DOI:10.2214/AJR.06.0584
AJR 2007; 188:W334-W340
© American Roentgen Ray Society
MR Angiography for Patient Surveillance After Endovascular Repair of Abdominal Aortic Aneurysms
Ryan B. Schwope1,
Huntley J. Alper,
Adam D. Talenfeld,
Emil I. Cohen and
Robert A. Lookstein
1 All authors: Department of Interventional Radiology, Mount Sinai Medical
Center, One Gustave L. Levy Pl., Box 1234, New York, NY 10029.
Received April 29, 2006;
accepted after revision August 1, 2006.
Address correspondence to R. A. Lookstein
(robert.lookstein{at}msnyuhealth.org).
WEB This is a Web exclusive article.
Abstract
OBJECTIVE. The objective of this article is to demonstrate how new
imaging sequences and techniques allow characterization of postoperative
complications after endovascular surgery and offer the physician more
information for planning treatment than ever before.
CONCLUSION. MR angiography is an excellent technique for the
surveillance of patients after endovascular repair of abdominal aortic
aneurysms because it is highly sensitive for the detection of postoperative
complications. A thorough knowledge of the physical properties of the
endovascular components is essential to choose the appropriate patients for
this form of surveillance.
Keywords: abdominal aortic aneurysms • dynamic MRI • MDCT • MR angiography • MRI • vascular stents
Introduction
Since first described in 1991
[1], endovascular aneurysm
repair has become increasingly used for the treatment of abdominal aortic
aneurysms. Lifelong surveillance of aortic endografts is imperative because
postoperative complications, including endoleaks, endotension, limb occlusion,
and device migration, are not rare. MDCT angiography (MDCTA) is currently the
standard in many institutions for postoperative surveillance. A technique
using MRI and MR angiography (MRI/MRA) offers an excellent alternative to
MDCTA for patient surveillance after endovascular aneurysm repair
[2]. This technique is most
useful in patients with nickel-titanium (nitinol)-based devices because this
type of endograft is clearly visualized with little or no artifact
[3].
Stainless steel devices produce significant metallic artifact, limiting
evaluation, and are not suited for postoperative surveillance with MRI. New MR
pulse sequences, including "white blood" imaging and time-resolved
imaging, allow assessment of the aneurysm sac with improved ability to
characterize endoleaks and endotension
[4-6].
MRI/MRA is ideal for patients with renal insufficiency and does not use
ionizing radiation. Contrast-enhanced MR angiography offers excellent
sensitivity for the detection of endoleaks and has been shown in previous
reports to be more sensitive than MDCTA
[2,
6].
Abdominal Aortic Endografts
Several endografts are currently available for endovascular repair. At
present, only nitinol-based devices are approved for MRI/MRA
[3]. The safety of stainless
steel endografts and their use with MRI/MRA has not been established. The
AneuRx endograft (Medtronic Medical), the Gore Excluder abdominal aortic
device (Gore Medical), and the Endologix endovascular graft (Endologix) are
currently approved by the U. S. Food and Drug Administration (FDA) for
endovascular aneurysm repair (Figs.
1A,
1B,
1C and
2A,
2B,
2C). All three devices are
composed of a nitinol skeleton and therefore create minimal metallic artifact.
The Medtronic Talent (Medtronic Medical) device is FDA-approved for
investigational use only (Fig.
3A,
3B,
3C) and is also composed of
nitinol, making it MRI/MRA compatible. The Zenith endovascular graft (Cook
Medical) is FDA-approved for endovascular aneurysm repair; however, due to the
stainless steel skeleton, this device is not approved for use with
MRI/MRA.
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Fig. 1A —AneuRx (Medtronic Medical) endovascular device designed to treat
infrarenal abdominal aortic aneurysms. Photograph (courtesy of Medtronic
Vascular, Santa Rosa, CA) (A), radiograph (B), and coronal
maximal-intensity-projection image from contrast-enhanced MR angiography
(C). Modular, self-expanding endovascular device designed to treat
infrarenal abdominal aortic aneurysms. This stent-graft module consists of
smooth woven polyester graft that is joined to nitinol exoskeleton.
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Fig. 1B —AneuRx (Medtronic Medical) endovascular device designed to treat
infrarenal abdominal aortic aneurysms. Photograph (courtesy of Medtronic
Vascular, Santa Rosa, CA) (A), radiograph (B), and coronal
maximal-intensity-projection image from contrast-enhanced MR angiography
(C). Modular, self-expanding endovascular device designed to treat
infrarenal abdominal aortic aneurysms. This stent-graft module consists of
smooth woven polyester graft that is joined to nitinol exoskeleton.
