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Current Directions in Biomedical Engineering

Joint Journal of the German Society for Biomedical Engineering in VDE and the Austrian and Swiss Societies for Biomedical Engineering

Editor-in-Chief: Dössel, Olaf

Editorial Board: Augat, Peter / Buzug, Thorsten M. / Haueisen, Jens / Jockenhoevel, Stefan / Knaup-Gregori, Petra / Kraft, Marc / Lenarz, Thomas / Leonhardt, Steffen / Malberg, Hagen / Penzel, Thomas / Plank, Gernot / Radermacher, Klaus M. / Schkommodau, Erik / Stieglitz, Thomas / Urban, Gerald A.


CiteScore 2018: 0.47

Source Normalized Impact per Paper (SNIP) 2018: 0.377

Open Access
Online
ISSN
2364-5504
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Increasing the visibility of thin NITINOL vascular implants

A. Boese / G. Rose / M. Friebe / T. Hoffmann / S. Serowy / M. Skalej / W. Mailänder / G. Cattaneo
Published Online: 2015-09-12 | DOI: https://doi.org/10.1515/cdbme-2015-0120

Abstract

New implants for vascular therapy like flow diverters are made of tiny braided wires. The radio opacity of these wires is poor, which makes assessment of implant expansion and exact positioning difficult. Additional markers only allow the estimation of the current device position, but they also induce artefacts that impair the assessment during the intervention and in follow-up examination. A new strategy to increase implant visibility is the braiding of composite wires with a radiopaque core along the whole implant. This paper shows some useful combinations of these new wires on a phantom study with five vascular implants.

Keywords: vascular implants; radio opacity; DFT; composite wires; visibility; imaging

1 Introduction

Radiographic visibility of vascular implants is a major requirement beside the mechanical performance [1]. New implants like flow diverters (FD) are made of braided small NITINOL wires [2]. Due to the thin material, the radio opacity of these devices is poor [3]. To enhance visibility, radiopaque markers made of gold, platinum, or tantalum are integrated in current implants [4]. A new approach is the use of wires produced by “Drawn Filled Tubing” (DFT) (Fort Wayne Metals, Fort Wayne, USA). We investigated the change in visibility of different variants of implants made of DFT wires compared to a state of the art FD in a phantom study.

2 Methods

A proper phantom for the visibility studies was created based on experience und preliminary work of our group [59]. A model of a human skull was filled with gelatine containing a small amount of contrast agent (CA) (Imeron 350, Bracco imaging, Konstanz, Germany) to imitate bone structures and soft tissue. A shrinking tube in a bended shape was placed in the gelatine as a model of the internal carotid artery. For the evaluation, five variants of FD where produced (Acandis GmbH & Co KG, Pforzheim, Germany). Four of the FD were partially or completely made of DFT wires. For fabrication of DFT wires a core of one metal, e.g. platinum, is placed in a tube of another metal, e.g. NITINOL. After thinning by pultrusion the material combination is fixed together [10]. The following variants of implants were used for visibility testing:

  1. . FD state of the art (SoA) made out of 40 NITINOL wires, two platinum wires and end markers

  2. . FD DFT 50 (50% of the wires (24pc) DFT: NITINOL wire with platinum core 70%/30%) and end markers

  3. . FD DFT 100 (100% of the wires (48pc) DFT: NITINOL wire with platinum core 70%/30%) and end markers

  4. . FD DFT 50 10 (50% of the wires (24pc) DFT: NITINOL wire with platinum core 90%/10%), no markers

  5. . FD DFT 100 10 (100% of the wires (48pc) DFT: NITINOL wire with platinum core 90%/10%), no markers

The implants where successively placed in the shrinking tube of the phantom under image guidance. To simulate a venous contrast injection the tube was filled with water first and flushed with a water CA mix (7%) afterwards. The phantom including the device was imaged with two different angiographic c-arm systems (Artis zeego, Artis Q, both SIEMENS Healthcare, Forchheim, Germany) under 2D fluoroscopic mode, 2D radiography, standard 3D cone beam CT and 3D cone beam CT micro (only Artis Q). The imaging setup is shown in Figure 1.

