AJR 2001; 177:607-614
© American Roentgen Ray Society
Endovascular Repair of Abdominal Aortic Aneurysms
Assessment with Multislice CT
Jonas Rydberg1,
Kenyon K. Kopecky1,
Matthew S. Johnson1,
Nilesh H. Patel1,
Scott A. Persohn1 and
Stephen G. Lalka2
1
Department of Radiology, Indiana University Hospital, Rm. 0279, 550 N.
University Blvd., Indianapolis, IN 46202-5253.
2
Department of Surgery, Indiana University Hospital, Indianapolis, IN
46202-5253.
Received August 2, 2000;
accepted after revision February 13, 2001.
Presented at the annual meeting of the American Roentgen Ray Society,
Washington, DC, May 2000.
Address correspondence to J. Rydberg.
Introduction
Ruptured abdominal aortic aneurysms are the 15th leading cause of death in
the United States and the 10th leading cause of death in men older than 55
[1]. For decades, the standard
treatment for abdominal aortic aneurysms has been open surgery. Many
investigators have worked to develop a less invasive treatment based on
stents, which has been used to treat arterial stenoses. In 1991 Parodi et al.
[2] reported the first series
of patients with abdominal aortic aneurysms treated with stent-grafts (covered
stents). Endovascular repair was first offered to patients with significant
comorbid conditions, who were at high risk for open surgical repair
[3]. Now endovascular repair is
being offered to many patients, regardless of their comorbidities.
Stent-Grafting Technique
Endovascular repair is performed via cutdown on the femoral arteries. A
stent-graft (Fig.
1A,1B),
consisting of a fabric sleeve with a supporting wire frame, is inserted over
guidewire into the aorta. The devices are either balloon-expandable or
self-expanding. The graft excludes the aneurysm from the aortic lumen, thereby
decreasing the pressure within the aneurysm and reducing the chance of
rupture.
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Fig. 1A. —Photographs of bifurcated unibody (A) and modular
multicomponent (B) stent-grafts. Ancure stent-graft (Guidant, Menlo
Park, CA) has supporting metal stents only at ends of device (solid
arrows). Radioopaque markers are attached to graft (open arrow).
(Courtesy of Guidant)
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Fig. 1B. —Photographs of bifurcated unibody (A) and modular
multicomponent (B) stent-grafts. Zenith stent-graft (Cook, Bloomington,
IN) (left) consists of polyester graft with sewed-on self-expanding stainless
steel wire frame. Beneath main body, separate "leg" (open
arrow) is seen, which is inserted into main body of stent-graft. Small
hooks (arrows) welded into wire frame make it possible to anchor
stent-graft to aortic wall above renal arteries. Fabric does not cover cranial
part of wire frame, to allow passage of blood to renal arteries. AneuRx
stent-graft (Medtronic, Santa Rosa, CA) (right) consists of polyester fabric
covered by self-expanding nickel—titanium wire frame. Separate leg can
be introduced and inserted into short leg of main device via contralateral
femoral artery. On March 16, 2001, Guidant suspended production of the Ancure
stent; the Food and Drug Administration (FDA) is continuing its evaluation as
the AJR goes to press. Also, as the journal goes to press, the FDA is
working with Medtronic AVE to obtain relevant data regarding problems reported
with the AneuRx stent.
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Stent-grafts may be classified into bifurcated or tubular unibody devices,
modular multicomponent devices, and aortouniiliac devices used in combination
with a conventional femoral-to-femoral bypass graft
[4].
Preplacement Imaging
The preoperative assessment of patients being considered for endovascular
repair requires high-quality imaging combined with accurate quantitative
measurements. The width of the aneurysm neck, the aneurysm's length, and the
diameter of relevant arteries have to be evaluated
[5]
(Fig. 2). Not all patients with
abdominal aortic aneurysms are candidates for this type of treatment, and
imaging must be able to accurately differentiate candidates from
noncandidates. For example, an iliac artery with a large diameter may preclude
graft fixation, and an iliac artery with a small diameter may not accommodate
the deployment sheath. Other examples of absolute or relative
contraindications to stent-graft surgery are tortuosity along the deployment
route and angulation or thrombosis within the aorta (Figs.
