AJR 2005; 184:1178-1180
© American Roentgen Ray Society
Gadolinium-Enhanced CT Angiography of Endovascular Stent-Grafts
John Thomas,
Tracy A. Jaffe and
Erik K. Paulson
Department of Radiology, Duke University Medical Center, Box 3808,
Durham, NC 27710.
Received May 11, 2004;
accepted after revision July 19, 2004.
Address correspondence to J. Thomas
(thoma120{at}mc.duke.edu).
Introduction
The management of patients with abdominal aortic aneurysms has
changed dramatically since the introduction of aortic endovascular
stent-grafts in 1994 [1]. The
main target group of patients for minimally invasive stent-graft implantation
is patients with comorbidities, in whom open surgical repair has significant
risk. The postoperative assessment of stent-grafts is based primarily on the
results of CT using IV iodinated contrast agents. However, the use of IV
iodinated contrast material is predicated on normal renal function. In
patients with renal insufficiency, MRI is an alternative to CT, but it is
limited by artifacts from metallic stents that may obscure both the flow lumen
and the aortic sac, thus limiting visualization of complications such as an
endoleak [2]. In such clinical
situations, it is feasible to perform enhanced CT using IV gadolinium instead
of iodinated contrast material because gadolinium is radiodense and less
nephrotoxic than iodinated agents
[3]. We report a patient with
an endovascular stent-graft who underwent CT using gadolinium as the contrast
agent.
Case Report
An 85-year-old man underwent aortic endovascular stent-graft placement
(AneuRx Bifurcated Stent-Graft System, Medtronic) for a 5.2-cm infrarenal
abdominal aortic aneurysm and was referred for imaging to rule out endoleak.
Because the patient had renal insufficiency (serum creatinine level, 3.7
mg/dL), he was initially referred for aortic sonography instead of
contrast-enhanced CT. Aortic sonography revealed an endovascular stent
appropriately positioned within the aneurysmal aortic lumen. Color and power
Doppler sonography revealed questionable flow within the sac near the proximal
aspect of the stentgraft (Fig.
1A), raising the possibility of an aortic endoleak. Because of the
concern of endoleak, the decision was made to attempt CT using gadolinium as
the IV contrast agent. The patient's renal insufficiency precluded the use of
IV iodinated contrast material. Furthermore, it was thought that MRI would be
suboptimal because blooming artifact would obscure the stent and limit
assessment of an endoleak.
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Fig. 1A. —85-year-old man after endovascular stent-graft placement who
was referred to assess for complications. Sagittal power Doppler image shows
flow (arrows) in aortic sac near proximal aspect of endovascular
stent (arrowheads).
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The scans were obtained on a 16-MDCT scanner (LightSpeed, GE Healthcare)
using the following protocol: An initial unenhanced CT scan and then a
contrast-enhanced CT scan were obtained with a 16 x 1.25 mm detector
configuration, pitch of 1.375:1, table speed of 27.5 mm per rotation, and
rotation time of 0.5 sec. The images were then reconstructed at 0.625-mm
thickness and reformatted in the coronal plane. Eighty milliliters (40 mmol)
of gadopentetate dimeglumine (Magnevist [0.5 mmol/mL], Berlex Laboratories)
was injected at 3 mL/sec after a 40-sec scanning delay interval, which was
chosen by manual bolus tracking.
The CT scans showed an aortic endovascular stent-graft originating 2 cm
below the ostia of the renal arteries (Figs.
1C and
1D). The graft was patent.
Contrast material was noted to extravasate into the proximal aspect of the
native aortic aneurysmal sac, consistent with a type I endovascular leak.
Compared with the preoperative CT scan obtained 6 months earlier, the
aneurysmal sac diameter was stable. Because the patient was asymptomatic, the
surgical team opted for conservative management with follow-up imaging in 6
months. The follow-up CT examination, which was also performed with IV
gadolinium, showed no change in the type I endoleak.
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Fig. 1C. —85-year-old man after endovascular stent-graft placement who
was referred to assess for complications. Gadolinium-enhanced axial CT image
shows high-attenuation material (arrows) extending from endovascular
stent (arrowheads) into aortic sac, consistent with type I endoleak.
Postcontrast attenuation was 82 H within stent and 106 H within aortic
sac.
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Fig. 1D. —85-year-old man after endovascular stent-graft placement who
was referred to assess for complications. Gadolinium-enhanced coronal
reformation confirms high attenuation (arrowhead) adjacent to
stentgraft consistent with type I endoleak.
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Discussion
Traditionally, patients with abdominal aortic aneurysms have been treated
with open surgery to decrease the likelihood of aortic rupture. In patients
with comorbid conditions such as cardiac, pulmonary, or renal disease, the
mortality rate associated with open surgical repair ranges from 5.7% to as
high as 31% [4]. Endovascular
aortic stent-graft placement is less invasive than open surgery and is
preferable in patients with comorbidities. In 1994, Scott and Chuter et al.
