Michel Nonent*, Ali Badra**
and Pierre Gouny**
*Department of Radiology and **Department of Vascular Surgery,
University Hospital Centre, Brest, France
Address for correspondence:
Michel Nonent, MD, Service de Radiologie,
Hôpital de la Cavale Blanche, CHU,
Bd Tanguy Prigent,
29609 Brest cedex,
France
Tel: +33 (0)2 98 34 75 20
Fax: +33 (0)2 98 34 78 25
Email: michel.nonent@chu-brest.fr
Abstract
Endovascular repair using stent-grafts for abdominal aortic aneurysms (AAAs) is becoming increasingly employed worldwide. Imaging has great importance in the assessment of AAAs and in decision-making.
Spiral-computed tomography (CT) is valuable in patient selection, morphologic assessment and follow-up. Recent improvements in technology (multislice CT) and in image processing (three-dimensional processing
software, CT simulation before stent-grafting) allow an opportunity to replace X-ray angiography in the
assessment of AAAs. Magnetic resonance imaging is becoming a favoured method, as it can be used without iodinated contrast agents; however, there are often
limitations in the availability of equipment. Doppler ultrasound is the safest imaging method, but gives
variable results and cannot be used alone. This article provides information and recommendations about the use of imaging techniques before and after
endovascular repair of AAAs.
Introduction
Imaging techniques are of tremendous importance in
the assessment and selection of patients for stent
grafting of abdominal aortic aneurysms (AAAs) (see
Table 1).4 Firstly, imaging helps to identify the lesion
and localize visceral arteries originating from the aorta
relative to the position of the aneurysmal sac, especially the main and accessory renal arteries. Secondly, images
allow precise measurement of the diameter and length
of the aneurysm, diameter of the proximal and distal
necks, and dimensions of the iliac arteries. Four imaging
modalities are available for the assessment of
stent-grafting in AAAs: spiral-computed tomography
(CT), X-ray angiography, magnetic resonance imaging
(MRI) and Doppler ultrasound. Most investigators
consider CT as the best imaging method. X-ray
angiography is most often used as a complementary
technique to CT, but may not be strictly necessary
in the future because of the development of
three-dimensional processing of CT and MRI. The use of
CT and X-ray angiography can be limited due to the risk
of nephropathy from the large volumes of iodinated
contrast media that may be required, although this risk
is dependent on the type of iodinated contrast media
employed.5 Doppler ultrasound is also important in the
follow-up of AAAs after endovascular repair. Finally,
magnetic resonance angiography (MRA) is becoming
favoured in the assessment of AAAs, because (i) it is a
non-invasive method, (ii) it uses a non-nephrotoxic and
well-tolerated contrast medium (gadolinium) and (iii)
because of recent improvements in acquisition techniques.
This paper reviews important information required by
radiologists and surgeons to select patients for stent
grafting of AAAs, and compares imaging modalities in
the assessment and follow-up of endovascular repair.
Morphologic assessment
Diameter
Endovascular repair is indicated for AAAs =50 mm in
diameter in patients considered to be at high risk from
open surgery. As a result, the external diameter of the
abdominal aorta must be carefully measured. For these
measurements, CT is the most accurate method due
to its efficacy in imaging all components of the
aneurysmal wall, especially thrombi and calcifications.
Doppler ultrasound has some limitations, particularly in
obese patients, or when gas in the intestinal tract
prevents aortic visualization. X-ray angiography can only
measure the diameter of the aneurysm lumen, which
can be misleading in the case of thick calcification of
the arterial wall. MRI could become the most favoured
method for the estimation of aortic diameter; however,
it still suffers from insufficient spatial resolution,
inadequate visualization of calcified plaques, variability
in processing software and poor availability of
equipment. Despite these problems, MRI – including
contrast-enhanced magnetic resonance angiography
(MRA) – is very useful in patients with chronic renal
insufficiency.
Proximal neck
Anatomic configuration of the proximal neck, the part
of aorta located below the renal arteries and above
the upper extremity of the AAA, is important for
decision-making in endovascular repair because the
stent-graft must be intimate with the arterial wall in
order to prevent a leak. The proximal neck must be
=15 mm in length and <30 mm in diameter and, to
allow the stent-graft to conform with the aortic curve,
the proximal neck must be at an angle of <60°. CT
is undoubtedly the best method for the complete
assessment of the proximal neck.