3,4,5).
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Fig. 2. —Schematic drawing showing measurements that must be obtained
before selection of stent-graft components. Key measurements are widths of
aneurysmal necks (D1 and D3), widths of common iliac arteries (D4 and D5),
proximity of aneurysm to renal arteries (L1), length of aneurysm (L2), and
length of common iliac arteries (L3 and L4). Maximal width of aneurysm (D2)
does not influence choice of stent-graft.
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Fig. 3. —76-year-old man with abdominal aortic aneurysms.
Maximum-intensity-projection image shows highly angulated infrarenal aneurysm
(lines show angle for measurement), which excluded patient from stent-graft
surgery.
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Fig. 4. —82-year-old man with abdominal aortic aneurysms. Coronal
reformat depicts circumferential thrombus (solid arrows) stretching
up to renal arteries (open arrows), which is relative
contraindication to stent-graft surgery.
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Fig. 5. —80-year-old man with abdominal aortic aneurysms. Oblique
surface rendering shows severe bilateral tortuosity of external iliac arteries
(arrows), suggesting major obstacle to access-wire passage. At
attempted stent-graft surgery, guidewires could not be passed through tortuous
segments.
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In addition, for patients who remain candidates, imaging must provide
accurate measurements to determine the diameters and lengths of the modular
stent-graft components that will be used in the repair
(Fig. 6). The most accurate
measurements are achieved from sagittal and coronal reformats. Measurements
off three-dimensional reconstructions can be misleading. A good quality
multislice CT scan will serve as a guide in selecting appropriate patients for
aortic stenting; this scan also is needed to tailor graft specifications to
patient anatomy. Additional studies, such as sonography, MR imaging, plain
radiography, and calibrated angiography, may supplement information obtained
on CT.
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Fig. 6. —87-year-old man with distal abdominal aortic aneurysms.
Curved coronal reformatted CT image depicts aneurysm neck lengths (first and
last segment), aneurysm length (middle segment), and common iliac artery
length (third segment), as well as diameters. Vessel diameters should be
measured perpendicular to long axis of vessel. Many CT vendors provide
software on workstations to make these measurements, but some controversy
exists regarding validation of measurements done on curved reformats.
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Multislice CT Technique
Multislice CT represents a major developmental step in CT imaging. The
technique allows for faster scanning, fewer motion artifacts, thinner slices,
and higher spatial resolution in the longitudinal plane compared with
single-slice CT. With four outgoing channels of data and a gantry rotation
time of 0.5 sec (Model MX 8000; Marconi Medical Systems, Cleveland, OH),
multislice CT examination speed is eight times faster than that of most
single-slice CT scanners.
In our institution, the preoperative anatomical evaluation of abdominal
aortic aneurysms is performed with multislice CT. Routine abdominal aortoiliac
multislice CT is performed in 25 sec with 2.5-mm slice thickness covering the
50-cm distance from T10 to the lesser trochanters of the femurs. The arterial
access routes (the femoral and iliac arteries) must be included in the
examination. The reconstruction interval is 1.3 mm. All images are sent to a
workstation (MX View; Marconi Medical Systems) for postprocessing. The image
postprocessing may include multiplanar reformats, maximum intensity
projection, shaded-surface display, volume rendering, and virtual endoscopy
(Figs. 7 and
8A,8B).
The same technical parameters are used for postoperative multislice CT as are
used for the preoperative examination. Three-dimensional renderings add
overview of the placement and size of the stent-graft (Fig.
9A,9B).
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Fig. 7. —69-year-old man with abdominal aortic aneurysms.
