Key words aorta - angiography - interventional procedures - aneurysm
Introduction
Endovascular repair (EVAR) of abdominal aortic aneurysms (AAA) was introduced by Parodi
and coworkers in 1991 [1 ]. In the last two decades, EVAR has emerged as an important alternative to open surgery
for selected patients [2 ]
[3 ]. Unlike patients who are treated with open AAA repair, patients undergoing EVAR
require lifelong image surveillance with cross-sectional imaging to monitor the position
of the device, to ensure stent graft function, and to detect complications. The occurrence
of endoleaks (EL) after EVAR remains one of its principal and most frequent complications
[4 ].
In most centers, bi- or triphasic MDCT is used for follow-up after EVAR with high
sensitivity for the presence of endoleaks [5 ]. Once an EL is detected, high-quality imaging for exact classification is warranted
to guide therapy [6 ]. Although CE ultrasound [7 ], time-resolved CT [8 ] and MRI [9 ] have shown promising results in the classification of EL that are difficult to assess
with standard follow-up, DSA is still considered the gold standard due to its high
temporal and spatial resolution and its ability to depict flow direction [5 ]. Another benefit is the ability to perform interventional treatment in the same
setting if necessary.
Disadvantages include arterial puncture and the lack of cross-sectional soft tissue
information since DSA provides only projection images. This often leads to multiple
views and contrast injections to clearly depict feeding arteries of an endoleak free
from superimposing structures such as the aorta, the stent graft or other arteries.
In contrast to its extremely high spatial resolution, DSA provides limited low-contrast
resolution. Therefore, strong contrast enhancement of the endoleak has to be obtained
to make it visible. It is well known that small, slow-flow endoleaks can be missed
by DSA alone [8 ]
[9 ].
Cone-beam C-arm CT facilitates acquisition and reconstruction of CT-like images in
a flat-panel angiography system. The ability to combine real-time fluoroscopy and
DSA with C-arm CT has helped to overcome shortcomings during many interventional procedures
[10 ]
[11 ]
[12 ]. For EVAR, there are case reports that demonstrated the feasibility of the intraoperative
use of C-arm CT during the EVAR procedure [13 ]
[14 ]
[15 ]. Binkert et al. and van Bindsbergen et al. used C-arm CT to guide translumbar endoleak
repair by direct puncture of the perfused aneurysm sac in three and five patients,
respectively [16 ]
[17 ]. However, to the best of our knowledge, the use of C-arm CT for endoleak classification
has not been described.
The aim of this study was to investigate the value of C-arm CT as an adjunct to DSA
for the classification of endoleaks that could not be classified with noninvasive
diagnostic tests.
Method
Patient population
Over a time period of two years, 12 patients with an endoleak detected on a triphasic
(unenhanced, arterial and late phase) MDCT follow-up exam 3 to 9 months after EVAR
that could not be classified [18 ] with MDCT were included in this retrospective study. All 12 patients were referred
to interventional radiology and underwent digital subtraction angiography (DSA) and
angiographic C-arm CT using a flat-panel C-Arm CT (C-arm CT, Axiom Artis dBA, Siemens,
Forchheim, Germany) according to a standard image acquisition protocol. Each patient
gave informed written consent to perform the angiographic study. The retrospective
evaluation was approved by the institutional review board with a waiver of consent
granted.
Imaging technique
DSA was performed using an angiographic system (Axiom Artis® , Siemens Medical, Forchheim, Germany) equipped with a 30 × 40 cm flat-panel detector.
