Technical notePost-stenotic plug-like jet with a vortex ring demonstrated by 4D flow MRI☆
Introduction
Time-resolved, three-directional phase-contrast magnetic resonance imaging (PC-MRI), also known as ‘4D flow MRI’, has been used to investigate various hemodynamic features in the heart and blood vessels [1], [2]. This imaging technique has been successfully employed to visualize streamlines and pathlines based on time-resolved 3D velocity data at each voxel [3], [4], [5]. Several hemodynamic phenomena or indices quantitatively measured using 4D flow MRI have been introduced to aid diagnoses, including wall shear stress distribution on the blood vessel [1], [6], [7], a vortex of blood flow in the main pulmonary artery [8], and a vortex ring in the left ventricle [9]. Understanding flow-related physiology in normal and abnormal states is expected to provide additional insights into a range of cardiovascular diseases [10].
In our current study, we describe the flow structure of a ‘vortex ring’ that was separated from the main stream during the decelerating period of diastole phase in a post-stenotic area. Typically, a vortex ring has been observed in a jet or slug flow after ejection from a nozzle. The presence of a vortex ring structure in the ventricular flow has been detected when the left ventricle is filling as the blood ‘jets’ from the atrium through the mitral valve. A vortex ring in mitral inflow was initially recognized by in vitro visualization [11], [12], [13], and then was confirmed by analyses based on color Doppler mapping [14], [15] and MRI [9], [16], [17].
Another generating mechanism of ‘vortex ring’ structure is found in stenotic flow. A pulsatile flow in stenotic vessel can make a vortex ring at the post-stenotic region during hydrodynamic deceleration at the post-systole phase [18], [19]. This separation of hydrodynamic momentum from the main stream results in a locally faster velocity region, even at near diastole phase, which is briefly described by a ‘plug-like jet’ [18] (see ‘b’ in Fig. 3B). The plug-like jet has been recently introduced in studies of computational fluid dynamics, but it has not yet been investigated using clinical machines, such as MRI, and its clinical importance has not been fully established. In our present study, the hydrodynamic structure of a plug-like jet with a vortex ring was quantitatively measured by 4D flow MRI in two different stenotic phantoms, and then its clinical significance was discussed.
Section snippets
Phantom preparation
The experimental setup consisted of a container to equip a stenotic phantom, a pulsatile pump, and a reservoir (Fig. 1). We tested a pulsatile Newtonian flow in two stenotic phantoms with a 50% or 75% reduction in area, which were designed with consideration of the shape of a sinusoidal function [18]. For more accurate shaping, phantoms were manufactured using a 3D printer (ProJet 3510 SD, 3D Systems, Rock Hill, SC) with VisiJet Crystal, a rigid material (Fig. 2A). They had a circular
Flow characteristics in an experimental setting
For overall observations of the stenotic flows in each phantom, the centerline velocity distribution based on the stream-wise position and pulsatile phase was assessed for each stenotic phantom (Fig. 2C and 2D). At the diastole phase, it is found that the post-stenotic region between + 1 D and + 2 D showed a relatively higher velocity, and this was only found in the 50% stenotic phantom (‘b’ in Fig. 2C). This higher local velocity was not quantitatively high compared to the highest value at the
Discussion
In our current study, the flow structure of a vortex ring was demonstrated in pulsatile stenotic flow phantoms by 4D flow MRI. A vortex ring could be observed at diastole phase in the post-stenotic region of 26.4 mm (1.2 D), but only in the 50% stenotic phantom. From the stream and span-wise velocity field (Fig. 4B and 4C), the detailed flow components of the vortex ring structure could be deduced. The sequential vorticity field map (Fig. 4D) showed how the vortex ring could develop based on the
Acknowledgments
This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF-2015R1A2A2A04003034 and NRF-2013R1A1A1058711). This research was also supported by a grant (2015-504) from the Asan Institute for Life Sciences, Asan Medical Center, Seoul, Republic of Korea.
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