[1] successfully placed a
bifurcated stent-graft in six patients with abdominal aortic aneurysms. Since
then, the management of abdominal aortic aneurysm has shifted from open
surgery to endovascular stent placement. Although less invasive than an open
approach, endovascular stent placement has its own complications including
stent migration, kinking, and endoleak. Consequently, these patients require
regular clinical and imaging follow-up. CT is necessary before and after
stent-graft implantation to determine stent-graft sizes and dimensions and to
exclude stent-graft leakages, respectively
[5,
6].
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Fig. 1B. —85-year-old man after endovascular stent-graft placement who
was referred to assess for complications. Precontrast axial CT image through
proximal aorta shows stentgraft (arrows) and surrounding aneurysmal
sac (arrowheads). Precontrast attenuation of aorta was 41 H.
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The AneuRx Stent-Graft System was used in our patient. This is a modular
stent with an external skeleton composed of nitinol and an internal graft
composed of polyethylene terephthalate fiber (Dacron, DuPont). In a review of
the MR appearance of stent-grafts by Merkle et al.
[2], this system is partly
MR-compatible. However, visualization of the aortic sac was obscured by
blooming artifact. Consequently, in our patient, we decided to use gadolinium
CT angiography instead of MR angiography.
One of the important complications related to endovascular stent-grafts is
endoleak. Endoleak refers to persistent blood flow outside the stent lumen but
within the native aneurysmal sac or adjacent vascular segment. A type I leak
is a perigraft leak where aortic flow escapes around the cuff of the stent
into the aneurysmal sac. A type II leak refers to retrograde flow through
collateral arteries into the aneurysmal sac. A type III leak occurs from flow
through the graft due to tears, disconnection, or disintegration of stent
fabric. A type IV endoleak refers to increased graft porosity. The presence of
an endoleak suggests that the procedure has failed to completely exclude the
aneurysm from the aortic circulation. Such failure, especially if systemic
arterial pressures are maintained within the aortic sac, may lead to continued
expansion of the aneurysmal sac and risk of rupture.
The postoperative assessment of stent-grafts is based primarily on the
results of CT using IV iodinated contrast material
[5]. We presume the diagnosis
of endoleak would have been established using either 4-, 8-, or 16-MDCT. An
alternative method of assessment of these grafts is color Doppler sonography
or MR angiography using IV gadolinium
[2]. The significant limitation
of color Doppler sonography is acoustic shadowing from the stent-graft itself
or overlying bowel gas that may obscure visualization of the aortic sac and
stent. Furthermore, the absence of color flow does not exclude endoleak,
because slow flow may not be detected on sonography. MR angiography is also
limited in some grafts because of metal artifact that may obscure the flow
lumen and aortic sac. Nevertheless, a large leak, like the one seen in our
patient, may be seen on MR angiography. In addition, MRI is contraindicated in
patients with pacemakers, claustrophobia, or recently placed metallic stents
or surgical clips.
The IV administration of gadolinium chelated with diethylenetriamine
pentaacetic acid (DTPA) has been found to be safe and well tolerated
[7]. The physiologic half-life
in patients with normal renal function is approximately 1.5 hr. The
manufacturer's recommended dose is 0.1 mmol/kg. Prince et al.
[7] reviewed patients who had
higher doses of gadolinium. The range of gadolinium chelate used in their
series was 0.2-0.4 mmol/kg. They concluded that such high doses are
significantly less nephrotoxic than IV iodinated contrast medium.
Quinn et al. [8] have shown
that for cranial CT, diagnostic vascular enhancement with gadopentetate
dimeglumine may be achieved with doses as high as 0.5 mmol/kg. Similarly,
enhancement of 100 H has been achieved in the thoracic aorta and its branches
using gadolinium doses of 0.5 mmol/kg
[9]. For our patient, a dose of
80 mL of gadolinium (0.4 mmol/kg) resulted in aortic enhancement of 61 H,
which was sufficient to show the type I endoleak. The decision to use 80 mL of
gadolinium was based in part on such work indicating that a dose of
approximately 0.5 mmol/kg is suitable for aortic opacification
[8,
9]. A higher dose of gadolinium
was not considered for fear of toxicity. The approximate charge for 80 mL of
gadolinium was $350-400. Similar enhancement was reproduced on the follow-up
scan 6 months later.
In the appropriate setting, patients with renal insufficiency (serum
creatinine level, > 2.0 mg/dL) will be considered for gadolinium-enhanced
CT. Although our report shows that CT using gadolinium as the contrast agent
is able to show the presence of an endoleak, there have been no studies
comparing the efficacy of gadolinium-enhanced CT with that of iodinated
contrast-enhanced CT for endoleak detection. Hence, the technique of choice in
the clinical setting of renal failure will depend on local imaging practice
patterns.
In conclusion, we report a patient with renal insufficiency in whom
enhanced CT using gadolinium successfully diagnosed a type I endoleak.
Although the enhancement after gadolinium administration was less vivid than
that expected with iodine, diagnostic images were obtained.
Gadolinium-enhanced CT angiography is an acceptable alternative to iodinated
contrast-enhanced CT angiography in selected scenarios in which imaging is
required to rule out endoleak.
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Introduction
Case Report