First, digital subtraction angiography of the aorta (5F Pigtail catheter, 30 mL of
iomeprol (Iomeron, Bracco); 300 mg I/mL; flow rate, 20 mL/s, catheter tip 1 cm above
the proximal stent graft margin) was performed. Second, the catheter was left in place
and a contrast-enhanced C-arm CT scan of the abdominal aorta and the pelvic arteries
was acquired (rotation time 8 s, total scan angle 240°, projection angle increment
0.5°, dose per pulse 0.36 µGy) in the arterial phase (contrast injection: 30 mL of
iomeprol diluted with 30 mL of saline for a iodine concentration of 150 mg I/mL; flow
rate, 8 mL/s; no delay). The cylindrical scan had a cranio-caudal coverage of 185 mm
and a transverse and sagittal scan range of 225 mm. For image reconstruction, the
raw dataset was sent to a dedicated 3 D image reconstruction workstation (X-Leonardo® , Siemens Healthcare, Erlangen, Germany) to generate an isotropic voxel dataset with
a typical voxel size of 0.4 mm. The dataset was visualized using multiplanar reconstructions,
maximum intensity projections as well as volume rendering techniques. The generation
of a 3 D dataset with a 512 × 512 matrix took less than 1 minute as a 100mBit/s network
connection between the C-arm system and the reconstruction workstation was used.
Data evaluation
During the procedure, DSA, fluoroscopy, and C-arm CT images were reviewed on a workstation
by one reader who had access to all available patient information and prior imaging
(pre- and post-EVAR MDCT). The type of endoleak was assessed based on all available
information first to make a treatment decision and to assess the optimal therapeutic
approach during the procedure and second to serve as the standard of reference (SOR)
for this study. Subsequently, a blinded second reader (not involved in the procedure)
retrospectively evaluated DSA and C-arm CT separately in random order to assess the
endoleak classification based on each of the two imaging methods alone.
In case of an interventional treatment (n = 6), the second reader subsequently assessed
the value of the C-arm CT images for procedure planning on a 4-point scale (1: essential information for procedure guidance, could not have done without it; 2: helpful information , did alter the course of the intervention, 3: additional information , did not alter the course of the intervention; 4: no additional information ).
Results
The EL could be visualized and classified successfully by use of the intraprocedural
imaging including C-arm CT and all prior imaging by reader 1 in all cases. In 4 patients,
a type 1 EL (proximal neck, type 1a: 1 patient; distal neck, type 1b: 3 patients,
[Fig. 1 ]) was diagnosed. In 7 patients, the EL could be determined as a type 2 EL with retrograde
filling of the aneurysm sac from either the inferior mesenteric artery (n = 2) or
the lumbar arteries (n = 5) ([Fig. 2 ]). In one patient, DSA at an early phase revealed a defect in the graft material
([Fig. 3 ]) resulting in a type 3 EL.
Fig. 1 Patient with a type 1B endoleak at the distal attachment site of the right iliac
limb of the graft. C-arm CT a shows the endoleak in between the two limbs (arrow in a ) without contrast-enhanced lumbar arteries as an indirect sign for incomplete attachment
of the endoprosthesis limb. b DSA shows a faint contrast extravasation between the endoprosthesis limbs (arrow
in b ), which was missed by the blinded reader. c DSA obtained after placement of a covered stent over iliac attachment site shows
no further leak.Abb. 1 Patient mit einem Typ-1B-Endoleak am distalen Ende des rechtsseitigen Prothesenschenkels.
Die C-Arm-CT a stellt neben dem Endoleak auch das unvollständige Anliegen der Endoprothese dar (Pfeil
in a ). Die DSA b zeigt einen schwach abgrenzbaren Kontrastmittelaustritt (Pfeil in b ) dar, der durch den zweiten Radiologen übersehen wurde. Die DSA c nach Implantation eines gecoverten Stents zeigt kein Endoleak mehr.
Fig. 2 Patient with type 2 endoleak (asterisk in a–c ) detected with MDCT (a transverse MIP, arterial phase). C-Arm CT (b, c transverse and coronal MIP) clearly depicts the feeding lumbar arteries (white arrows
in b, c ) at the same slice position. Coronal MIP c of C-arm CT visualizes lumbar arteries as well as a third feeding artery arising
from the left internal iliac artery (black arrow in c ).Abb. 2 Patient mit in der MDCT (a transversale MIP in der arteriellen Phase) detektiertem Typ-2-Endoleak (Stern in
a–c ). In der C-Arm-CT (b, c transversale und koronare MIP) in gleicher Position stellen sich das Endoleak füllende
Lumbalarterien(weiße Pfeile in b, c ) dar. Die coronare MIP der C-Arm CT zeigt neben den Lumbalarterien zusätzlich eine
dritte speisende Arterie, die aus der linken A. iliaca interna hervorgeht (schwarzer
Pfeil in c ).