The 3D cone beam CT micro is a scan mode with a small volume of interest but a high resolution due to a 1x1 binning of the detector [11]. The scan parameters of all image protocols are shown in Table 1.

Table 1

Scan parameters off the performed imaging protocols

Imaging setup with skull phantom on the Artis zeego
Figure 1

Imaging setup with skull phantom on the Artis zeego

The resulting images of both systems, all image protocols and of the different implants where prepared for comparison. For 2D images lateral views with the skull base and soft tissue overlaying the implant were acquired. For 3D imaging a slice in the center line view of the implant was chosen. These images where compared with respect to their visibility in 2D and 3D images (Figure 2, 3, 4).

Fluoroscopic images of the implants in a phantom model.
Figure 2

Fluoroscopic images of the implants in a phantom model.

Radiography of the implants in a phantom model
Figure 3

Radiography of the implants in a phantom model

3D cone beam CT micro images of the scanned implants in a phantom model (voxel size 0,118)
Figure 4

3D cone beam CT micro images of the scanned implants in a phantom model (voxel size 0,118)

3 Results

Comparing both imaging systems, a marked increase in spatial resolution can be stated with the new Artis Q, especially in the 3D micro mode. Regarding the visualisation of the FD, the marker of the SoA FD are proper visible in 2D imaging, whereas an impression of the 3D location is hard to get. In 3D imaging the platinum wires cause artefacts, which limit the assessment of the wall apposition or complete opening. DFT wires generally leads to a nearly homogenous appearance of the device. The DFT 50, DFT 100 and DFT 100 10 are well depicted with fluoroscopy and radiography, but the visibility of the DFT 50 10 is very poor in 2D. Reasons for this are the lack of markers and the low amount of platinum inside the wires. With 3D imaging, all variants showed good results. The DFT 100 causes minor artefacts, but these don’t have an impact on the estimation of the wall apposition. The contrast ratio (C) was calculated between the implant corpus (I) and bone structure (B) or soft tissue (S) and between implant marker (M) and bone structure or soft tissue.

Table 2

Evaluation of contrast ratio (C) of the implant corpus (I) and bone structure (B) or soft tissue (S) and between implant marker (M) and bone structure or soft tissue

C=(HCLC)/(HC+LC)

C: Contrast ratio HC: High contrast structure LC: Low contrast structure

The numerical evaluation is shown in Table 2 for all implants and both imaging systems.

4 Conclusion

For an increase of visibility of thin wire implants a new strategy is the use of DFT wires. The experiments showed a good correlation between percentage of DFT and visibility. Important for a sufficient representation in 2D and 3D imaging is the percentage of platinum. Larger amounts of platinum increase the visibility in 2D imaging, but artefacts in 3D imaging are more pronounced. In this regard, a good compromise was shown for both FD designs DFT 50 and DFT 100 10. Moreover, influence of wire composition on mechanical performance should also be considered for final implant design.

Acknowledgement

The work of this paper is partly funded by the Federal Ministry of Education and Research within the Forschungscampus STIMULATE under grant number ’13GW0095A’ and INKA (03IPT7100X).

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About the article

A. Boese: Department of Medical Engineering, Otto-von-Guericke-University, Magdeburg, Germany, tel.: +493916719366


Published Online: 2015-09-12

Published in Print: 2015-09-01


Author's Statement

Conflict of interest: Authors state no conflict of interest. Material and Methods: Informed consent: Informed consent has been obtained from all individuals included in this study. Ethical approval: The research related to human use has been complied with all the relevant national regulations, institutional policies and in accordance the tenets of the Helsinki Declaration, and has been approved by the authors’ institutional review board or equivalent committee.


Citation Information: Current Directions in Biomedical Engineering, Volume 1, Issue 1, Pages 503–506, ISSN (Online) 2364-5504, DOI: https://doi.org/10.1515/cdbme-2015-0120.

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