Multislice CT angiography in abdominal aortic aneurysm treated with endovascular stent grafts: evaluation of 2D and 3D visualisations
Z Sun, PhD
Department of Imaging and Applied Physics, Curtin University
of Technology, Perth, Australia
ABSTRACT
Endovascular repair of abdominal aortic aneurysms has been
introduced into the clinical practice for more than a decade and has been
confirmed to be an effective alternative to conventional open surgery,
especially in patients with co-morbid medical conditions. Helical CT
angiography is the preferred imaging method in the follow-up of endovascular
repair. Recent introduction of multislice CT scanners has augmented its
diagnostic role in this area. Diagnostic value of multlislice CT has been
complemented by a series of 3D post-processings, which assist vascular surgeons
in accurately assessing the effect of endovascular repair by providing
additional information when compared to conventional 2D axial images. These
reconstructions include multiplanar reformation, curved multiplanar
reconstruction, shaded surface display, maximum intensity projection, volume
rendering and virtual endoscopy. This article aims to demonstrate the
generation of these 2D/3D reconstructions based on multislice CT data acquired
from a group of patients with abdominal aortic aneurysm following endovascular
repair. A brief introduction of generating each reconstruction was provided;
potential clinical applications of each reconstruction were briefly discussed.
Images were presented in a dynamic format with the aim of allowing the reader
to easily understand the post-processing of these reconstructions. © 2007
Biomedical Imaging and Intervention Journal. All rights reserved.
Keywords: Stent-graft, computed tomography, 3D reconstruction, visualisation, suprarenal fixation
INTRODUCTION
Conventional surgical repair is considered the gold standard
technique in the treatment of patients with abdominal aortic aneurysm (AAA).
However, it is not only an invasive technique but also carries a potential
danger of perioperative and postoperative complications. Thus, less invasive
methods have been investigated and endovascular repair has been confirmed to be
an effective alternative since its first introduction into clinical practice in
1991 by Parodi et al [1-3]. Unlike conventional surgery, the success of
endovascular repair mainly depends on medical imaging and multislice CT
angiography (MSCTA) is considered to be the preferred modality in both
pre-operative planning and post-operative follow-up [4-6]. Besides conventional
2D axial images, MSCTA has been complemented by a series of 3D reconstructions,
which allows the viewer to evaluate the relationship between stent grafts and
arterial branches. Moreover, dynamic visualisation has become possible with the
development of CT scanning techniques and computer software. This article
presents a series of reconstructed visualisations based on CTA data sets in a
group of patients with AAA treated with endovascular stent grafts. These
include multiplanar reformation, curved multiplanar reconstruction, shaded
surface display, maximum-intensity projection, volume rendering, and virtual
endoscopy. Most of the images were presented in a dynamic format with the aim
of assisting readers to be familiar with variable reconstruction visualisations
and help them to choose the appropriate visualisation technique when dealing
with similar situations in clinical practice.
Patients' data selection
18 patients (15 men and 3 women, mean age: 75 years, age
range: 63-84) undergoing endovascular repair of AAA were included in the study.
Vascular surgeons recommended that the patients receive endovascular stent
graft repair because they were unsuitable for open surgical repair due to
comorbid medical conditions. All patients were treated with the Zenith AAA
Endovascular Graft (William Cook Europe, Bjaeverskov, Denmark) with a
suprarenal uncovered component placed above the renal arteries for obtaining
proximal fixation. MSCTA was performed on a 16-slice scanner (Toshiba Medical
Systems Europe, Netherlands) with the scanning protocol as follows: 1 mm
section thickness, pitch 2.0 and reconstruction interval of 1 mm. MSCTA was
performed with uniphasic arterial phase scans: an intravenous injection of 100
ml of non-ionic contrast media (Niopam 300, Bracco UK Ltd. High Wycombe) was
administered at a rate of 2 ml/second with a scan delay of 30 seconds.