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Fig. 1C —AneuRx (Medtronic Medical) endovascular device designed to treat
infrarenal abdominal aortic aneurysms. Photograph (courtesy of Medtronic
Vascular, Santa Rosa, CA) (A), radiograph (B), and coronal
maximal-intensity-projection image from contrast-enhanced MR angiography
(C). Modular, self-expanding endovascular device designed to treat
infrarenal abdominal aortic aneurysms. This stent-graft module consists of
smooth woven polyester graft that is joined to nitinol exoskeleton.
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Fig. 2A —Gore Excluder (Gore Medical) abdominal aortic device. Photograph
(courtesy of Gore Medical) (A), radiograph (B), and coronal
maximal-intensity-projection image from contrast-enhanced MR angiography
(C). Abdominal aortic device is modular bifurcated endovascular system.
Skeleton is made of nitinol, which spans length of each component.
Polytetrafluoroethylene (PTFE) graft material is attached to nitinol with
polyethylene tape.
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Fig. 2B —Gore Excluder (Gore Medical) abdominal aortic device. Photograph
(courtesy of Gore Medical) (A), radiograph (B), and coronal
maximal-intensity-projection image from contrast-enhanced MR angiography
(C). Abdominal aortic device is modular bifurcated endovascular system.
Skeleton is made of nitinol, which spans length of each component.
Polytetrafluoroethylene (PTFE) graft material is attached to nitinol with
polyethylene tape.
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Fig. 2C —Gore Excluder (Gore Medical) abdominal aortic device. Photograph
(courtesy of Gore Medical) (A), radiograph (B), and coronal
maximal-intensity-projection image from contrast-enhanced MR angiography
(C). Abdominal aortic device is modular bifurcated endovascular system.
Skeleton is made of nitinol, which spans length of each component.
Polytetrafluoroethylene (PTFE) graft material is attached to nitinol with
polyethylene tape.
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Fig. 3A —Medtronic Talent (Medtronic Medical) stent-graft system. Photograph
(courtesy of Medtronic Vascular, Santa Rosa, CA) (A), radiograph
(B), and coronal maximal-intensity-projection image from
contrast-enhanced MR angiography (C). Endograft is self-expanding
modular stent-graft system composed of serpentine-shaped nitinol stents inlaid
in woven polyester fabric.
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Fig. 3B —Medtronic Talent (Medtronic Medical) stent-graft system. Photograph
(courtesy of Medtronic Vascular, Santa Rosa, CA) (A), radiograph
(B), and coronal maximal-intensity-projection image from
contrast-enhanced MR angiography (C). Endograft is self-expanding
modular stent-graft system composed of serpentine-shaped nitinol stents inlaid
in woven polyester fabric.
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Fig. 3C —Medtronic Talent (Medtronic Medical) stent-graft system. Photograph
(courtesy of Medtronic Vascular, Santa Rosa, CA) (A), radiograph
(B), and coronal maximal-intensity-projection image from
contrast-enhanced MR angiography (C). Endograft is self-expanding
modular stent-graft system composed of serpentine-shaped nitinol stents inlaid
in woven polyester fabric.
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Postoperative Complications
Endoleak
The most common complication, occurring in 10-25% of cases
[7], is an endoleak or
continued perfusion of the aneurysm sac, possibly leading to its progressive
expansion. Several endoleak types have been described. A type 1 endoleak is an
incomplete seal of the proximal or distal endograft attachment site
(Fig. 4). A type 2 endoleak is
related to retro-grade flow into the aneurysm from a patent side branch, most
commonly the inferior mesenteric (Fig.
5) or lumbar arteries. A type 3 endoleak results from separation
of the modular components or a tear in the endograft fabric. A type 4 endoleak
is related to the porosity of the device fabric and is rarely seen in the
postoperative setting.
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Fig. 4 —81-year-old man with proximal type 1 endoleak 6 months after
endovascular aneurysm repair with Talent (Medtronic Medical) bifurcated
device. Coronal maximal-intensity-projection image from contrast-enhanced MR
angiography shows large collection of contrast material to left of device,
originating (arrow) from proximal seal zone.
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Fig. 5 —76-year-old man with type 2 endoleak after endovascular aneurysm
repair with Talent (Medtronic Medical) device. Selected frame from coronal
contrast-enhanced MR angiography shows perfusion of aneurysm sac and patent
inferior mesenteric artery (arrow).