Distal neck
Similar considerations are required for assessment of
the distal neck – the part of aorta located between the inferior extremity of the AAA and the aorto-iliac
bifurcation. As for the proximal neck, the distal neck
must be =15 mm in length for treatment using an
aortic tube alone. CT is also accurate enough for the
assessment of the inferior neck.
Iliac arteries
Visualization of the iliac arteries is strictly necessary
for endovascular repair, especially when a bifurcated
stent-graft is required. At least 10 mm of common iliac
artery above the internal iliac arteries is necessary to
place a bifurcated stent-graft. It is not permissible to
cover both of the internal iliac arteries because of the
high risk of colonic necrosis, due to the absence of
collateral pathways supplying the inferior mesenteric
artery territory. The diameters of common and external
iliac arteries must be measured to evaluate the
feasibility of stent-graft access and advancement.
A diameter of =12 mm for common iliac arteries and
=6 mm for external iliac arteries is generally required
for safe vascular access. It is also important to consider
calcifications and tortuosities before an endovascular
repair decision. Thick calcifications of the arterial wall
or excessive tortuosities may prevent catheterization.
Most reviews on this topic point out that CT images
give relatively low visualization of the tortuosities of
iliac arteries. For this reason, X-ray angiography using
a graduated catheter is still recommended for the
assessment of iliac arteries, but contrast-enhanced CT,
especially multislice CT, is being recognised as an
effective technique. Using thin-slice collimation and
overlapped reconstructions, multislice CT provides
multiplanar reconstructions (MPR), maximal intensity
projections (MIP) and volume rendering technique (VRT)
reconstructions. These images are sufficiently accurate for
the estimation of diameters, lengths and curves and for
accurately locating the iliac artery bifurcation. CT images
effectively demonstrate calcifications that do not mask
the arterial lumen, when specific processing software such
as VRT with transparency is used.
Follow-up
The reported rate of conversion to open surgery for
stent-grafted AAAs is 1% in the first year, increasing to
3.7% in the second year post-implant. Post-implant
surgery carries a mortality risk of approximately 25%
irrespective of the reason for graft failure.6 Imaging is
essential for the follow-up of AAAs after endovascular
repair. Four issues must be checked by imaging
following endovascular treatment: aneurysm size,
detection of an endoleak, reduction of limb flow and
migration of the device. Early detection of an endoleak
is particularly important as it can cause failure and
delayed rupture. Faries et al. recently reported results
from 597 patients treated by stent graft, which
showed that endoleaks requiring secondary treatment
developed in 11.7% of patients.7 In a registry of
2464 patients, delayed rupture was reported at a rate
of 1% over 1 year. In 1046 patients recruited in an
investigational device study, 2% experienced delayed
rupture 3 years post procedure.8
A distinction must be made between primary endoleaks
occurring immediately after the procedure and
secondary endoleaks detected during follow-up.
White et al. have defined a commonly used
classification of endoleaks: type I is an endoleak related
to an incompetent seal at one of the graft attachment
sites; type II is retrograde filling of the aneurysm by
collaterals; type III is the result of perforation or
modular dysjunction; and type IV is porosity of the
fabric used in the graft.9 A non-graft related type II
endoleak must be treated only in the case of
enlargement of the aneurysmal sac.10
Secondary endoleaks are detected by different imaging
techniques with varying results. CT is often considered
the best modality for detection of endoleaks. However,
some investigators have found better sensitivity with
MRA, especially in the case of a low-flow endoleak.
Kramer et al. observed similar performances for CT and
MRA in the detection of leaks, but a better result with
MRA for visualization of collaterals.11 Doppler ultrasound
is the least invasive method, but the variability of
inter- and intra-observer results is the main drawback of
this technique. Despite these problems, recent studies
have shown high sensitivity with Doppler ultrasound in
the detection of endoleaks, especially if ultrasound
contrast media are used.12–14
In France, an expert panel of the Agency of Security
in Health Products (AFSSAPS) has published
recommendations for imaging in the follow-up of
AAAs managed with stent-grafts.3 Apart from the
angiographic control performed immediately at the
end of the procedure, the protocol recommended by
the agency is the following:
• Plain radiographs (antero-posterior, lateral, obliques)
and contrast-enhanced CT before discharge
• Same protocol at 3, 6, 12, 18 and 24 months
• In usual cases, after a delay of 24 months following
endovascular treatment, a CT and plain radiograph
must be obtained every year
• In the case of an endoleak, dislocation of the
endoprosthesis or aneurysm growth, a repair
treatment must be discussed. If a repair is not
necessary, the follow-up is repeated 3 months later,
otherwise corrective treatment may be indicated.