Fig. 3 C-arm CT a and DSA b show contrast in the aneurysm sac (arrowhead in a and b) in close proximity to a
lumbar artery leading to the diagnosis of type 2 endoleak in this patient. Early phase
DSA c shows an area of contrast leakage from the mid-portion of the right limb of the stent
graft (arrowhead in c ). Selective contrast injection in the right iliac limb d shows filling of the endoleak (arrowhead in d ) prior to any feeding arteries leading to the diagnosis of type 3 endoleak.Abb. 3 Die Darstellung einer Kontrastmittelanreicherung im Aneurysmasack (Pfeilspitze in
a , b ) in der C-Arm CT a und in der DSA b in unmittelbarer Nähe zu einer Lumbalarterie führten bei diesem Patienten zur Diagnose
eines Typ-2-Endoleaks. Die DSA in einer frühen Phase c stellt einen vom mittleren Drittel des rechtsseitigen Prothesenschenkels ausgehenden
Kontrastmittelaustritt dar (Pfeilspitze in c ). Die selektive Kontrastmittelinjektion in den rechten iliakalen Prothesenschenkel
d führt zu einer Füllung de Endoleaks (Pfeilspitze in d) ohne eine Kontrastierung potentiell
füllender Arterien. Dadurch konnte die Diagnose eines Typ-3-Endoleaks gesichert werden.
The absolute numbers of patients with agreement or disagreement of endoleak classification
of DSA alone or C-Arm CT alone compared to SOR are shown in [Table 1 ]. With DSA alone, 9 ELs could be correctly classified. In 3 patients, 1 type 1 and
2 type 2 ELs were missed. This was probably due to slow filling of the EL, DSA artifacts
and superimposition of the stent graft.
Table 1
Endoleak classification in DSA and C-arm CT.Tab. 1 Endoleak-Klassifikation in der DSA und im C-Arm CT.
endoleak classification
patients (n)
SOR
DSA only
C-arm CT only
Σ = 8
agreement
1
type 1a
=
=
2
type 1b
=
=
5
type 2
=
=
Σ = 3
disagreement of SOR and DSA
1
type 1b
ND*
=
2
type 2
ND*
=
Σ = 1
disagreement of SOR and C-arm CT
1
type 3
=
type 2*
SOR: Standard of reference, *: Endoleak classification disagreed with the SOR, = :
Endoleak classification agreed with the SOR, ND: Not diagnosed/no classification assessed.
SOR: Referenzstandard, *: Endoleak-Klassifikation stimmt nicht mit dem Referenzstandard
überein, = : Endoleak-Klassifikation stimmt mit dem Referenzstandard überein, ND:
keine Diagnose möglich/keine Klassifikation möglich.
Using the monophasic cross-sectional C-arm CT data, 11 ELs could be correctly classified.
In one patient, C-arm CT misleadingly demonstrated a type 2 endoleak with contrast
in the sac in close proximity to a lumbar artery ([Fig. 3 ]). However, DSA at an early phase revealed a defect in the graft material ([Fig. 3c ]) resulting in a type 3 endoleak classification.
All patients with type 1 EL were treated. In 3 patients, the stent graft was extended
proximally (n = 1) or distally (n = 2, [Fig. 1 ]). In one patient, an angioplasty of the distal neck was successfully performed.
In 6 of 7 patients with type 2 EL, the diameter of the aneurysm sac was stable when
compared to the post-EVAR MDCT. In these patients, no immediate therapy was performed.
In one case, an increasing AAA sac diameter was noted and a translumbar embolization
of the aneurysm sac was performed using C-arm CT-based needle tracking. In the patient
with a type 3 endoleak, the fabric tear in the iliac limb was successfully covered
using a stent graft extension. Subsequently, filling of the endoleak was eliminated
which proved the diagnosis of a type 3 endoleak.
