Intravascular optical coherence tomography (OCT) is an established tool for the characterization of vascular pathologies, e.g. plaques, in cardiology . Its diagnostic benefit has been proven in different studies [2–6]. It was also used for the assessment of the wall apposition of stents [7, 8]. Conventional angiographic X-ray systems cannot or only insufficient resolve fine implant structures in fluoroscopy, radiography or 3D imaging. This is due to the diameter of stent struts (for flow diverters less than 40 µm ) and metal artifacts in 3D reconstruction . For those cases an additional imaging of the implants by OCT would be beneficial. In our study, we determined the ability of OCT to image structural information of different vascular stents in a phantom study.
2 Materials and methods
A plastic model with bores of different diameters for the in-take of 3 vascular implants was manufactured (see Figure 1). The model has a geometrical extend of 80 mm×50 mm× 15 mm and a straight course of the bores. A translucent plastic material was selected for a low absorption of near infrared light generated by the OCT system. Objects of investigation were 3 vascular stents with different geometrical and structural properties (see Table 1). Implant 1 and 3 were manufactured by laser cutting. The material composition was nitinol with gold markers at the ends. Implant 2 (flow diverter) was a braiding from nitinol wires with two platinum wires as radiopaque markers. The implants were manually deployed in bores with a diameter that was 0.5 mm smaller than the implant’s diameter. For measurement purposes, the model was placed in a saline bath to create appropriate conditions for OCT imaging. A flat panel SIEMENS Artis Zeego System (SIEMENS AG Health-care, Forchheim, Germany) was used for X-ray imaging. Radiography and high resolution cone beam CT (convolution kernel: HU/sharp) were performed. For intravascular OCT a Terumo LUNAWAVE™ System with a Terumo FastView™ catheter (both Terumo Corporation, Shibuya, Japan) was used. The pullback length was 129.8 mm with an image number of 1024. Spatial resolutions of 3D X-ray and OCT datasets are shown in Table 2. Images were analyzed with MeVisLab (MeVis Medical Solutions AG, Fraunhofer MEVIS, Bremen, Germany). To accomplish a better impression of the OCT and CT image information, a virtual projection (variance mode) of the image stacks perpendicular to the catheter axis was done (see Figure 2). This technique enables the visualization of the stent structure in a single image. Although this technique may not be used for in vivo imaging due to the higher tissue absorption, it was useful for the spatial assessment of the depicted implant structure in comparison to the evaluation of single 2D slices. A calculation of the image contrast in the body and marker region was done to objectify the visibility of stent body and marker area based on equation 1 (Michelson contrast method).(1)
Maximum (Imax) and minimum (Imin) intensities were automatically determined in the selected region of interest (stent body or marker region).
Comparison between the generated data sets of all implants showed that OCT images contain at least the same information (visual appearance) as radiography and cone beam CT data sets. Figure 3 A shows a photography of the investigated stent (implant 1). The corpus of the wire frame with its repeating structures can be seen. Also connecting struts between the single segments are visible. Figure 3 B depicts a virtual projection of the OCT image stack. Due to the maximum imaging diameter of the OCT system, it was not possible to detect the whole structure of the stent. Nevertheless, intravascular OCT is able to depict the struts of the implant. Figure 3 D shows the projection of a cone beam CT 3D data set. Stent struts and markers are visible, but metal artifacts impair image quality with thickening and blurring of the stent struts. It is possible to identify stent struts, single connecting struts and radiopaque marker in all four images. Figure 4 A shows a photograph of the flow diverter implant. The corpus structure braided of single nitinol wires, two radiopaque platinum wires and the marker can be seen. Virtual projection of the OCT image shows faintly the structure of the radiopaque wire and the braiding structure at the less tightly braided wires at the end of the device, see figure 4 B. Tight braided structures of the implant corpus cannot be discerned in the virtual projection image. X-ray radiography only shows radiopaque platinum wires and marker structures, but not braided nitinol corpus wires, see figure 4 C. Virtual projection of the cone beam CT does not show nitinol wires but clearly platinum wire structures and visible markers, see figure 4 D. An identification of the third implant based upon the radiography image of the third implant could not be done caused by insufficient contrast (see Figure 5, C). Only radiopaque markers could be seen in the image. Virtual projection of the OCT image could depict all stent struts and markers. Projection of cone beam CT only showed radiopaque markers thickened by artifacts. Contrast values are listed in Table 3 for implant body and marker structures. Contrasts between different imaging techniques are not comparable due to different system parameters. Noise values were measured in areas beside the implant structure.