In the case of stability or a decrease in aneurysm
size, a 6-month follow-up is mandatory.
In our opinion, plain radiographs are not necessary if CT
is performed with an up-to-date processing technique,
and especially with volume rendering, which allows
effective visualization and control of the structure and
position of the stent itself. In our experience, threedimensional
VRT with carefully selected densities and
multiple-view angles, is at least as effective as plain
radiographs for detecting a distortion of the stent.
Doppler ultrasound must also be systematically performed
in association with CT. The risk of delayed rupture is
sufficient to justify the need for indefinite monitoring of
AAAs in patients treated with endovascular grafts.8
Practical considerations of multislice CT in
AAA assessment
Acquisition protocols
With multislice CT, it is now possible to scan 4, 8 or
16 slices per rotation depending on the equipment
available. Acquisition with multislice CT is faster than
with conventional computed tomography techniques,
and there is significant improvement of temporal and
spatial resolution. Multislice CT provides the
opportunity to choose the slice thickness and gap size
after the scan. For vascular imaging with multislice CT,
it is necessary to overlap the slices and to choose a
soft filter to obtain high quality three-dimensional
reconstructions. It is now possible to concomitantly
explore the abdominal aorta and the lower limb arteries
and run-off vessels.With four-slice equipment, our
protocol for abdominal aorta and iliac artery
exploration is a 4 x 2.5 mm acquisition after
injection of 120 ml iodinated contrast medium
(300 mg iodine/ml) at a flow rate of 3 ml/second,
with a delay of 30 seconds between injection and
acquisition. Reconstructions are performed with a
3 mm slice thickness and a slice increment of 1.5 mm.
With 16-slice equipment, we use a 16 x 1.5 mm
acquisition, 80 ml of iodinated contrast medium at
a flow rate of 3 ml/second, delay-optimised with
automatic detection of contrast material, and
reconstruction with a 2.5–3.0 mm slice thickness
and 1.25–1.50 mm slice increment.
Image processing
 |
| Figure 1. Abdominal aortic aneurysm:multislice computed tomography with maximal intensity projection reconstructions and measurements of diameters and lengths. |
 |
 |
| Figure 2a. Abdominal aortic aneurysm treated by a stent graft: multislice computed tomography with volume rendering reconstruction. The location and structure of the endoprosthesis is well defined | Figure 2b. Transparency imaging allows visualization of the aorto-iliac lumen inside the stent graft |
Three-dimensional VRT is also useful for the assessment of calcified iliac arteries and for verifying the patency of the aorta and iliac arteries through the endoprosthesis. The other interesting imaging technique for the follow-up of stent-grafting procedures is MPR reconstruction through the axis of the vessel, also known as vessel analysis, mastercut or vessel view, which allows accurate estimation of the diameter and length of the aorto-iliac axis (Figures 3a and b). ‘Virtual endograft’ software is also available and is an interesting tool for simulating endografts according to the specific vascular anatomy of the patient.4
 |
 |
| Figure 3a. MPR reconstruction of the aorta and right iliac artery: image through the axis of the vessel (MasterCut, Philips software) | Figure 3b. Reformatted image (3a) is extracted from this three-dimensional VRT reconstruction (the line inside the vessel represents the axis) |
Measurements of DMAX cannot detect changes in aneurysm size following endovascular repair in over one-third of cases. In 37% of DMAX measurements, discordance was found between DMAX and volume measurements. Using DMAX, a decrease in aneurysm size was missed in 14% of cases and an increase was missed in 19% of cases.15 Volumetric measurement is more accurate than DMAX assessment for CT and MRA.16 According to Pollock et al., a 10% change in sac volume is considered significant for an increased risk of rupture. Aneurysms that increase in volume are significantly associated with type I and III endoleaks.17
| Conclusion Imaging provides an expanding array of tools, allowing increasingly accurate assessment of AAAs and patient selection for endovascular repair. Surgeons and radiologists in this field must be aware of the technological improvements in each imaging modality, to make the right choices before, during and after stent-grafting (see Table 2). |
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