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Several studies have shown that MR angiography is more sensitive for
endoleak detection than MDCTA
[2,
6]. Similar research has shown
MR angiography to be more specific for characterizing the extent of an
endoleak and determining its type
[2]. Time-resolved or dynamic
contrast-enhanced MR angiography allows rapid visualization of vascular flow
from which a virtual MR flush angiogram is created
[4-6].
This allows visualization of flow direction in an endoleak and, therefore,
determination of the endoleak type (Fig.
6A,
6B,
6C). In one study at a single
institution from June 2002 to June 2003, 12 patients with documented endoleaks
after endovascular repair of aortic aneurysms (10 abdominal and two thoracic)
underwent time-resolved MR angiography to identify and characterize the
endoleak [4]. All time-resolved
MR angiography findings correlated with conventional angiography in
characterization of the endoleak type.
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Fig. 6A —69-year-old man with type 2 endoleak after endovascular aneurysm
repair with Gore Excluder (Gore Medical). Selected frames from coronal
time-resolved MR angiography sequence show this endoleak originating from
inferior mesenteric artery, which is perfused via arc of Riolan
(arrow, B) to left of endograft.
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Fig. 6B —69-year-old man with type 2 endoleak after endovascular aneurysm
repair with Gore Excluder (Gore Medical). Selected frames from coronal
time-resolved MR angiography sequence show this endoleak originating from
inferior mesenteric artery, which is perfused via arc of Riolan
(arrow, B) to left of endograft.
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Fig. 6C —69-year-old man with type 2 endoleak after endovascular aneurysm
repair with Gore Excluder (Gore Medical). Selected frames from coronal
time-resolved MR angiography sequence show this endoleak originating from
inferior mesenteric artery, which is perfused via arc of Riolan
(arrow, B) to left of endograft.
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Endotension
Previous authors have suggested that aneurysm sac expansion can occur even
in the absence of an endoleak
[7]. This rare phenomenon is
known as endotension, and its mechanism remains controversial. In the
strictest sense, endotension is defined as persistent elevated pressure within
the excluded aneurysm sac in the absence of any demonstrable endoleak, with or
without growth in sac diameter. It has been postulated that endotension may
result from transudation of serous blood components and pressure through the
stent-graft and thrombus, which seal the aneurysm sac from flowing blood. This
fluid accumulation is conceptually analogous to a hygroma (Fig.
7A,
7B,
7C,
7D,
7E,
7F). Other proposed causes of
endotension include missed endoleaks and endograft migration.
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Fig. 7A —82-year-old woman with enlarging aneurysm sac after thoracic
endovascular aneurysm repair with Gore Excluder (Gore Medical). CT angiogram
from July 1999 (A) shows aneurysm diameter of 7.0 cm; CT angiogram from
July 2003 (B) shows aneurysm diameter of 10.0 cm.
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Fig. 7B —82-year-old woman with enlarging aneurysm sac after thoracic
endovascular aneurysm repair with Gore Excluder (Gore Medical). CT angiogram
from July 1999 (A) shows aneurysm diameter of 7.0 cm; CT angiogram from
July 2003 (B) shows aneurysm diameter of 10.0 cm.
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Fig. 7C —82-year-old woman with enlarging aneurysm sac after thoracic
endovascular aneurysm repair with Gore Excluder (Gore Medical). Axial,
contrast-enhanced fat-saturated T1-weighted MR image from August 2003
(C) and axial steady-state free procession (SSFP) image from August
2003 (D). Four years after thoracic endovascular aneurysm repair, CT
angiogram (B) shows no evidence of endoleak, but aneurysm sac has grown
by 3.0 cm. Axial, contrast-enhanced fat-saturated T1-weighted MR image
(C) also shows no endoleak, but axial SSFP image (D) shows
diffuse high signal within aneurysm sac, suggesting endotension.
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Fig. 7D —82-year-old woman with enlarging aneurysm sac after thoracic
endovascular aneurysm repair with Gore Excluder (Gore Medical). Axial,
contrast-enhanced fat-saturated T1-weighted MR image from August 2003
(C) and axial steady-state free procession (SSFP) image from August
2003 (D). Four years after thoracic endovascular aneurysm repair, CT
angiogram (B) shows no evidence of endoleak, but aneurysm sac has grown
by 3.0 cm. Axial, contrast-enhanced fat-saturated T1-weighted MR image
(C) also shows no endoleak, but axial SSFP image (D) shows
diffuse high signal within aneurysm sac, suggesting endotension.