Maximum-intensity-projection (MIP) data set can be viewed from any angle for
optimal evaluation of vessel anatomy. Oblique anterior MIP shows both major
renal arteries as well as bilateral accessory renal arteries
(arrows). Accessory renal arteries were not depicted on standard
anterior MIP.
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Fig. 8A. —70-year-old man with abdominal aortic aneurysms and iliac
artery stenosis. Maximum-intensity-projection (MIP) image shows aortic wall
calcifications, contrast-filled lumen of aorta, and adjoining vessels
including stenotic segment (arrow). But MIPs do not depict thrombosed
part of aneurysm.
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Fig. 8B. —70-year-old man with abdominal aortic aneurysms and iliac
artery stenosis. Curved coronal reformat provides better visualization of
internal anatomy of iliac artery at site of stenosis (arrows).
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Fig. 9A. —77-year-old man with abdominal aortic aneurysms repaired with
Zenith stent-graft. Coronal maximum-intensity-projection (MIP) image depicts
contrast-filled vessels, calcifications, and stent-graft position.
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Fig. 9B. —77-year-old man with abdominal aortic aneurysms repaired with
Zenith stent-graft. Shaded-surface display (SSD) image in same projection as
A. Note that less of wire frame is shown with SSD technique compared
with MIP image. MIP image represents sum of all metal in specific projection,
whereas SSD depicts only surface.
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Understanding CT of Stent-Grafts
To interpret and report on radiologic images of man-made devices, a
thorough understanding of the devices and the surgical procedure is necessary.
The wire frames of the stent-grafts are always visible on CT images. The
covering fabric cannot be depicted unless it is covered by contrast material
on both sides; in those instances it appears as a thin black wall
(Fig. 10 H). The proximal end
of the wire frame in some stent-grafts is not covered with fabric, making it
possible to anchor the stent-graft above the renal arteries without occluding
them [6] (Figs.
1B,
9A,9B,
and
10A,10B,10C).
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Fig. 10H. —85-year-old man with Chuter modular multicomponent
stent-graft (T. Chuter, San Francisco, CA). Awareness of stent-graft design
details is emphasized (A-C) and an endoleak is shown (D-H).
Axial image from multislice CT 14 days after procedure shows complete
resolution of endoleak. Bulging of fabric is still present (long
arrows). Visualization of fabric (open arrows) is result of
contrast media surrounding fabric.
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Fig. 10A. —85-year-old man with Chuter modular multi-component
stent-graft (T. Chuter, San Francisco, CA). Awareness of stent-graft design
details is emphasized (A-C) and an endoleak is shown (D-H).
Axial CT image obtained at level of left renal artery shows that stent-graft
seems to cover renal artery (arrow) and possibly indicates incorrect
placement of endograft.
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Fig. 10B. —85-year-old man with Chuter modular multi-component
stent-graft (T. Chuter, San Francisco, CA). Awareness of stent-graft design
details is emphasized (A-C) and an endoleak is shown (D-H).
Schematic drawing of bimodular stent-graft shows that proximal end of
wire-frame is not covered by fabric, thus allowing placement of wire frame at
level of renal arteries (compare with Zenith stent-graft (Cook, Bloomington,
IN) in Fig. 1B.)
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Fig. 10C. —85-year-old man with Chuter modular multi-component
stent-graft (T. Chuter, San Francisco, CA). Awareness of stent-graft design
details is emphasized (A-C) and an endoleak is shown (D-H).
Oblique coronal shaded-surface—display image confirms correct placement
of wire frame (yellow) at level of left renal artery, allowing for
blood flow to kidney.
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Imaging After Stent-Graft Placement
Endoleak, that is, persistent blood flow in the perigraft aneurysm, occurs
in 12-44% of patients after stent-graft surgery
[7]. In most cases the endoleak
ceases spontaneously, but it may persist and result in stent-graft loosening
and continued enlargement of the aneurysm
[7]. Endoleaks may be
graft-related (Figs.
