AJR 2002; 178:641-648
© American Roentgen Ray Society
MR Angiographic Findings in Patients with Aortic Endoprostheses
Elmar M. Merkle1,2,
Stefan Klein1,
Stefan C. Krämer1 and
Christian Wisianowsky1
1
Department of Radiology, University Hospitals of Ulm,
Steinhövelstr. 9, 89075 Ulm, Germany.
2
Department of Radiology, University Hospitals of Cleveland/Case Western
Reserve University, 11100 Euclid Ave., Cleveland, OH 44106.
Received July 30, 2001;
accepted after revision September 17, 2001.
Address for correspondence to E. M. Merkle.
Introduction
During the past 10 years, transfemoral endovascular stent-graft placement
has gained increasing acceptance as an alternative treatment to conventional
surgery for aortic disease. Contrary to conventional surgery, stent-graft
placement is associated with procedure-related complications such as stent
migration, kinking, and leakage in the aneurysmal sac that necessitate regular
follow-up. Currently, multislice CT represents the imaging gold standard after
endovascular repair of aortic aneurysms. MR angiography may become an
alternative imaging modality because the development of improved hardware and
software and the bolus-triggered application of contrast medium now permit
satisfactory visualization of both the arterial and venous vascular systems.
An important advantage of MR imaging relates to the low toxicity of its
contrast agents; hence, MR angiography can be used instead of CT, particularly
in patients with renal insufficiency. Finally, the amount of radiation
exposure associated with an imaging modality must be considered, particularly
in young patients, when selecting an imaging modality for a patient who has
undergone endovascular repair of traumatic aortic rupture (Fig.
1A,1B).
View larger version (137K):
[in this window]
[in a new window]
[as a PowerPoint slide]
|
Fig. 1A. —12-year-old girl with traumatic aortic rupture and Excluder
endograft (Gore, Flagstaff, AZ) insertion. Maximum-intensity-projection CT
scan shows proximal stent end at origin of left subclavian artery
(arrow).
|
|
View larger version (110K):
[in this window]
[in a new window]
[as a PowerPoint slide]
|
Fig. 1B. —12-year-old girl with traumatic aortic rupture and Excluder
endograft (Gore, Flagstaff, AZ) insertion. Contrast-enhanced three-dimensional
maximum-intensity-projection MR angiogram reveals interior lumen without
significant susceptibility artifacts. Note patency of left subclavian artery.
Origin of left subclavian artery has been overstented by bare spring portion
of endoprosthesis.
|
|
Technical Considerations and Safety Issues
MR angiography is not commonly used in patients with vascular stents for
many reasons. The image quality of MR angiography is often severely
compromised by susceptibility artifacts that are associated with the stent. In
addition to the composition of the stent alloy, other factors—such as
the magnetic field strength (B0), the orientation of the stent in
relation to the magnetic field, and individual sequence parameters (e.g.,
sequence type, slice thickness, and TE)—may also affect image quality
[1]. In addition, the shielding
effects of a conductive metallic stent allow eddy currents, which may cause
signal reduction from the interior lumen
[2]. Furthermore, certain
safety issues must be considered. These issues include the potential movement
or dislodging of the stent by magnetic field interactions and the heating of
the endoprosthesis by radiofrequency power deposition
[3]. However, because stents
are increasingly manufactured with MR-compatible materials, even MR
imaging—guided percutaneous stent implantation will become increasingly
possible [4].
Radiologists must become familiar with the appearance of specific
metal-related susceptibility artifacts on MR imaging; these artifacts may
differ significantly among the various stent-graft systems. Thus, the most
important factor for correct MR image interpretation is a detailed knowledge
of all inserted endoprostheses (Fig.
2A,2B,2C)
and their compositions (e.g., stent alloys).