In all 6 patients who underwent interventional therapy, the information provided by
C-arm CT to plan the respective procedure was assessed by the second reader to be
essential in 3 patients and helpful in 2, mainly due to exact localization of the
EL. In one patient, C-arm CT provided additional information with respect to stent
position and anatomy but had no influence on the course of the intervention.
Discussion
Digital flat detectors allow CT-like soft tissue images in the angiography suite,
adding a third dimension to the normally planar DSA images. Although somewhat limited
in field of view and image quality when compared to MDCT, the seamless integration
of C-arm CT in the interventional suite offers a tremendous improvement in the workflow
during complex procedures [19 ].
There are a few case reports on the use of C-arm CT in the realm of EVAR procedures
that demonstrated the feasibility during implantation [13 ]
[14 ]
[15 ]
[20 ]. Two case studies reported the use of C-arm CT guidance for endoleak repair [16 ]
[17 ]. In one study [17 ], contrast-enhanced C-arm CT was used to visualize the endoleak nidus. However, to
the best of our knowledge, the use of C-arm CT to classify endoleaks that are difficult
to assess by MDCT has not been described.
With any imaging method, the presence of contrast material in the excluded aneurysm
sac after EVAR leads to the diagnosis of an endoleak. Once detected, endoleak classification
is critical for patient care. Studies using CTA and MRA alone or in combination yield
a high number of undetermined ELs ranging from 40 to 60 percent with relatively poor
interobserver agreement [9 ]
[21 ]
[22 ]. All imaging modalities have specific shortcomings that can make reliable classification
difficult. With both, CTA and MRA, scan timing can be challenging. This has led to
studies using dynamic CTA with promising results [8 ] but a relatively high radiation dose. MRA has shown some potential for EL classification
but is limited to grafts that do not create a susceptibility artifact [9 ]
[23 ]
[24 ]. CEUS is a dynamic study that overcomes timing issues with CTA and MRA but has difficulties
visualizing both endoleak nidus and feeding vessels, especially in obese patients
[5 ]
[7 ]
[23 ]. Therefore, in some cases exact classification requires the use of DSA which is,
despite some shortcomings of its own, still considered the gold standard for endoleak
classification, and should be performed especially in cases with a growing sac and
equivocal CTA [5 ]
[25 ]. DSA provides high spatial and temporal resolution and is able to depict flow direction
[26 ]. However, it is recognized to have limited sensitivity for identifying small ELs
with slow flow [5 ]. Here, additional 3 D C-arm CT information in combination with intra-arterial contrast
injection adds valuable information. In our study, we successfully acquired C-arm
CT images on the angiography table with an intra-arterial angiographic catheter facilitating
high intra-arterial contrast medium density in comparison to conventional MDCT. We
were able to determine the presence of contrast agent in the sac in all patients and
could correctly classify the endoleak in all but one case based on C-arm CT only.
Moreover, the full integration of angiography and C-Arm CT provided information that
helped to guide immediate therapy with the patient never leaving the angio suite.
The main limitations of our study were the small number of patients referred to DSA
with undetermined endoleaks and the retrospective evaluation. Although a prospective
study with a larger series would be beneficial to assess the technique, we believe
that, due to our standardized approach for both DSA and C-arm CT, the results are
still meaningful. Another limitation is that the reference standard is based on reading
of one operator during the intervention. However, this reading was performed using
all available information including DSA and fluoroscopy with the latter not being
saved. Therefore, a second reading was not possible. However, in contrast to many
studies on endoleak follow-up that did not have a reference standard, we had a true
gold standard for 6 patients who were treated in the same setting with a negative
finding for the presence of contrast media on follow-up after treatment.
Conclusion
Endovascular repair of aortic aneurysms is performed with increasing frequency. Imaging
surveillance is mandatory in EVAR patients to detect complications. Even with reduced
follow-up protocols, there will always be endoleaks that require exact classification,
especially if expansion of the aneurysm sac is observed [5 ]
[27 ]. For such cases, C-arm CT offers an ideal adjunct to DSA as it helps to localize
and classify endoleaks.