Intravascular OCT has the ability to image implant structures which cannot or only hardly be seen by conventional X-ray imaging techniques. For less radiopaque implants OCT seems to be a good choice to verify wall apposition, stent expansion as well as possible torsion and structural defects. Its extraordinary high spatial resolution of less than 15 µm enables the imaging of small details like struts of flow diverters and stents. Cone beam CT and radiography cannot exactly depict stent struts due to metal artifacts, less radio opacity and spatial resolutions higher than 150 µm. However OCT imaging is an invasive imaging technique with the possibilty of permanent morbidity. So, this method should only be used, if the benefit markedly outweighs the risk.
The Terumo LUNAWAVE™ imaging system was trial equipment given by the Terumo Corporation.
Funding: This work was partly funded by the Federal Ministry of Education and Research within the ForschungscampusSTIMULATE under grant number 13GW0095A.
Tearney, Guillermo J.; Jang, Ik-Kyung; Bouma, Brett E. (2006): Optical coherence tomography for imaging the vulnerable plaque. In: Journal of biomedical optics 11 (2), pp. 021002-021002-10. Google Scholar
Kawasaki, M.; Bouma, B. E.; Bressner, J.; Houser, S. L.; Nadkarni, S. K.; MacNeill, B. D. et al. (2006): Diagnostic accuracy of optical coherence tomography and integrated backscatter intravascular ultrasound images for tissue characterization of human coronary plaques. In: Journal of the American College of Cardiology 48 (1), pp. 81–88. Google Scholar
Kume, T.; Okura, H.; Kawamoto, T.; Akasaka, T.; Toyota, E.; Watanabe, N. et al. (2008): Relationship between coronary remodeling and plaque characterization in patients without clinical evidence of coronary artery disease. In: Atherosclerosis 197 (2), pp. 799–805.Google Scholar
Manfrini, O.; Mont, E.; Leone, O.; Arbustini, E.; Eusebi, V.; Virmani, R.; Bugiardini, R. (2006): Sources of error and interpretation of plaque morphology by optical coherence tomography. In: The American journal of cardiology 98 (2), pp. 156–159.Google Scholar
Rieber, J.; Meissner, O.; Babaryka, G.; Reim, S.; Oswald, M.; Koenig, A. et al. (2006): Diagnostic accuracy of optical coherence tomography and intravascular ultrasound for the detection and characterization of atherosclerotic plaque composition in ex-vivo coronary specimens: a comparison with histology. In: Coronary artery disease 17 (5), pp. 425–430.Google Scholar
Yabushita, H.; Bouma, B. E.; Houser, S. L.; Aretz, H. T.; Jang, I.; Schlendorf, K. H. et al. (2002): Characterization of human atherosclerosis by optical coherence tomography. In: Circulation 106 (13), pp. 1640–1645. Google Scholar
van der Marel, K.; Gounis, M.; King, R.; Wakhloo, A.; Puri, A. (2014): P-001 High-Resolution Optical and Angiographic CT Imaging of Flow-Diverter Stents for Assessment of Vessel Wall Apposition. In: Journal of neurointerventional surgery 6 (Suppl 1), pp. A21-A21. Google Scholar
Bouma, B. E.; Tearney, G. J.; Yabushita, H.; Shishkov, M.; Kauff-man, C. R.; Gauthier, D. DeJoseph et al. (2003): Evaluation of intracoronary stenting by intravascular optical coherence tomography. In: Heart 89 (3), pp. 317–320. Google Scholar
Fischer, S.; Vajda, Z.; Perez, M. A.; Schmid, E.; Hopf, N.; Bäzner, H.; Henkes, H. (2012): Pipeline embolization device (PED) for neurovascular reconstruction: initial experience in the treatment of 101 intracranial aneurysms and dissections. In: Neuro-radiology 54 (4), pp. 369–382. Google Scholar
Zhang, X.; Wang, J.; Xing, L. (2011): Metal artifact reduction in x-ray computed tomography (CT) by constrained optimization. In: Medical physics 38 (2), pp. 701–711. Google Scholar
About the article
Published Online: 2015-09-12
Published in Print: 2015-09-01
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.