View larger version (84K):
[in this window]
[in a new window]
[as a PowerPoint slide]
|
Fig. 2A. —72-year-old man with infrarenal aortic aneurysm, Excluder
stent-graft (Gore, Flagstaff, AZ) insertion, and overstenting of left junction
zone using Easy Wallstent (Boston Scientific, Nattick, MA). Conventional
radiograph shows Excluder stent-graft in aorta. Note Easy Wallstent
(arrows) can also be seen.
|
|
View larger version (85K):
[in this window]
[in a new window]
[as a PowerPoint slide]
|
Fig. 2B. —72-year-old man with infrarenal aortic aneurysm, Excluder
stent-graft (Gore, Flagstaff, AZ) insertion, and overstenting of left junction
zone using Easy Wallstent (Boston Scientific, Nattick, MA). Contrast-enhanced
three-dimensional maximum-intensity-projection MR angiogram obtained during
early phase reveals interior lumen. Note that area covered by Easy Wallstent
shows complete signal loss (arrow).
|
|
View larger version (97K):
[in this window]
[in a new window]
[as a PowerPoint slide]
|
Fig. 2C. —72-year-old man with infrarenal aortic aneurysm, Excluder
stent-graft (Gore, Flagstaff, AZ) insertion, and overstenting of left junction
zone using Easy Wallstent (Boston Scientific, Nattick, MA).
Maximum-intensity-projection MR angiogram obtained during delayed phase shows
complete signal loss (arrow) in area covered by Easy Wallstent.
|
|
Four different levels of MR safety and compatibility for endoprostheses can
be defined. One level, the worst-case scenario, includes the non—MR safe
endoprothesis, which means that the stent is dangerous to the patient because
of its potential for heating, movement, or dislodgment caused by magnetic
field interactions. The other three levels consist of endoprotheses that are
MR-safe, but each level of endoprostheses exhibits a different level of MR
compatibility. MR compatibility includes imaging considerations, such as
imaging quality issues, in situations in which the endoprosthesis is included
in the field of view. Susceptibility artifacts of fully MR-compatible stents
are minimal, allowing not only visualization of structures that are adjacent
to the stent but also clear depiction of the stent lumen (Fig.
3A,3B,3C,3D).
Such endografts are usually composed of nitinol (a nickel—titanium
alloy). Other endoprostheses permit adequate visualization of the structures
that are adjacent to the stent, but these endoprostheses cause complete MR
signal reduction from the interior stent lumen and sharp demarcation of the
cranial and caudal ends (Fig.
4A,4B,4C,4D).
These endoprostheses are usually composed of Elgiloy, a cobalt-based alloy. In
these patients with this type of endoprosthesis, MR angiography is not capable
of showing even a severe stenosis within the stent. However, visualization of
a sharply demarcated high signal intensity distal to a signal void in the
course of a vessel makes the presence of a partly MR-compatible stent likely.
In doubtful cases, obtaining a conventional radiograph of the region of
interest is helpful. Finally, stainless steel-based, non-MR compatible
endoprostheses do not permit visualization of either the vessel lumen or the
adjacent structures (Fig.
5A,5b,5C).
View larger version (81K):
[in this window]
[in a new window]
[as a PowerPoint slide]
|
Fig. 3A. —76-year-old man with infrarenal aortic aneurysm, Excluder
stent-graft (Gore, Flagstaff, AZ) insertion, and non—graft-related
retrograde endoleak (i.e., type II endoleak). Contrast-enhanced
three-dimensional maximum-intensity-projection MR angiogram obtained during
early phase reveals interior lumen with no significant susceptibility
artifacts.
|
|
View larger version (78K):
[in this window]
[in a new window]
[as a PowerPoint slide]
|
Fig. 3B. —76-year-old man with infrarenal aortic aneurysm, Excluder
stent-graft (Gore, Flagstaff, AZ) insertion, and non—graft-related
retrograde endoleak (i.e., type II endoleak). Type II endoleak
(arrow) is depicted only on this maximum-intensity-projection MR
angiogram that was obtained during delayed phase.
|
|
View larger version (80K):
[in this window]
[in a new window]
[as a PowerPoint slide]
|
Fig. 3C. —76-year-old man with infrarenal aortic aneurysm, Excluder
stent-graft (Gore, Flagstaff, AZ) insertion, and non—graft-related
retrograde endoleak (i.e., type II endoleak). Unenhanced axial T1-weighted
gradient-recalled echo MR image shows aortic aneurysm treated with Excluder
stent-graft. Note small susceptibility artifacts.
|
|
View larger version (81K):
[in this window]
[in a new window]
[as a PowerPoint slide]
|
Fig. 3D. —76-year-old man with infrarenal aortic aneurysm, Excluder
stent-graft (Gore, Flagstaff, AZ) insertion, and non—graft-related
retrograde endoleak (i.e., type II endoleak). Contrast-enhanced axial
T1-weighted gradient-recalled echo MR image shows type II endoleak
(arrow).
|
|
View larger version (69K):
[in this window]
[in a new window]
[as a PowerPoint slide]
|
Fig. 4D. —51-year-old man with lower extremity atherosclerotic disease.