10D,10E,10F,10G
and
11A,11B,11C,11D,11E,11F)
or non—graft-related. Classifications of endoleaks have been established
[4,
8]. Analysis of CT of endoleaks
should define the path of communication, which will help the surgeon or
interventionalist plan further treatment. Endoleaks have inflow and outflow
channels; arteriography may be needed to show the direction of flow (Figs.
11E and
11F). As a result of bulging
of the fabric, contrast filling may occur seemingly outside the wire frame
(Figs. 10F and
10H) and mimic endoleak.
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Fig. 10D. —85-year-old man with Chuter modular multicomponent
stent-graft (T. Chuter, San Francisco, CA). Awareness of stent-graft design
details is emphasized (A-C) and an endoleak is shown (D-H). On
axial CT image obtained five days after procedure, large endoleak was
discovered (short arrow). Contiguous filling of proximal lumbar
arteries (long straight arrow) indicated this was out-flow component
of endoleak.
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Fig. 10E. —85-year-old man with Chuter modular multicomponent
stent-graft (T. Chuter, San Francisco, CA). Awareness of stent-graft design
details is emphasized (A-C) and an endoleak is shown (D-H).
Axial CT image reveals broad interface with stent-graft (curved
arrows) in middle of 6-cm-long endoleak (short arrows), This
area coincided with junction zone between two stent-graft modules (B), making
it likely that endoleak was stent-graft related. (Reprinted with permission
from [9])
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Fig. 10F. —85-year-old man with Chuter modular multicomponent
stent-graft (T. Chuter, San Francisco, CA). Awareness of stent-graft design
details is emphasized (A-C) and an endoleak is shown (D-H).
Axial CT image shows contrast media filling peripherally to wire frame
(asterisk). This filling is not due to endoleak (short
arrows). Contrast media accumulation relates to bulging of fabric of
stent-graft.
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Fig. 10G. —85-year-old man with Chuter modular multicomponent
stent-graft (T. Chuter, San Francisco, CA). Awareness of stent-graft design
details is emphasized (A-C) and an endoleak is shown (D-H).
Sagittal reformat showing vertical extent of endoleak (short arrows).
(Reprinted with permission from
[9].)
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Fig. 11A. —68-year-old man with Zenith stent-graft (Cook, Bloomington,
IN) showing graft-related endoleak stretching from right common iliac artery
to inferior mesenteric artery. Localizations of inflow and outflow channels
were suggested on CT findings and confirmed using angiography. Axial CT image
obtained from top of aneurysm shows some patchy contrast (arrows)
filling anterior to stent-graft.
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Fig. 11B. —68-year-old man with Zenith stent-graft (Cook, Bloomington,
IN) showing graft-related endoleak stretching from right common iliac artery
to inferior mesenteric artery. Localizations of inflow and outflow channels
were suggested on CT findings and confirmed using angiography. Coronal CT
reformat (15-mm-thick slice) shows contrast-filled endoleak (arrows)
inside right aspect of thrombosed aneurysm.
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Fig. 11C. —68-year-old man with Zenith stent-graft (Cook, Bloomington,
IN) showing graft-related endoleak stretching from right common iliac artery
to inferior mesenteric artery. Localizations of inflow and outflow channels
were suggested on CT findings and confirmed using angiography. Axial image
obtained from level of distal end of modular stent-graft shows that endoleak
(long arrow) can be followed distally to this point between wire
frame and large mural calcification (short arrow).
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Fig. 11D. —68-year-old man with Zenith stent-graft (Cook, Bloomington,
IN) showing graft-related endoleak stretching from right common iliac artery
to inferior mesenteric artery. Localizations of inflow and outflow channels
were suggested on CT findings and confirmed using angiography. Sagittal
maximum-intensity-projection image of right stent-graft leg shows abundant
mural calcifications (short arrow) at level of distal part of wire
frame (long arrow) in right common iliac artery. Calcifications
prevent tight seal of lumen between vessel wall and wire frame facilitating
endoleak.