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Fig. 7E —82-year-old woman with enlarging aneurysm sac after thoracic
endovascular aneurysm repair with Gore Excluder (Gore Medical). To confirm
diagnosis of endotension, patient underwent direct aneurysm sac aspiration
with CT guidance (arrow, E) to sample contents of sac.
Approximately 150 mL of serosanguineous fluid was sampled and is shown in
photograph (F). This confirmed diagnosis of sac hygroma, which is one
possible cause of endotension.
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Fig. 7F —82-year-old woman with enlarging aneurysm sac after thoracic
endovascular aneurysm repair with Gore Excluder (Gore Medical). To confirm
diagnosis of endotension, patient underwent direct aneurysm sac aspiration
with CT guidance (arrow, E) to sample contents of sac.
Approximately 150 mL of serosanguineous fluid was sampled and is shown in
photograph (F). This confirmed diagnosis of sac hygroma, which is one
possible cause of endotension.
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Embolization Follow-Up
The radiologist performing MR surveillance must be cognizant of previous
embolization procedures because stainless steel coils create significant bloom
artifact, which limits evaluation of the device and the aortic aneurysm (Fig.
8A,
8B). Platinum embolization
coils are nearly transparent on MRI/MRA and are well suited for patients
undergoing postoperative surveillance (Fig.
9A,
9B).
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Fig. 8A —82-year-old man after endovascular aneurysm repair with AneuRx
endograft (Medtronic Medical). Follow-up imaging shows enlarging aneurysm sac.
Any stainless steel products (coils, bare stents, and covered stents) produce
significant metallic artifact on MRI, preventing evaluation of aneurysm sac
with this technique, as shown in this axial, contrast-enhanced fat-saturated
T1-weighted MR image (A). Patient underwent embolization of one
internal iliac artery with four stainless steel coils. Patient eventually
underwent MDCT angiography (B), which revealed complex type 1 endoleak.
As result, many institutions have shifted to use of only platinum coils for
embolization procedures.
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Fig. 8B —82-year-old man after endovascular aneurysm repair with AneuRx
endograft (Medtronic Medical). Follow-up imaging shows enlarging aneurysm sac.
Any stainless steel products (coils, bare stents, and covered stents) produce
significant metallic artifact on MRI, preventing evaluation of aneurysm sac
with this technique, as shown in this axial, contrast-enhanced fat-saturated
T1-weighted MR image (A). Patient underwent embolization of one
internal iliac artery with four stainless steel coils. Patient eventually
underwent MDCT angiography (B), which revealed complex type 1 endoleak.
As result, many institutions have shifted to use of only platinum coils for
embolization procedures.
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Fig. 9A —72-year-old man after endovascular aneurysm repair with Talent
(Medtronic Medical) device and embolization of type 2 endoleak with platinum
coils. MDCT angiography follow-up shows significant metallic artifact,
preventing detailed visualization and analysis of aneurysm sac to determine if
there is persistent endoleak.
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Fig. 9B —72-year-old man after endovascular aneurysm repair with Talent
(Medtronic Medical) device and embolization of type 2 endoleak with platinum
coils. Axial, contrast-enhanced, fat-saturated T1-weighted MR image provides
optimal visualization of the aneurysm sac because platinum coils produce
little or no artifact on MRI. No endoleak is noted.
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Limb Occlusion
Limb occlusions are less common than endoleaks, occurring in approximately
3% of aneurysms treated with endografts
[8]. This complication occurs
when the limbs of the endograft become twisted or kinked, obscuring blood flow
to the ipsilateral lower extremity and possibly causing acute limb ischemia
(Fig. 10A,
10B).
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Fig. 10A —77-year-old man with sudden onset of left leg pain 3 months after
endovascular aneurysm repair with AneuRx endograft (Medtronic Medical).
Coronal maximal-intensity-projection image from contrast-enhanced MR
angiography (A) and axial contrast-enhanced fat-saturated T1-weighted
MR image (B) show occluded left iliac limb (arrow, B).
Patient underwent urgent femoral-femoral bypass to treat acute limb
ischemia.
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Fig. 10B —77-year-old man with sudden onset of left leg pain 3 months after
endovascular aneurysm repair with AneuRx endograft (Medtronic Medical).
Coronal maximal-intensity-projection image from contrast-enhanced MR
angiography (A) and axial contrast-enhanced fat-saturated T1-weighted
MR image (B) show occluded left iliac limb (arrow, B).
Patient underwent urgent femoral-femoral bypass to treat acute limb
ischemia.
