AJR 2000; 174:820-822
© American Roentgen Ray Society
Usefulness of CT Angiography with Volume Rendering After Carotid Angioplasty and Stenting
X. Leclerc1,
J. Y. Gauvrit and
J. P. Pruvo
1
All authors: Department of Neuroradiology,
Hôpital Roger Salengro, University Hospital of
Lille, France
Received April 16, 1999;
accepted after revision August 16, 1999.
Address correspondence to X. Leclerc, Service de Neuroradiologie,
Hôpital Roger Salengro, Boulevard du Professeur
Leclercq, 59037 Lille, France.
Introduction
Previous reports have shown the effectiveness of helical CT with
three-dimensional reconstructions to evaluate carotid artery stenosis,
although calcifications of the vessel wall may obscure the visualization of
the residual lumen on maximum-intensity-projection images
[1,
2]. Volume-rendering technique
is a new three-dimensional postprocessing algorithm based on a selection of
voxels of the image with adjustment of opacity for each selected material to
allow change in the transparency
[3]. This method has recently
shown its usefulness to assess the residual arterial lumen through
arterial-wall calcifications
[4]. Another potential
application of volume-rendering technique is using helical CT for follow-up of
patients treated with carotid angioplasty and stenting because the arterial
lumen can be analyzed theoretically despite the high-attenuation values of the
stent.
The purpose of this study was to evaluate the feasibility of using the
volume-rendering technique for visualization of the arterial lumen through the
stent wall.
Subjects and Methods
From January 1998 to February 1999, we performed carotid angioplasty with
stenting in six patients (six men; age range, 47-74 years; median age, 73
years). All patients were referred from the vascular surgery department for a
high-grade carotid restenosis that had occurred after endarterectomy. Patients
underwent Doppler sonography and conventional angiography before angioplasty.
Endovascular treatment was performed by femoral approach using a 3- or 4-cm
self-expandable stent (Easy Wall-stent; Schneider, Minneapolis, MN). Follow-up
helical CT angiography was performed at a median delay of 2 months after the
procedure (range, 1-14 months). Informed consent was obtained from all
patients for helical CT examinations.
CT was performed on a Somatom Plus 4A scanner (Siemens, Erlangen, Germany)
with continuous data acquired for 40 sec and started approximately 2 cm below
the stent. A total volume of 120 ml of nonionic contrast material was
administered IV at a rate of 3 ml/sec, using a power injector. Helical
scanning (2-mm collimation, 3-mm/sec table speed, 12-cm field of view, 120 kV,
200 mA, 512 x 512 matrix) was automatically triggered by a tracking
acquisition that continuously monitored the attenuation values from the volume
of interest in the common carotid artery. A threshold of 25 H was
predetermined and continuous images were acquired at the lower part of the
acquisition volume every 3 sec for 30 sec with a 10-sec scan delay after the
start of the contrast material injection. When the attenuation value increased
at the arrival of contrast material and exceeded the defined threshold value,
the helical scanning automatically started. The axial images were
reconstructed at 0.5-mm increments using a 180° linear-interpolation
algorithm. A high-spatial-frequency convolution algorithm was used. The window
level was preset between 150 and 300 H, with a width of between 400 and 800
H.
CT data were transferred to an independent workstation (Magic View;
Siemens) for three-dimensional reconstructions including
maximum-intensity-projection and volume-rendering technique algorithms.
Regions of interest were manually selected on axial source images. As
previously reported [4],
volume-rendering technique is based on a statistical classification using
trapezoid of each voxel according to the different materials present in the
voxel. The position and the shape of the trapezoid determine the number and
the attenuation of the voxels incorporated into the image, whereas the opacity
value determines their relative transparency. Parameters were determined in a
preliminary study. The first trapezoid related to the enhanced arterial lumen
and ranged from 200 to 600 H, and the second trapezoid related to the wall of
the stent and ranged from 780 to 3000 H
(Fig. 1). An opacity value was
then determined to define the relative transparency of each material. We chose
by empiric approach a maximal opacity value (100%) for the contrast material
and a minimal opacity value (5%) for the stent to visualize the enhanced
arterial lumen through the stent wall. The time required by technologists to
manually select the images and to display maximum-intensity-projection and
volume-rendered images was approximately 15 min.