Contrast-enhanced three-dimensional maximum-intensity-projection MR angiogram
shows complete signal void within stent with sharp demarcation of cranial and
caudal ends. Note high signal intensity in vessel distal to signal loss in
combination with sharp demarcation; this MR appearance suggests presence of
partly MR-compatible stent-graft. However, even severe stenosis cannot be
ruled out within stent. Adjacent vessels are still visible
(arrow).
|
|
View larger version (145K):
[in this window]
[in a new window]
[as a PowerPoint slide]
|
Fig. 5B. —68-year-old man with infrarenal aortic aneurysm and Zenith
stent-graft (Cook, Bloomington, IN) insertion. Coronal gradient-echo MR image
obtained using true fast imaging with steady-state precession shows huge
susceptibility artifacts that preclude visualization of either interior stent
lumen or adjacent structures.
|
|
The Variety of Endoprostheses
At least 13 different endograft systems have been used for treatment of
aortic disease [5].
Wallstent
Noncovered wallstents, such as Easy Wallstent (Boston Scientific, Nattick,
MA), are usually made of a cobalt-based alloy and are commonly used for
treatment of aortic stenosis (Figs.
2A,2B,2C
and
4A,4B,4C,4D).
Most wallstents are partially MR compatible and allow visualization of the
structures that are adjacent to the stent; however, wallstents cause signal
reduction from the interior lumen
[2].
Excluder Endograft
The Excluder (Gore, Flagstaff, AZ) is a stent-graft with a spiral frame
that is composed of nitinol and covered inside and out by a polyester graft
fabric that is heat-sealed. This system is fully MR compatible and allows
clear visualization of both the interior vessel lumen as well as the adjacent
structures [6] (Figs.
1A,1B,2A,2B,2C,3A,3B,3C,3D).
Ancure Stent-Graft
The Ancure device (EVT/Guidant, Menlo Park, CA) is a nonsupported
stent-graft with proximal and distal hooklike fixation devices made of
Elgiloy. The graft is made of Dacron (DuPont, Wilmington, DE) and has crimped
legs. Only adjacent structures can be visualized, whereas the interior lumen
appears dark because of signal reduction (Kampschulte A et al., presented at
the German Roentgen Ray Society meeting, May 2001).
AneuRx Stent-Graft
The AneuRx stent-graft (Medtronic, Sunnyvale, CA) is a modular stent-graft
with an external skeleton that is composed of nitinol and an internal graft
that is composed of Dacron. This system is partly MR compatible; visualization
of the interior vessel lumen is insufficient, but the depiction of the
adjacent structures is excellent (Fig.
6A,6B,6C,6D).
View larger version (100K):
[in this window]
[in a new window]
[as a PowerPoint slide]
|
Fig. 6A. —68-year-old man with infrarenal aortic aneurysm and AneuRx
stent-graft (Medtronic, Sunnyvale, CA) insertion. Conventional radiograph
shows AneuRx stent with figure eight—shaped metallic connections
(arrows).
|
|
View larger version (70K):
[in this window]
[in a new window]
[as a PowerPoint slide]
|
Fig. 6B. —68-year-old man with infrarenal aortic aneurysm and AneuRx
stent-graft (Medtronic, Sunnyvale, CA) insertion. Contrast-enhanced
three-dimensional maximum-intensity-projection MR angiogram obtained during
early phase shows multiple bandlike signal voids in both stent limbs
(arrows). These voids probably result from figure
8A,8B,8C,8D—shaped
metallic connections between Dacron (DuPont, Wilmington, DE) graft and nitinol
skeleton.
|
|
View larger version (80K):
[in this window]
[in a new window]
[as a PowerPoint slide]
|
Fig. 8A. —68-year-old man with infrarenal aortic aneurysm treated by
Vanguard stent-graft (Boston Scientific, Nattick, MA) insertion. Right iliac
limb was extended by insertion of additional Talent stent-graft (World Medical
Manufacturing, Sunrise, FL). Conventional radiograph shows Vanguard
stent-graft (solid arrows) and Talent stent (dashed
arrows).
