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Impact of an Aortic Nitinol Stent Graft on Flow Measurements by Time-resolved Three-dimensional Velocity-encoded MRI

Posted Feb 29, 2012 Updated Apr 10, 2023
By Fabian Rengier , Michael Delles , Rol Unterhinninghofen PhD , Sebastian Ley MD , Sasan Partovi MD , Rüdiger Dillmann PhD , Hans-Ulrich Kauczor MD , Hendrik von Tengg-Kobligk MD
9 min read

Rationale and Objectives

Three-dimensional (3D) velocity-encoded cine (VEC) magnetic resonance imaging (MRI) has the potential to quantify 3D hemodynamic aspects known from computational fluid dynamics and to be used to identify hemodynamic risk factors for complications of endovascular aortic repair. The purpose of this study was to investigate the impact of an aortic nickel-titanium (nitinol) stent graft on the accuracy of flow measurements by 3D VEC MRI.

Materials and Methods

A pump generated pulsatile aortic flow in an elastic tube phantom mimicking the aorta. Stacked two-dimensional three-directional VEC MRI (stacked-2D-3dir-MRI), 3D three-directional VEC MRI (3D-3dir-MRI), and gold-standard 2D through-plane VEC MRI were applied before and after the insertion of an aortic nitinol stent graft. Six equidistant levels were analyzed twice by the same reader. The percentage difference of the measured flow rate from the gold standard was defined as the parameter of accuracy.

Results

The overall accuracy of in-stent flow measurements related to the gold standard was −5.4% for stacked-2D-3dir-MRI and −4.1% for 3D-3dir-MRI, demonstrating significant overall underestimation compared to the gold standard ( P = .016 and P = .013). However, flow measurements with the stent graft were significantly overestimated by 4.1% using stacked-2D-3dir-MRI ( P < .001) and by 5.4% using 3D-3dir-MRI ( P = .003) compared to identical measurements without the stent graft. In stacked-2D-3dir-MRI, this positive bias was significantly greater at the proximal and distal ends of the stent graft ( P = .025). In 3D-3dir-MRI, measurements along the whole length of the stent graft were affected ( P = .006). Intraobserver agreement was excellent, with intraclass correlation coefficients of 0.94 for stacked-2D-3dir-MRI ( P < .001) and 0.90 for 3D-3dir-MRI ( P < .001).

Conclusions

Flow measurements within an aortic nitinol stent graft by 3D VEC MRI are feasible, but stent grafts may cause a significant positive bias.

Endovascular repair (EVR) of the thoracic and abdominal aorta has evolved into an accepted alternative to open surgical repair for most adult aortic diseases . Complications of EVR include endoleaks, stent graft migration, and thrombus formation . Previous studies using computational fluid dynamics reported that those complications may be caused by three-dimensional (3D) hemodynamic changes after EVR .

It has been shown that increased pressure in a stent graft leads to an increased drag force on the stent graft that in turn may be followed by stent graft migration . Furthermore, a type III endoleak may occur in cases with high wall stress at the junction of two stent grafts . Finally, areas of reduced velocity within the stent graft might be associated with thrombus formation .

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Materials and methods

Phantom Setup

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Figure 1, Adjusted pump setting to mimic the aortic target waveform on the basis of a volunteer. The difference between the pump setting and the two-dimensional through-plane velocity-encoded cine magnetic resonance imaging measurement is due mostly to the tubing connecting the pump outlet with the elastic aortic tube. The additional late systolic peak is probably caused by a wave reflection within the phantom. Note that the shape of the flow waveform does not influence the average flow rate over the cycle and thus the values calculated in this study.

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Image Acquisition

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Figure 2, Diagram of the six levels and the three stent graft positions: (a) before and after stent graft, (b) proximal and distal ends of stent graft, (c) within stent graft.

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Image Analysis

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Statistical Analysis

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3D VEC MRI with stent graft versus gold standard with stent graft

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3D VEC MRI with stent graft versus 3D VEC MRI without stent graft

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Intraobserver agreement

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Results

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Figure 3, Sagittal view along the tube. Arrows indicate the first and last stent graft struts causing small susceptibility artifacts. The flow direction is from left to right. MRI, magnetic resonance imaging; 3D, three-dimensional; 3dir, three-directional; 2D, two-dimensional.

Figure 4, Representative cross-sectional magnitude and phase images within the stent graft (level 4). The large circle is the region of interest. The smaller circles along the circumference of the large circle can be used to adjust the size of the region of interest. In the phase image, black represents a velocity of −150 cm/s and white +150 cm/s. Arrows indicate velocity noise, with slightly increased velocities in the area of the stent graft struts. The stent graft struts cannot be clearly seen on the single cross-sectional magnitude image in contrast to the sagittal view. MRI, magnetic resonance imaging; 3D, three-dimensional; 3dir, three-directional; 2D, two-dimensional.

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3D VEC MRI with Stent Graft Versus Gold Standard with Stent Graft

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3D VEC MRI with Stent Graft Versus 3D VEC MRI without Stent Graft

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Table 1

Impact of Stent Graft Position

Position 2D Through-plane VEC MRI Stacked-2D-3dir-MRI 3D-3dir-MRI mL/s mL/s % mL/s % Mean 49.9 ± 1.0 2.0 ± 1.9 ∗ 4.1 ± 3.7 ∗ 2.7 ± 3.8 ∗ 5.4 ± 7.4 ∗ A (outside stent graft) 49.3 ± 1.0 0.8 ± 0.7 2.2 ± 1.7 −1.5 ± 2.7 −2.7 ± 5.7 B (ends of stent graft) 50.3 ± 1.2 3.9 ± 1.3 † 7.7 ± 2.7 † 4.4 ± 2.7 † 8.7 ± 5.3 † C (within stent graft) 50.1 ± 0.3 1.2 ± 1.9 2.5 ± 3.6 5.2 ± 1.4 † 10.3 ± 2.8 †

MRI, magnetic resonance imaging; 3D, three-dimensional; 2D, two-dimensional.

Differences between flow measurements by stacked-2D-3dir-MRI and 3D-3dir-MRI in the setup with the stent graft and flow measurements in the setup without the stent graft are given in millilitres per second and percentages for the three stent graft positions.

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Intraobserver Agreement

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Discussion

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Conclusions

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Acknowledgments

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Academic Radiology, Volume 19, Issue 3
Journals General Radiology
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