| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
1 All authors: Department of Clinical Radiology, University of Münster, Albert-Schweitzer-Str. 33, 48129 Münster, Germany.
Received January 29, 2002;
accepted after revision April 17, 2002.
Address correspondence to D. Maintz.
Abstract
 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
MATERIALS AND METHODS. Ten different metallic stents (made of steel, nitinol, tantalum, or cobalt) were implanted in plastic tubes (8 mm). The stent lumina were partially obstructed by wax (CT density, -30 H) resulting in 50-60% in-stent stenoses. The tubes were filled with diluted contrast material (25 mmol/L of gadopentetate dimeglumine or 6 mg I/mL of iodinated contrast material) and placed in a plastic container filled with oil or water, respectively. CT angiography was performed on a four-detector CT scanner (detector collimation, 4 x 1 mm; slice thickness, 1.25 mm; table feed, 4 mm per rotation). MR angiography was performed on a 1.5-T system with a three-dimensional gradient-echo sequence (TR/TE, 4.6/1.8; flip angle, 30°; slice thickness, 1.88 mm). Axial and longitudinal reformations of CT and MR imaging data were evaluated regarding the in-stent attenuation and signal intensity, the visible lumen diameter inside the stent, and the delineation of the stenoses. For comparison, digital radiographic angiography was performed as the gold standard.
RESULTS. The degree and character of stent-related artifacts differed in CT angiography and MR angiography. In CT angiography, only the tantalum stent caused artifacts that obscured the stenosis; in all other cases, the stenoses were visible. In MR angiography, depiction of stenoses was impaired in two steel stents but possible in the tantalum and most nitinol stents.
CONCLUSION. CT angiography is suited for detection of relevant stenoses in steel, cobalt-based, and nitinol stents. MR angiography is superior only in tantalum products.
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
On CT, stents also cause artifacts, which are characterized by an artificial luminal narrowing and decreased intraluminal attenuation values [9]. These artifacts are caused by hardening of the X-ray beam. Although the degree and expression of artifacts for different iliac artery stents have been evaluated with CT and MR imaging [6, 9, 10], to our knowledge no studies have yet assessed the detectability of in-stent stenoses. The purpose of our study was to investigate the detectability of in-stent stenoses in various iliac artery stents on multidetector CT (MDCT) angiography and MR angiography compared with the gold standard, digital radiographic angiography.
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
50%). The exact degrees of stenosis
were determined by digital radiographic angiography. A tube with a wax
stenosis without a stent served as a control.
CT Parameters
For CT, the tubes were filled with iopromide (Ultravist 300; Schering,
Berlin, Germany) diluted with isotonic saline solution to 200 H, closed at
both ends, and positioned in a plastic container filled with commercially
available vegetable oil. The density of the oil was adjusted to -70 H by the
addition of iodized oil (Lipiodol; Guerbet, Sulzbach, Germany) to simulate
perivascular fat. The stents were oriented parallel to the z-axis of
the scanner and placed in the center of the gantry. CT was performed on a
four-detector scanner (Somatom Volume Zoom; Siemens, Forchheim, Germany) using
a detector collimation of 4 x 1 mm, a table feed of 4 mm per rotation
(pitch: 1), a rotation time of 500 msec, a tube current of 160 mAs, and a tube
voltage of 120 kV. A field of view of 100 mm with a 512 x 512 matrix was
chosen, resulting in an in-plane resolution of approximately 0.2 x 0.2
mm2.
Volume data sets with an effective slice thickness of 1.25 mm and a 50% reconstruction overlap were obtained using the scanner's standard adaptive axial image reconstruction algorithm and a medium smooth body kernel (fix indication, 30 f). In addition, multiplanar reformations in the longitudinal axis of the stents were created (thickness, 1.3 mm).
MR Imaging Parameters
For MR angiography, the tubes were filled with gadopentetate dimeglumine
(Magnevist; Schering) and water with a ratio of 1:20. The resulting
concentration of 25 mmol/L, which provides the highest intraluminal signal for
dilution, was chosen according to previous publications
[9,
10]. The tubes were closed at
both ends and positioned in a plastic container filled with water.