|
|
View larger version (111K):
[in this window]
[in a new window]
[as a PowerPoint slide]
|
Fig. 8B. —68-year-old man with infrarenal aortic aneurysm treated by
Vanguard stent-graft (Boston Scientific, Nattick, MA) insertion. Right iliac
limb was extended by insertion of additional Talent stent-graft (World Medical
Manufacturing, Sunrise, FL). Contrast-enhanced three-dimensional
maximum-intensity-projection MR angiogram obtained during early phase reveals
interior lumen without significant susceptibility artifacts. Note
kinking-related stenosis (arrow) at origin of right common iliac
artery.
|
|
View larger version (125K):
[in this window]
[in a new window]
[as a PowerPoint slide]
|
Fig. 8C. —68-year-old man with infrarenal aortic aneurysm treated by
Vanguard stent-graft (Boston Scientific, Nattick, MA) insertion. Right iliac
limb was extended by insertion of additional Talent stent-graft (World Medical
Manufacturing, Sunrise, FL). Contrast-enhanced three-dimensional
maximum-intensity-projection MR angiogram obtained during delayed phase more
clearly depicts occlusion of left common iliac artery (arrow)
together with refilling of external iliac artery than image obtained during
early phase (C).
|
|
View larger version (76K):
[in this window]
[in a new window]
[as a PowerPoint slide]
|
Fig. 8D. —68-year-old man with infrarenal aortic aneurysm treated by
Vanguard stent-graft (Boston Scientific, Nattick, MA) insertion. Right iliac
limb was extended by insertion of additional Talent stent-graft (World Medical
Manufacturing, Sunrise, FL). Maximum-intensity-projection CT scan shows
occlusion (arrow) of common iliac artery on left side with refilling
of external iliac artery via collateral vessels. High-density values in
occluded artery represent calcified plaques.
|
|
Lifepath
The Lifepath stent-graft (Baxter, Morton Grove, IL) is a combination of
self-expandable and balloon-expandable devices made with a metal frame of
Elgiloy and stainless steel. The stent system is MR safe, but not MR
compatible, thus prohibiting adequate visualization of both the interior lumen
and the adjacent structures (Kampschulte A et al., presented at the German
Roentgen Ray Society meeting, May 2001).
Talent Stent-Graft
The Talent System (World Medical Manufacturing, Sunrise, FL) is a
custom-made, supported, self-expandable device with a metal frame composed of
multiple nitinol stents inside the main body of the Dacron graft. The system
is fully MR compatible [6]
(Fig.
7A,7B,7C,7D).
Of note is the marked but homogeneous signal decrease within the stent.
View larger version (105K):
[in this window]
[in a new window]
[as a PowerPoint slide]
|
Fig. 7B. —82-year-old man with thoracic aortic aneurysm and Talent
stent-graft (World Medical Manufacturing, Sunrise, FL) insertion.
Contrast-enhanced maximum-intensity-projection CT scan reveals stent-graft in
close proximity to origin of left subclavian artery (arrow).
|
|
View larger version (85K):
[in this window]
[in a new window]
[as a PowerPoint slide]
|
Fig. 7C. —82-year-old man with thoracic aortic aneurysm and Talent
stent-graft (World Medical Manufacturing, Sunrise, FL) insertion.
Contrast-enhanced three-dimensional maximum-intensity-projection MR angiogram
obtained during early phase reveals decreased signal of interior lumen without
significant susceptibility artifacts. Note truncus bicaroticus as anatomic
variant.
|
|
View larger version (100K):
[in this window]
[in a new window]
[as a PowerPoint slide]
|
Fig. 7D. —82-year-old man with thoracic aortic aneurysm and Talent
stent-graft (World Medical Manufacturing, Sunrise, FL) insertion.
Contrast-enhanced three-dimensional maximum-intensity-projection MR angiogram
obtained during delayed phase shows refilling of overstented left subclavian
artery (arrow) via left vertebral artery.
|
|
Vanguard Stent-Graft
The Vanguard stent-graft (Boston Scientific) is a supported,
self-expandable device made of a nitinol frame covered by thin Dacron graft
material. The system is fully MR compatible
[7,8,9]
(Fig.
8A,8B,8C,8D).
Zenith Stent-Graft
The Zenith device (Cook, Bloomington, IN) is a supported, self-expandable
stent-graft with multiple stainless steel Z stents placed inside the graft.
The stent system is MR safe but not MR compatible because it causes major
susceptibility artifacts (Fig.
5A,5B,5C).