Results of patient follow-up
The patients were routinely followed-up within one week of
the stent graft implantation, and 1 month, 3 months, 6 months, 12 months and
yearly thereafter. The follow-up time ranged from 24 months to 54 months, with
a mean period of 40 + 7.6 months. 2D axial CT images were most commonly used to
follow-up these cases and 3D reconstructions were generated to provide
additional information, if required, for further assessment of potential
complications. Endoleaks were found in five cases, with type I and III in one
patient each, respectively, and type II in three cases. The renal function was
evaluated by measuring the serum creatinine levels - it was not significantly
affected in all patients, except in one patient who developed chronic renal
failure because of an atrophic left renal artery and had to receive renal
dialysis. Distal stent graft migration occurred in four patients and 3D image
visualisations demonstrated superiority in the detection of migration, which
was discussed in the following section of 3D reconstructions. All of the
stent-covered aortic branches remained patent and no stenosis or occlusion was
observed.
Generation of reconstructed images
CT volume data were converted from original DICOM (Digital
Imaging and Communication in Medicine) images with a commercially available
software Analyze V 6.0 (www.analyzedirect.com Mayo Clinic, USA). Spatial
resolution of the axial CT images was 512x512 matrix with a voxel size of 0.68
to 0.74. The data were interpolated for the purpose of generating a
cubic/isotropic data set. The total number of slices within the volume of data
ranged from 300 to 550 depending on the scanning coverage in each patient. As a
result, a typical processed dataset had a size ranging from 40 to 65 Mbytes.
Although all of the image reconstructions were performed with Analyze V 6.0
software in this study, there are a number of other commercial software
packages, which allow for performance of similar post-processings.
Post-processing of 2D and 3D visualisations
Multiplanar reformation
Multiplanar reformation (MPR) allows the user to quickly and
easily view image data either in the plane of acquisition or any orthogonal
plane. Movies 1 and 2 show that images were demonstrated in a sequence of
coronal and sagittal MPR views in a patient with AAA after endovascular aortic
repair. Besides orthogonal orientations, the intersecting sections allow the
interactive display of intersecting orthogonal sections. Movie 3 shows a series
of images visualised by selecting interactive sections in the same patient as
in movie 1. Planes can be interactively sliced away in the three orthogonal
orientations to reveal interior sections of the cube (volume data). This
enhances viewers’ understanding of the complex anatomy of aortic aneurysm and
its relationship to the stent grafts.
Curved multiplanar reconstruction
As most of the aortic aneurysms are tortuous and angulated
to some extent, it is not aligned with the orthogonal axes of the 3D volume.
Conventional multiplanar reformation is not always able to reveal the
anatomical information required for assessment. Curved multiplanar reformation
(CVR) interactively generates any arbitrary plane through a volume and allows
reformatting of a series or the entire volume. This is especially useful in the
assessment of patients with tortuous aneurysms as CVR allows the viewer to
generate images along the centreline of the abdominal aorta. Movie 4 shows a
series of CVR images in a patient after endovascular repair of AAA. The stent
graft and arterial branches are clearly demonstrated in these images, even if
the aneurysm is angulated because a line can be placed in the centre of the
abdominal aorta, which allows generation of reconstructed images along the
centre of the aorta. In this case, the suprarenal stents were deployed above
the renal arteries, which appeared patent. CVR is useful in the evaluation of
tortuous or angulated aortic aneurysms and the relationship between suprarenal
stent struts and renal arteries.
Shaded surface display
Shaded surface display (SSD) represents the surface of a
structure within a volume dataset. It is fast to generate because it relies on
simple thresholding. It has the superiority in speed and flexibility in image
rendering in the visualisation of stent graft and arterial branches. Movie 5
demonstrates a series of SSD images generated with an interval of 15o
along the z-axis in a patient after endovascular repair of AAA. Because of the
inherent limitation of this visualisation, not all of the information in the
volume data is used. Moreover, it is difficult to differentiate high-density
calcification from stent wires on SSD images, as observed in the images. Thus,
SSD has little role to play in the assessment of endovascular repair since it
provides little additional information when compared to 2D axial images [7].