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Fig. 11E. —68-year-old man with Zenith stent-graft (Cook, Bloomington,
IN) showing graft-related endoleak stretching from right common iliac artery
to inferior mesenteric artery. Localizations of inflow and outflow channels
were suggested on CT findings and confirmed using angiography. Early image
from arteriography shows catheter (arrows) in right common iliac
artery and contrast media filling right common iliac artery and distal
aorta.
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Fig. 11F. —68-year-old man with Zenith stent-graft (Cook, Bloomington,
IN) showing graft-related endoleak stretching from right common iliac artery
to inferior mesenteric artery. Localizations of inflow and outflow channels
were suggested on CT findings and confirmed using angiography. Later image
from angiography shows filling of endoleak (long arrows) in cephalad
direction along stent-graft. At top of aneurysm endoleak communicates
(open arrow) with inferior mesenteric artery (short
arrows).
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Other complications that may accompany stent-graft surgery are stenosis
(Fig.
12A,12B),
thrombosis, migration (Fig.
13A,13B),
angulation of graft, enlargement of aneurysm, aortoenteric fistula and
hemorrhage, fabric tear, detachment of the contralateral iliac limb, enlarging
sac size without visible endoleak (endotension), metal fatigue, suture
failure, infection, and branch-vessel compromise (with suprarenal fixation).
To understand stent-graft complications, one has to be aware of the
geometrical changes an aneurysm may undergo over time while the more rigid
stent-graft remains unchanged. Multislice CT is ideal in the diagnosis of
complications, as well as for following the diameter of the aneurysm. The
thin-slice technique facilitates visualization of the wire frame, adjacent
blood flow, and surrounding tissues. The long z-axis coverage of
multislice CT provides fine detail and excellent vascular enhancement.
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Fig. 12A. —71-year-old man with Zenith stent-graft (Cook, Bloomington,
IN). Stenosis of stent-graft limb. During deployment stent-graft had to be
rotated 180° along its long axis because of incorrect introduction in
femoral artery. Under fluoroscopic guidance, attempts were made to rotate
stent-graft into correct position. Immediate postoperative axial image showed
stenosis (arrow) of "left limb."
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Fig. 12B. —71-year-old man with Zenith stent-graft (Cook, Bloomington,
IN). Stenosis of stent-graft limb. During deployment stent-graft had to be
rotated 180° along its long axis because of incorrect introduction in
femoral artery. Under fluoroscopic guidance, attempts were made to rotate
stent-graft into correct position. Coronal reformat located stenosis
(arrow) to most proximal part of "left limb."
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Fig. 13A. —76-year-old man with stent-graft (Corvita Europe, Brussels,
Belgium) showing migration and enlargement. Anterior
maximum-intensity-projection image 6 months after placement reveals that
device has migrated caudally.
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Fig. 13B. —76-year-old man with stent-graft (Corvita Europe, Brussels,
Belgium) showing migration and enlargement. Anterior
maximum-intensity-projection image with view identical to A, obtained 3
years after placement, shows that stent has migrated farther caudally,
shortened along long axis, and widened in diameter. Stent changes are result
of changes in aneurysm size and length that occurred after stent-graft
placement (e.g., aneurysm remodeling).
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Patient Safety
The two relative contraindications to CT angiography are renal
insufficiency and severe allergy to iodinated contrast media. When CT is
contraindicated, alternative preoperative imaging procedures are sonography
and MR angiography.
Conclusion
Stent-grafting of abdominal aortic aneurysms is a minimally invasive
procedure with a low patient mortality risk that requires only a short
hospital stay. Multislice CT is a fast, safe, and minimally invasive
diagnostic method that can answer most pertinent preoperative and
postoperative diagnostic questions.
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Top
Introduction
Stent-Grafting Technique