Corvita Endovascular Graft
The Corvita endovascular graft (Boston Scientific) is a cylindric
self-expandable device with flared ends. It is made of Elgiloy and is lined
with polycarbonate urethane microfibrils. Only the adjacent structures can be
seen on MR imaging because the interior lumen appears dark as a result of
signal reduction (Merkle EM, unpublished data).
Additional Devices
In the general literature, to the best of our knowledge, no information
exists regarding MR compatibility for other stent-grafts. For example, MR
compatibility is possible but has not yet been proven for the Quantum LP
stent-graft (Cordis, Warren, NJ), which is a nitinol-based device. With an
Algiloy-based device, such as the Endologyx device (Bard, Covington, GA), we
have been able to see the adjacent structures, but this observation has not
yet been proven. The Anaconda stent-graft (Sulzer Vascutech, Bad Soden,
Germany), a nitinol-based device, might have full MR compatibility but has not
yet been proven. The Ella stent-graft (Ella-CS; Hradec Kralove, Czech
Republic), a stainless steel—based device, might be non-MR
compatible.
Combination of Numerous Stent Systems
Treatment of kinking, leakage, or migration oftentimes requires the
percutaneous insertion of additional stent-grafts, thus causing additional
susceptibility artifacts and occasionally leading to pitfalls in MR imaging.
Obtaining a conventional radiograph of the region of interest may be helpful
in cases for which MR findings are indeterminate (Figs.
2A,2B,2C
and
8A,8B,8C,8D).
References
-
Lewin JS, Duerk JL, Jain VR, Petersilge CA, Chao CP, Haaga JR.
Needle localization in MR-guided biopsy and aspiration: effects of field
strength, sequence design, and magnetic field orientation.
AJR
1996;166:1337
-1345[Abstract/Free Full Text]
-
Klemm T, Duda S, Machann J, et al. MR imaging in the presence of
vascular stents: a systematic assessment of artifacts for various stent
orientations, sequence types, and field strengths. J Magn Reson
Imaging 2000;12:606
-615[Medline]
-
Shellock FG, Shellock VJ. Metallic stents: evaluation of MR imaging
safety. AJR
1999;173:543
-547[Abstract/Free Full Text]
-
Buecker A, Neuerburg JM, Adam GB, et al. Real-time MR fluoroscopy
for MR-guided iliac artery stent placement. J Magn Reson
Imaging 2000;12:616
-622[Medline]
-
Uflacker R, Robison J. Endovascular treatment of abdominal aortic
aneurysms: a review. Eur Radiol
2001;11:739
-753[Medline]
-
Merkle EM, Klein S, Wisianowsky C, et al. Magnetic resonance
imaging versus multi-slice computed tomography in patients with vascular
endoprostheses in the thoracic aorta: in vitro measurements and in vivo
results in 13 patients. (in press) J Endovasc
Ther
-
Engellau L, Larsson EM, Albrechtsson U, et al. Magnetic resonance
imaging and MR angiography of endoluminally treated abdominal aortic
aneurysms. Eur J Vasc Endovasc Surg
1998;15:212
-219[Medline]
-
Hilfiker PR, Quick HH, Pfammatter T, Schmidt M, Debatin JF.
Three-dimensional MR angiography of a nitinol-based abdominal aortic stent
graft: assessment of heating and imaging characteristics. Eur
Radiol 1999;9:1775
-1780[Medline]
-
Engellau L, Olsrud J, Brockstedt S, et al. MR evaluation ex vivo
and in vivo of a covered stent graft for abdominal aortic aneurysms:
ferromagnetism, heating, artifacts, and velocity mapping. J Magn
Reson Imaging 2000;12:112
-121[Medline]
CiteULike 
Complore 
Connotea 
Del.icio.us 
Digg 
Reddit 
Technorati What's this?
This article has been cited by other articles:
|
|
|
|
 
S. W. Stavropoulos and S. R. Charagundla
Imaging Techniques for Detection and Management of Endoleaks after Endovascular Aortic Aneurysm Repair
Radiology,
June 1, 2007;
243(3):
641 - 655.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
J. Thomas, T. A. Jaffe, and E. K. Paulson
Gadolinium-Enhanced CT Angiography of Endovascular Stent-Grafts
Am. J. Roentgenol.,
April 1, 2005;
184(4):
1178 - 1180.
[Full Text]
[PDF]
|
|
|
Top
Introduction
Technical Considerations and...