Maximum-intensity projection
Maximum-intensity projection (MIP) has been considered as
the preferred visualisation in vascular imaging because it is independent of
threshold selection and only the highest pixel value in a volume data can be
displayed in the final projected images. Structures such as calcification,
contrast-enhanced arteries, metal stent wires and bones are easily visualised
on MIP images. Due to this reason, editing of the volume data is necessary to
avoid overlapping of bony components and arteries as well as stent wires.
Manual editing is time-consuming while semiautomatic/automatic editing makes post-processing
a feasible and an acceptable technique in clinical practice. The Analyze
software that was used in this study and other commercially available software
packages for 3D post-processing allow the user to clip the volume data from any
angles (x, y or z axis), which makes the generation of MIP images much quicker
and more efficient. Movie 6 shows a group of MIP images generated with manual
removal of the bones and soft tissue while keeping the arteries and stent wires
(post-processing time is 30 minutes), whereas movie 7 shows a similar group of
MIP images generated with semi-automatic clipping method by removing some
structures of the volume data from the y-axis, while the x and z-axis remain
unchanged in the views (post-processing time is 3 minutes). The
contrast-enhanced arteries, stent wires and kidney are clearly shown in these
images, which were presented at an interval of 15o along the z-axis.
MIP has been found to be valuable in the follow-up of endovascular repair, e.g.
detection of endoleaks and assessment of stent migration [8]. Figure 1 is an
example of a patient developing endoleak after endovascular repair of AAA,
while Figure 2 (A, B) is another case showing the distal migration of stent
graft, which is accurately measured on sagittal MIP images.
Volume rendering
Different from SSD and MIP, volume rendering (VR) uses all
of the information of a volume data and is able to demonstrate the anatomical
structures within a volume such as calcification, aortic arterial branches and
stent graft. Moreover, a colour and degree of opacity are assigned to each
particular structure, which makes them easily identified and differentiated. Movie
8 shows volume rendered images displaying the variable structures within a
volume with different colours in a patient following endovascular repair of
AAA. As shown in the images, metal stent wires, arterial branches and bones can
all be displayed in one image. VR has been accepted as the favoured 3D
visualisation in assessment of the relationship between arterial branches and
stent graft as it is able to demonstrate the 3D relationship between these
structures and the processing time is very short, which takes only a few
minutes, making it acceptable as a routine clinical tool.
Virtual endoscopy
Unlike previously mentioned 3D reconstructions, virtual
endoscopy (VE) provides unique intraluminal information such as aortic ostium,
intraluminal stent wires and their relation to the ostium. Moreover, based on
our previous experience, the degree of encroachment to aortic ostium, mainly
interference of the renal ostium by stent wires can be accurately assessed on
VE images. In VE, contrast-enhanced blood needs to be removed from the vessel
before performing intraluminal fly-through. The details of generation of VE
images of the aortic ostia and stent wires have been described elsewhere [9].
It has been reported that VE enhances the understanding of the effect of
endovascular repair on aortic branches, especially in patients with AAA treated
with suprarenal stent grafts [7-10]. Figure 3 shows VE images of aortic stent
wires relative to the aortic ostium. The degree and configuration of
encroachment to the renal and superior mesenteric artery ostia can be clearly
visualised in these images, which are valuable for vascular surgeons to assess
the treatment of suprarenal stent grafting. Figure 2 (C, D) is an example
showing that VE accurately detects the distal migration of stent grafts in a
patient after endovascular repair of AAA. Movie 9 is a short video
demonstrating the virtual fly-through in a patient with AAA pre-stent grafting,
while movie 10 shows a virtual fly-through in a post-stent grafting patient. As
shown in movie 10, the viewing point starts at the top of the suprarenal stents
and moves caudally towards the bifurcated section located in the common iliac
arteries.