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MR Protocol and Technique
The routine protocol used at our institution for postoperative endovascular
aneurysm repair surveillance includes unenhanced and contrast-enhanced
T1-weighted images and axial T2-weighted and steady-state free procession
(SSFP) images. The contrast material is administered dynamically and divided
into two injections. The first injection is used for time-resolved MR
angiography to aid in endoleak detection, and the second injection is used
with higher-spatial-resolution MR angiography for better depiction of
anatomy.
SSFP imaging produces high signal-to-noise ratio images with T2 and T1
weighting
[4-6].
Chronic thrombus shows low signal intensity, and noncoagulated blood exhibits
high signal intensity on this sequence, making it an ideal tool for
characterizing aneurysm sac contents and determining whether there is an
endoleak or endotension (Fig.
11A,
11B,
11C).
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Fig. 11A —Three different patients (72-year-old woman [A], 68-year-old
man [B], and 73-year-old man [C]) who underwent endovascular
aneurysm repair. Steady-state free procession (SSFP) images of AneuRx
(Medtronic Medical) (A), Talent (Medtronic Medical) (B), and
Gore (Gore Medical) (C) endografts show aneurysm sac and characterize
its contents without artifact from stent-graft. Sac contents appear as low
signal when there is no endoleak and thrombus has begun to involute
(A). When there is endoleak, there is heterogeneous signal within
aneurysm sac, with focal areas of high signal (B). When aneurysm sac
contents are uniformly bright in signal (C) and sac either is stable in
diameter or has started to expand, endotension is thought to be present. In
this scenario, there is typically no enhancement of sac contents after
contrast administration.
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Fig. 11B —Three different patients (72-year-old woman [A], 68-year-old
man [B], and 73-year-old man [C]) who underwent endovascular
aneurysm repair. Steady-state free procession (SSFP) images of AneuRx
(Medtronic Medical) (A), Talent (Medtronic Medical) (B), and
Gore (Gore Medical) (C) endografts show aneurysm sac and characterize
its contents without artifact from stent-graft. Sac contents appear as low
signal when there is no endoleak and thrombus has begun to involute
(A). When there is endoleak, there is heterogeneous signal within
aneurysm sac, with focal areas of high signal (B). When aneurysm sac
contents are uniformly bright in signal (C) and sac either is stable in
diameter or has started to expand, endotension is thought to be present. In
this scenario, there is typically no enhancement of sac contents after
contrast administration.
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Fig. 11C —Three different patients (72-year-old woman [A], 68-year-old
man [B], and 73-year-old man [C]) who underwent endovascular
aneurysm repair. Steady-state free procession (SSFP) images of AneuRx
(Medtronic Medical) (A), Talent (Medtronic Medical) (B), and
Gore (Gore Medical) (C) endografts show aneurysm sac and characterize
its contents without artifact from stent-graft. Sac contents appear as low
signal when there is no endoleak and thrombus has begun to involute
(A). When there is endoleak, there is heterogeneous signal within
aneurysm sac, with focal areas of high signal (B). When aneurysm sac
contents are uniformly bright in signal (C) and sac either is stable in
diameter or has started to expand, endotension is thought to be present. In
this scenario, there is typically no enhancement of sac contents after
contrast administration.
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Imaging Characteristics in the Postoperative Patient
If there is no endoleak or endotension post-operatively, the aneurysm sac
contents will show hypointensity on both T1-weighted and T2-weighted images,
no areas of enhancement after dynamic contrast administration, and diffuse low
signal on SSFP imaging. Typical MRI findings associated with endoleaks include
aneurysm sac contents with intermixed areas of variably increased or decreased
intensity on T1-weighted and T2-weighted images and areas of enhancement after
dynamic contrast administration. These heterogeneous areas appear as increased
signal on SSFP imaging, and correlation with site and direction of flow
enables characterization of the endoleak type. MRI characteristics of
endotension include aneurysm sac contents that show low T1 signal and high T2
signal, no areas of enhancement after dynamic contrast administration, and
uniform high signal on SSFP imaging (Figs.
7A,
7B,
7C,
7D,
7E,
7F and
11A,
11B,
11C). In the setting of limb
occlusion, there is no flow identified in the stent-graft (Fig.
10A,
10B).
Summary
MR angiography and CT angiography are both used for the surveillance of
patients after abdominal aortic aneurysm endovascular repair. Although both
techniques are noninvasive, MR angiography offers improved sensitivity and
specificity for detecting complications in patients after endograft placement
compared with CT angiography. Time-resolved MR angiography characterizes the
endoleak type, and SSFP pulse sequencing detects the presence of an endoleak
or endotension with-out the use of IV contrast material.
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Abstract
Introduction
