany). The aortic root diameter in the sinuses of Valsalva was estimated on cine SSFP images at peak systole. Aortic flow was assessed using 4D phase-contrast MRI (PC-MRI). Scanning parameters were as follows: TR = 10 ms, TE = 2.7 ms, flip angle = 7˚, temporal resolution = 40 ms, spatial resolution = (1.36-1.44) × (1.36-1.44) × 3.5 mm3, and venc = 1.5 m/s in three directions. The 3D blood flow visualization was achieved by reconstructing a 3D PC MR angiogram (MRA) from 4D flow data in a commercial software (EnSight 9.2, CEI Inc., Apex, NC). Fourteen 2D planes along the aorta were manually placed on the MRA and were perpendicular to long axis of aorta (Fig. 1). The quantity of the dot product of velocity ( ) and vorticity ( ) vectors refers to the helicity density: , where . Relative helicity h, is defined as . A helicity_AED mapping is constructed from the relative helicity values at different planes (horizontal) and cardiac frames (vertical) (Fig. 2). The signs of A, E, D were three specific relative helicity cores characterized in the helicity_AED mapping: E sign represents the relative helicity cores shown in the early diastole whereas D sign represents the relative helicity core shown time delay and downstream. The A sign represents an additional relative helicity core other than D, E, or the normal single cores (Fig. 2). We further characterized the helicity_AED mapping by: 1) the center of core, 2) a distance from the core center to the origin (Fig. 2a).The aortic root diameter of MFS group was larger than normal group (38.9 ± 8.9 mm vs. 25.9 ± 4.5 mm, P < 0.05). In figure 3, normal subjects generally met a single-core rule, where the local maximum relative helicity core appeared in AA during mid systole to end systole. For MFS patients, A, E, and/or D signs could be marked in the individual helicity_AED mappings. As listed in Table 1, the single core center of normal group occurred at plane 10.4 ± 1.1 and cardiac frame 6.3 ± 1.0. In patient group, the E sign was presented in AAo (plane = 2.3 ± 0.8, P < 0.001) at early diastolic phase (cardiac frame = 11.7 ± 2.7, P < 0.001), while the D sign displayed cores centers downstream (plane = 11.6 ± 0.7, P < 0.001) and time-delayed (cardiac frame = 8.2 ± 2.1, P < 0.001). The distance index of A and D signs of helicity_AED mappings in MFS patients exhibited significant larger values than normal cores (normal core: 12.1 ± 1.4; A: 15.5 ± 4.2, P < 0.05; D: 14.3 ± 1.5, P < 0.001).

Unlike the visualization of helical flow by streamlines [3, 4], relative helicity is a quantified index for evaluating blood flow rotation. An individual 2D helicity_AED mapping exhibited both temporal and spatial distribution of relative helicity. The D sign indicated the formation of helical flow in DAo of MFS patients which is consistent with the results reported by Geiger et al [5]. However, we found an additional helical flow (A sign) occurred in AA or DAo in 45% MFS patients and this finding is inconsistent with the results by Geiger et al. which indicated an additional local helix flow in the AAo. This can be attributed to the fact that we recruited symptomatic MFS patients other than asymptomatic MFS recruited by Geiger et al. The higher distance index in MFS patients indicated pathological delay or altered eccentricity of relative helicity core in patient group. In conclusion, the quantitative indices of helicity_AED mapping clearly distinguished the altered flow patterns in MFS patients and provided promising approaches for patient managements in the future.