Discussion and conclusion
The role of CT angiography in aortic stent grafting has been
enhanced with the introduction of MSCT. In contrast to single slice CT, CT
angiography can be performed more efficiently with MSCT scanners due to faster
scanning speed and higher spatial and temporal resolution [11-13]. With the
latest 64-slice CT scanner, almost isotropic volume data can be obtained with
MSCTA, which significantly improves the image quality of post-processing 3D
reconstructions, mainly in cardiovascular imaging. Previous studies have
demonstrated the diagnostic value of 3D CT visualisations in patients with AAA
following endovascular repair when compared to 2D images [7-10]. However, no
systematic analysis was performed with regard to the advantages and
disadvantages of each image visualisation. This article has presented a series
of most commonly used 2D/3D reconstruction visualisations, which were
demonstrated in a dynamic format (short videos) with the aim of helping readers
to be familiar with the generation and application of these visualisations in
clinical practice. 3D MSCT post-processing has become a routine protocol in
these authors’ daily clinical practice, and it has been widely used as an effective
alternative to conventional angiography in many areas. MSCTA has replaced
conventional angiography in pre-operative and post-operative assessment of
endovascular repair of AAA. Therefore, accurate selection of the appropriate
visualisation method is of paramount importance in clinical practice since it
not only provides additional information when compared to 2D images, but also
avoids unnecessary examinations and reduces the workload for imaging staff. Table
1 summarises advantages and disadvantages of these 2D and 3D visualisations in
the evaluation of post-aortic stent grafting. It is noted that 2D axial images
play an essential role in most of the routine follow-up, e.g. detection of
endoleaks, monitoring of the aneurysm sac size, and assessment of patency of
stent-covered renal arteries, which are the three main factors to determine the
success of endovascular repair. However, 3D visualisations provide additional
information, which is considered valuable for follow-up of endovascular repair.
In comparison to 2D axial images, MIP and VE visualisations demonstrate
superiority in the evaluation of stent graft migration and encroachment of
aortic ostium by suprarenal stent wires. In contrast, VR is a very useful tool
for demonstrating the 3D relationship between stent grafts and aortic branches.
SSD has a limited role to play in post-stent grafting, while CVR is of help in
patients with angulated or tortuous aneurysms. As the long-term effect of
aortic stent grafting is unclear, the additional information provided by 3D
visualisations in the current study is considered to be valuable for vascular
surgeons to assess the outcomes of endovascular repair.
In conclusion, this article has presented a series of 2D and
3D reconstruction visualisations generated from MSCT angiography based on a
group of patients undergoing endovascular repair of AAA. While 2D axial CT
images remain the main visualisation in most of the post-stent grafting
situations, some kinds of 3D reconstructions are required to be generated for providing
necessary information for accurately assessing the treatment of endovascular
repair. Readers are expected to be familiar with these reconstructions and able
to choose the appropriate one for diagnostic purposes while dealing with
similar situations in their daily practice, especially for those heavily
involved in MSCT imaging and aortic stent grafting.
ACKNOWLEDGEMENT
The author would like to thank Dr. Peter Ellis and Dr. Anton
Collins for clinical cooperation.
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Received 20 July 2006; received in revised form 13 November 2006; accepted 9 May 2007
Correspondence: Discipline of Medical Imaging, Department of Imaging and Applied Physics, Curtin University of Technology, GPO Box, U1987, Perth, WA 6845, Australia. Tel.: +61-8-9266 7509; Fax: +61-8-9266 4344; E-mail: z.sun@curtin.edu.au (Zhonghua Sun).
Please cite as: Sun Z,
Multislice CT angiography in abdominal aortic aneurysm treated with endovascular stent grafts: evaluation of 2D and 3D visualisations, Biomed Imaging Interv J 2007; 3(4):e20
<URL: http://www.biij.org/2007/4/e20/>
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