MR angiography was performed on a 1.5-T MR imaging system (Magnetom Vision; Siemens, Erlangen, Germany). The phantom was placed in the center of the magnet parallel to the z-axis. Signals were acquired using the body coil.
After localizing sequences in three orthogonal planes, we performed a three-dimensional gradient-echo sequence with a spoiler gradient (fast low-angle shot) in the longitudinal vessel direction (TR/TE, 4.6/1.8; flip angle, 30°; field of view, 390 x 195; matrix, 140 x 512; and resulting in-plane pixel size, 0.8 x 1.4 mm2). Thirty-two partitions of a 60-mm slab resulted in an effective slice thickness of 1.88 mm. Multiplanar reformations through the whole object in the axial and longitudinal directions were calculated (thickness, 3 mm).
Digital Radiographic Angiography
Digital radiographic angiography was performed on an Integris V 3000 system
(Philips, Best, The Netherlands). The tubes were imaged longitudinally and
positioned parallel to the stenoses shown under fluoroscopy (so that the
stenoses were visible in their maximal degree)
(Fig. 1). Images were acquired
in this position with 50 kV and 125 mAs and displayed as hard copies. The
exact degree of lumen narrowing caused by the stenoses was measured by hand
exactly in the middle of the stenosis. These measurements were repeated three
times, and the mean values were calculated.
|
Evaluation of Stenoses and Artifacts
Axial CT images and longitudinal CT reformations as well as axial and
longitudinal MR imaging reformations were assessed. CT angiograms were
displayed with a window width of 450 H and a center of 100 H.
For characterization of intraluminal artifacts, we measured attenuation values on CT angiography and signal intensities on MR angiography using a region-of-interest technique. The region of interest was placed inside the visible stent lumen without covering parts of the stent struts or the stenoses. Lumen reductions by artifactually thickened stent struts were graded on a 5-point scale: 0, no lumen narrowing; 1, lumen narrowing of 0-25%; 2, lumen reduction of 25-50%; 3, lumen reduction of 50-75%; and 4, lumen reduction of 75-100%.
The visibility of stenoses was evaluated by three experienced radiologists in a consensus decision. Stenoses were described as visible, visible with limitations, or not visible.
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
CT Angiography
Axial CT images and longitudinal reformations of all examined stents
revealed that stenosis visibility varies depending on the type of stent
(Fig. 2). Artifactual luminal
narrowing, in-stent attenuation, and visibility of stenoses are summarized in
Table 1. The Strecker tantalum
stent caused the most severe artifacts, with an artificial luminal narrowing
of more than 50% and hyperdense beamlike artifacts inside and outside the
stent. The material placed in the stent lumen for stenosis modeling was not
depicted. All other stents were characterized by decreased intraluminal
attenuation and by 25-50% lumen reduction due to thickening of artifactual
stent strut. Nevertheless, the implanted stenoses were easily visible in these
cases. No significant differences in artifact degree were shown for steel,
nitinol, and cobalt-based alloy stents.
|
|
The gold markers at the ends of the ZA-Stent caused beamlike artifacts inside and outside the stent lumen. The platinum marker of the Symphony stent did not cause a significant artifact.
MR Angiography
Axial and longitudinal multiplanar reformations of all examined stents
revealed visibility of stenosis varied depending on the type of stent
(Fig. 3). Artifactual luminal
narrowing, in-stent signal intensities, and visibility of stenoses are
summarized in Table 2. The
Strecker tantalum stent caused the least extensive artifacts in MR
angiography. Nitinol stents (Strecker, Symphony, Memotherm, Smart, and
ZA-Stent) showed better results on MR angiography than steel stents, although
decreased signal intensity inside the Symphony and the Smart stents made
stenosis delineation difficult. In the Memotherm stent, the intraluminal
signal was decreased compared with the control, but the stenosis was clearly
visible. For the Strecker nitinol stent, the ZA-Stent, and the Wallstent, only
minor luminal narrowing and high intraluminal signal were seen; stenosis
visibility was excellent.
|
|
Steel stents caused pronounced artifacts and decreased intraluminal signal. Stenoses were completely obscured in the Palmaz P394 and P424 stents but visible with limitations in the Perflex stent. The cobalt-based Easy Wallstent caused significant artifacts at the ends of the stent. However, the stenosis in the middle of the stent was clearly detectable.
