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Quantification of helical flow and aortic tortuosity using 4D Flow MRI1Department of Medical and Health Sciences, Linköping University, Linköping, Sweden, 2Center for Medical Image Science and Visualization (CMIV), Linköping, Sweden, 3University Hospital Linköping, Linköping, Sweden
Synopsis
Due to the complex anatomy of the heart, its valves, and the aorta, the blood flow in the aorta is similarly complex and can exhibit a swirling, or helical flow pattern. Previous studies have shown that aortic geometry changes with age. As the shape of the aorta is complex and evolves over time, the aim of this study is to examine the relationship between the shape of the aorta and helical flow. The results show that the aorta gets more tortuous with age, and that the increased tortuosity is associated with increased helicity.
Introduction
The large volume of blood that is being pumped through the body exits the heart at high velocity, and sometimes exhibits spiral shaped flow, also known as helical flow. Helical flow can be quantified using 4D Flow MRI and previous studies have indicated that flow helicity is related to age and whether the aorta is gothic-shaped, crook-shaped, or cubic-shaped1,2. We hypothesized that the tortuosity of the aorta is an age-related descriptor of the shape of the aorta that predicts the degree of flow helicity.Method
4D Flow MRI velocity data covering the whole aorta from aortic valve to the iliac bifurcation was acquired for 10 young (age: 23±2 y. o.) and 12 older (age: 70±3 y. o.) male volunteers with no prior history of cardiovascular disease using a 3T Philips Ingenia scanner (Philips Healthcare, Best, the Netherlands) using a free-breathing, navigator gated sequence. Subjects received a gadolinium contrast agent prior to the acquisition. Scan parameters included: Candy cane view adjusted to cover the whole aorta, VENC 100-180 cm/s, flip angle 15°, echo time 2.5-3.1 ms, repetition time 4.4-5.0 ms, and spatial resolution 2.5 x 2.5 x 2.5 mm. Flow data was corrected for background phase offset errors using 4th order polynomial fitting.
Aortic tortuosity was computed by separating the aorta into three distinct segments: the ascending aorta, defined as the region between the sinotubular junction and the brachiocephalic trunk; the aortic arch, defined as the region between the brachiocephalic trunk and the left subclavian artery; and the descending aorta, defined as the region between the left subclavian artery and the aortic bifurcation (see Figure 1). Tortuosity was defined as the ratio between the straight-line distance and the centerline distance of each segment which represent the mean flow path (see Figure 1). A tortuosity value of 1 indicates a perfectly straight aorta. A sensitivity test was performed to assess the degree to which variations in the definition of the three aortic segments affected the tortuosity computation. This was performed by randomly moving each anatomical landmark within a ± 15 mm range and computing the resulting maximum error as the standard error.
Helicity, or the direction of the helical flow, was computed as the localized normalized helicity (LNH), $$$ = v\cdot\omega/(|v||\omega|) $$$ where v is the local velocity vector and is the vorticity, which is the curl of the velocity. LNH is a pseudovector where positive and negative values indicate counterclockwise and clockwise rotation respectively.
A one-way analysis of variance (ANOVA) test was used to assess the difference between the two age groups. Linear regression was used to assess the relationship between LNH and tortuosity. A p-value < 0.05 was considered significant.
Results
The mean ± standard deviation for aortic tortuosity was 0.89±0.04 and 0.86±0.03 for younger and older normal volunteers, respectively. This difference was borderline significant (p=0.052). The sensitivity test assessing the manual definition of aortic segments used for tortuosity computations revealed that changes up to 15 mm had a minor influence on tortuosity. Segmental average LNH increased with age and had a significant difference (p < 0.05) in all aorta segments except the ascending aorta (Table 1). LNH was correlated to segmental tortuosity with R2 = 0.48 (Figure 2).Discussion and Conclusion
Adding to the work by Garcia et al that estimated the absolute volumetric LNH in normal volunteers and patients with bicuspid aortic valves3, this study measured the average LNH, which retains directional information, in two well-defined age groups. Our data helps define reference values for both tortuosity and LNH in young and old subjects without cardiovascular disease.
In conclusion, this study showed that aortic tortuosity changes with age, and that an increase in tortuosity is associated with increased flow helicity. This supports the hypothesis that tortuosity of the aorta is an age-related descriptor of the shape of the aorta that is related to the degree of flow helicity.
Acknowledgements
No acknowledgement found.References
1. Lorenz R, Bock J, Barker AJ, von Knobelsdorff-Brenkenhoff F, Wallis W, Korvink JG, Bissell MM, Schulz-Menger J, Markl M. 4D flow magnetic resonance imaging in bicuspid aortic valve disease demonstrates altered distribution of aortic blood flow helicity. Magn. Reson. Med 2014;71:1542–1553. doi: 10.1002/mrm.24802.
2. Frydrychowicz A, Berger A, Munoz del Rio A, Russe MF, Bock J, Harloff A, Markl M. Interdependencies of aortic arch secondary flow patterns, geometry, and age analysed by 4-dimensional phase contrast magnetic resonance imaging at 3 Tesla. Eur. Radiol 2012;22:1122–1130. doi: 10.1007/s00330-011-2353-6.
3. Garcia J, Barker AJ, Collins JD, Carr JC, Markl M. Volumetric quantification of absolute local normalized helicity in patients with bicuspid aortic valve and aortic dilatation. Magn. Reson. Med. 2016. doi: 10.1002/mrm.26387.
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