Introduction

Helical flow has significant effect on human blood circulation, and is prominent in arterial circulation [1-4]. However, most studies of venous flow patterns rely on hydrodynamic calculations, and clinical reports related to the investigation of helical flow in the human venous system are extremely rare. As shown with central vein computational fluidics with catheter placement, flow status might be related to thrombus formation in the superior vena cava (SVC)[5, 6]. In this study, we aimed to investigate whether helical flow exists in the SVC of healthy young males, and to describe the development and characteristics of this flow pattern with 4D flow MRI.

Methods

Eight healthy young males (age: 25.1 ± 4.9 years) underwent MRI on a 3T MRI (MAGNETOM Skyra, Siemens Healthcare, Erlangen, Germany). Data acquisition consisted of a prototype 4D flow sequence with retrospective ECG gating covering the SVC and brachiocephalic veins (BVs). Typical imaging parameters included: isotropic voxels (1.8 mm×2.2 mm×1.8 mm), 3D field-of-view of 276 mm×340 mm (FH x AP). TE = 2.69 - 2.79 ms, TR = 40.72 - 41.76 ms, α = 8⁰, velocity encoding = 80 cm/s, GRAPPA acceleration factor = 3, Pixel bandwidth = 490 Hz, and approximate scan time = 19 min. Hemodynamic parameters in the SVC, including velocity, velocity vectors, pathlines, streamlines, and flow waveforms, were obtained with a specialized commercial post-processing software（cvi42, version 5.11.3; Circle Cardiovascular Imaging Inc., Calgary, Canada). Data were analyzed by two experienced observers to interpret the flow pattern visually.

Results

Figure 1 summarizes streamline visualization results across all subjects in the cardiac diastole, while Figure 2 shows velocity maps and streamlines chosen at 4 cardiac timepoints in two subjects. These figures demonstrate that helical flow is present in normal SVC flow, occurring in diastole. This phenomenon is mainly located at the upper and middle segments of SVC. With decreased blood velocity, helical flow area had the tendency of gradual extension. Flow pathlines originating from the left and right BVs formed helical flows, respectively, and there were two development types (Figure 3): twining (Type 1) and untwining (Type 2). Type 1 was observed in 5 cases (V2, 5-8), and involved mutual entwining of BV pathlines. Type 2 was observed in the remaining cases, and involved BV pathlines moving along the corresponding side of lumen with nearly no interference. SVC helical flow direction (Figure 4) was identified as counter-clockwise (V1-3, 5-6, left-handed) or clockwise (V4, 7-8, right-handed). SVC flow velocity was lower in the center than at the periphery. The SVC flow waveform was biphasic with some cases having centrifugal diastolic blood flow. Except for V6, the first peak of SVC flow was at cardiac time = 60.9 ± 28.5 ms (30 - 112 ms) later than peak ascending aorta flow. The emergence period of helical flow was divided into three periods corresponding to the 3 red sections in Figure 5E: initially formed helical flow, matured helical flow, and disrupted helical flow. The two BV flow waveforms were similar, but RBV flow rate was greater than that of LBV. In Type 1 helical flow, the difference between the peak flow of the two BVs was less than that of the Type 2 (8.6 ± 5.8 mL/s vs. 25.9 ± 14.5 mL/s, p=0.08).

Discussion

In this study, the helical flow of SVC mainly started at the confluence of two BVs, which always appeared in pairs before gradually converging. It is considered that this natural phenomenon is caused by the mutual influence of both BVs. Aside from the peak flow difference of the two BVs, no obvious differences were observed between Type 1 and Type 2 flow patterns. Helical flow has high transport efficiency, which may help to improve the venous return rate and avoid the reduction of cardiac output [7]. Helical flow has also been recognized to inhibit blood flow disorders and instability [8, 9], which can prevent the development of chronic venous diseases by inhibiting inflammation and thrombosis. In other words, the loss of helical flow may mean an increased risk of diseases such as SVC obstruction.

Conclusion

SVC visualized helical flow in healthy young males through 4D flow MRI might be a characteristic of normal physiological function in this vessel. This phenomenon has the potential to be a reference for predicting early SVC disease.

Acknowledgements

This work was supported by the National Natural Science Foundation of China [grant number 81601462], 1·3·5 project for disciplines of excellence, West China Hospital, Sichuan University [ZYGD18013] and Sichuan Province Science and Technology Support plan [grant number 2016FZ0107].

References

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