In contrast to CT, the gold and platinum markers of the Symphony and the ZA-Stent did not cause relevant artifacts.
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
The vascular phantom was designed to simulate in vivo conditions. Nevertheless, some limitations have to be considered. The stents were scanned only in one direction, parallel to the z-axis. Usually, the iliac arteries have an oblique course in patients with tortuosities, possibly even parallel to the axial plane. On CT and MR angiography, a different stent orientation might influence the expression of artifacts. Schulte et al. [11] found that the degree of artifacts caused by coronary stents on MDCT angiography depends significantly on the orientation of the stent relative to the imaging plane. Dimensions of iliac artery stents are different, but comparable effects can be expected. The dependency of stent-related artifacts on stent orientation and gradient orientation in MR imaging has been shown [7].
All our stenoses were handmade. The shape and degree of the stenoses were comparable, but minor variations that should not significantly affect the results of this study were inevitable.
Our experiments were performed with a static fluid model. The effect of flow on artifact expression should be negligible on CT angiography because this technique works with differences in radiopacity and not with flow parameters. On MR angiography, flow features are relevant for the appearance of stent-related artifacts because of intravoxel dephasing. Nevertheless, the effect of flow is not as important a component as susceptibility and radiofrequency eddy currents [8]. Another possible source of artifacts on both CT and MR angiography under in vivo conditions is heavy calcifications in the vessel wall. Further investigation in patient studies might be required to study the impact of calcifications. Parameters for MDCT can vary. In a pilot study, we showed that higher pitches led to stronger artifacts and that the collimation mainly influenced the sharpness of the image, less the degree of artifacts [9]. The parameters used in our study constitute a compromise of a reasonable temporal resolution and low artifacts.
In conclusion, knowledge of the stent type is important in choosing the right imaging technique for noninvasive evaluation and will help in the interpretation of CT or MR images. MDCT is suitable to detect relevant stenoses inside steel, nitinol, and cobalt-based stents. MR angiography is superior to CT only in tantalum stents, comparable to CT in some nitinol stents (Strecker nitinol and ZA-Stent), and inferior in all other tested products. In the future, stent designers should be aware of the influence of materials and designs on imaging artifacts for both CT and MR imaging.
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
CiteULike 
Complore 
Connotea 
Del.icio.us 
Digg 
Reddit 
Technorati What's this?
This article has been cited by other articles:
|
|
 |
|
  M. Lettau, A. Sauer, S. Heiland, S. Rohde, M. Bendszus, and S. Hahnel Carotid Artery Stents: In Vitro Comparison of Different Stent Designs and Sizes Using CT Angiography and Contrast-Enhanced MR Angiography at 1.5T and 3T AJNR Am. J. Neuroradiol., November 1, 2009; 30(10): 1993 - 1997. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. G. Eichhorn, C. Jourdan, S. L. Hill, S. V. Raman, J. P. Cheatham, and F. R. Long CT of Pediatric Vascular Stents Used to Treat Congenital Heart Disease Am. J. Roentgenol., May 1, 2008; 190(5): 1241 - 1246. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. B. Blum, M. Schmook, R. Schernthaner, G. Edelhauser, S. Puchner, J. Lammer, and M. A. Funovics Quantification and Detectability of In-Stent Stenosis with CT Angiography and MR Angiography in Arterial Stents In Vitro Am. J. Roentgenol., November 1, 2007; 189(5): 1238 - 1242. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 O W Hamer, I Borisch, C Paetzel, W R Nitz, J Seitz, S Feuerbach, and N Zorger In vitro evaluation of stent patency and in-stent stenoses in 10 metallic stents using MR angiography Br. J. Radiol., August 1, 2006; 79(944): 636 - 643. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
