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Imaging Artifacts of Novel Laser-cut Venous Stents: Dependence between Stent Design and Radiofrequency Shielding1Division of Medical Physics, Dept. of Diagnostic and Interventional Radiology, Medical Physics, University Medical Center Freiburg, Faculty of Medicine, University of Freiburg, Freiburg, Germany, 2Dept. of Diagnostic and Interventional Radiology, Medical Physics, University Medical Center Freiburg, Faculty of Medicine, University of Freiburg, Freiburg, Germany
Synopsis
Keywords: In Silico, Artifacts
Motivation: RF shielding up to 95% in stents creates image artifacts that can impact an accurate diagnosis of vessel patency after stent implantation.
Goal(s): The association between design parameters of novel venous stents and their RF shielding properties were investigated.
Approach: Therefore, field simulations and measurements were performed of the relative induced B1 in venous stents with different lengths and cell geometries.
Results: Edge length of the stent cells and their orientation relative to the B1 transmit field determine the amount of RF shielding.
Impact: The work provides information about the correlation of venous stent geometries and RF shielding artifacts. Sequence parameters e.g. excitation flip angle could be adapted for different stent models to possibly achieve higher intraluminal signal in post-implantation MRI venography.
Introduction
MRI is increasingly used for imaging of venous compression syndromes and, consequently, could provide additional information about vessel patency after stent implantation1. Aside from the risk of radiofrequency (RF)-induced heating, image artifacts from RF shielding may impair the diagnostic value of MRI post stent implantation2-3. Image artifacts of all currently available dedicated venous stents showed only minor susceptibility-induced artifacts at 1.5 T4; however, shielding of the transmit B1 field by the stent reduces the intraluminal signal intensity depending on the type of stent and geometry. This work further analyzes these findings and complements them by a systematic finite difference time domain (FDTD) simulation study of the B1 interaction of laser-cut venous stents at both the 1.5 and the 3 T Larmor frequencies.Methods
18 different digitized models of venous stents were simulated with the FDTD software Sim4Life (v7.2.3, ZürichMedTech AG, Zürich, Switzerland). The models mimic laser-cut venous stents, i.e., they were comprised of a regular grid pattern on a cylindrical surface. A cylinder diameter of 16 mm was chosen which corresponds to a typical venous stent in the common iliac vein. The RF shielding was assessed for different stent parameters: cell width (l1), cell length (l2), total length, cell structure, orientation relative to B0 (referred to as orthogonal and parallel in terms of the stents‘ longitudinal axis), and field strength (1.5 T and 3 T). The stents were simulated as perfect electric conductors (PEC) immersed in water. To assess the dependency on the polarization of the transmit field, simulations were performed with both, linear (for 1.5 T) and circular polarized (for 1.5 and 3 T) RF fields. To quantify the RF shielding, B1 within a small ROI in the stent center (B1,ROI) and the undisturbed B1 outside the stent were determined, and the relative induced B1 field (Brel1,ind) in the stent was calculated: Brel1,ind = (B1- B1,ROI)/B1. The results were compared to previous results of in vitro experiments at a 1.5 T system (Siemens Symphony)5.Results
The total length of the stents had no effect on Brel1,ind for both stent orientations (Fig. 1a). Also, Brel1,ind was similar for stents with shifted or regular cell structure (Fig. 1b). However, the stents oriented parallel to B0 showed a linear dependence Brel1,ind on cell width l1 for both, simulation (Rsim2 = 0.99) and experiment (Rexp2 = 0.96) with negative slope (Fig. 2a). For stents positioned orthogonal to B0, no correlation of Brel1,ind and l1 could be found (Rsim2 = 0.68, Rexp2 = 0.64). No correlation was found between Brel1,ind and the cell length l2, regardless of the stent orientation. Stents with the same cell dimensions mostly exhibited a stronger shielding for the simulation compared to the measurement, where the difference remained constant by approximately 30% for identical l1. Furthermore, B0 did not influence Brel1,ind regardless of stent orientation or size (Fig. 3a). In linearly polarized transmit fields Brel1,ind depended linearly on l2 if the stents were oriented orthogonal to B0 (Fig. 3b).Discussion & Conclusion
The results show that the cell size of venous stents strongly influence their RF shielding property. A negative linear correlation was found between the cell length and Brel1,ind in both, measurements and simulations, which was independent of other design parameters (overall stent length, grid structure, Larmor frequency). The simulations with linear polarized B1 fields revealed that only the edge length of the cells parallel to the B1 component of the transmit field determines the amount of RF shielding. As circular polarized transmit fields are used in birdcage body coils, a linear dependence was only found in measurements when the stent was oriented parallel to B0, i.e. both B1 components were parallel to the cell width l1. If the stent is orthogonal to B0, the B1 field is polarized along both the cell length and width such that both parameters influence the RF shielding. The lower Brel1,ind values found in the measurements may be a result of the overestimation of the stent conductivity in the simulations: realistic stents have a finite non-vanishing resistance which reduces the induced currents and, thus, the RF shielding. The study highlights the importance of the stent cell geometry of novel venous stents. With these results, sequence parameters such as the excitation flip angle may be adapted for different stents to achieve a higher intraluminal signal in post-implantation MRI venography.Acknowledgements
This project received funding by The Federal Ministry of Education and Research (BMBF) under project number 13GW0366C.References
1. Zucker EJ, Ganguli S, Ghoshhajra BB, Gupta R, Prabhakar AM. Imaging of venous compression syndromes. Cardiovasc Diagn Ther. 2016;6(6):519-532.
2. Gross D, Scandling B, Leewood AR, Simonetti OP. Computational Modeling of the Thermal Effects of Flow on Radio Frequency-Induced Heating of Peripheral Vascular Stents during MRI. Biomed Phys Eng Express. October 2023.
3. Bartels LW, Smits HFM, Bakker CJG, Viergever MA. MR Imaging of Vascular Stents: Effects of Susceptibility, Flow, and Radiofrequency Eddy Currents. J Vasc Interv Radiol. 2001;12(3):365-371.
4. Guo Y, Jiang X. Simulations of the Stent Artifacts in Magnetic Resonance Imaging. IEEE Trans Magn. 2012;48(2):659-662.
5. Reiss S, Özen AC, Lottner T, Reichert A, Massmann A, Bock M. Magnetic Resonance Imaging of Venous Stents at 1.5 T: Susceptibility Artifacts and Radiofrequency Shielding. Invest Radiol. 2020;55(11):741-746.
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