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Fig. 1. —Graph shows trapezoid parameters for visualization of arterial lumen
(left) and stent (right). Points A and D represent minimum and maximum
attenuation values, respectively. Points B and C represent maximum variation
of attenuation of contrast material.
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Results
Using the bolus-tracking technique, arterial contrast enhancement appeared
optimal in all patients and enabled us to perform a complete evaluation of the
arterial lumen from the common carotid artery below the stent to the upper
portion of the internal carotid artery at the skull base. No motion artifact
was observed. The use of a high-spatial-frequency convolution algorithm and
appropriate windows allowed us to differentiate the wall of the stent from the
enhanced arterial lumen within the prosthesis in all patients. However, axial
source images showed an apparent increase in the thickness of the stent wall
(Fig. 2A).
Maximum-intensity-projection reconstructions did not allow the visualization
of the arterial lumen owing to the higher attenuation values of the stent
compared with those of the enhanced arterial lumen
(Fig. 2B). Volume-rendered
images showed accurate delineation of the enhanced arterial lumen through the
stent despite the thickness of the stent wall, which appeared uniformly
increased (Fig. 2C).
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Fig. 2A. —47-year-old man 1 month after carotid angioplasty and stenting.
Axial CT scan obtained at mid portion of stent shows accurate differentiation
between high-attenuation values of stent (arrow) and enhanced
arterial lumen (arrowhead).
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Fig. 2B. —47-year-old man 1 month after carotid angioplasty and stenting.
Maximum-intensity-projection CT scan shows that high-attenuation values of
stent (arrow) obscure arterial lumen of internal carotid artery.
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Fig. 2C. —47-year-old man 1 month after carotid angioplasty and stenting.
Volume-rendered CT scan reveals accurate delineation of arterial lumen
(black arrow) through stent despite apparent increase in thickness of
stent wall (white arrow).
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Discussion
Percutaneous transluminal angioplasty with stenting constitutes a promising
therapeutic approach for carotid artery stenosis that is currently under
investigation [5]. Noninvasive
techniques such as duplex sonography, MR angiography, or helical CT are
required for follow-up to avoid catheter angiography. Duplex sonography
provides both morphologic and hemodynamic data
[6], but the superior portion
of the stent may be difficult to evaluate because of the skull base.
Contrast-enhanced MR angiography constitutes another noninvasive technique
that has proved its effectiveness for the evaluation of carotid stenosis
[7] but susceptibility
artifacts may hinder complete visualization of the stent. CT angiography with
volume rendering is a third technique that can allow visualization of the
arterial lumen through the stent. Parameters of acquisition used in our study
were similar to those usually used by other investigators for imaging carotid
arteries [1,
2,
4]. However, a bolus-tracking
acquisition technique was included in our protocol to optimize contrast
enhancement during the arterial phase. This technique enabled us to
differentiate the enhanced arterial lumen from the wall of the stent on axial
source images in all patients.
By applying a volume-rendering algorithm, we classified voxels according to
the probability that they contained a tissue type. The two selected materials
(contrast material and stent) were retained by using trapezoid functions with
specific adjustments of parameters. However, this method led to an
overestimation of the thickness of the stent wall in all patients despite
having separate trapezoids for the arterial lumen and the stent wall. This
overestimation was probably related to a partial volume effect of voxels at
the border of the stent including portions of both the arterial lumen and the
stent wall [8]. This
misclassification of voxels depends on the position of the trapezoids and can
be minimized by using thin slices, overlapping reconstruction, and a
high-resolution matrix. Despite this limitation, the high-attenuation values
related to the stent could be assessed in their transparency by applying a
low-opacity value; this technique allowed accurate delineation of the arterial
lumen through the stent.
In conclusion, volume-rendered images provided three-dimensional
angiographic appearance with good analysis of the relationship between
arterial lumen and stent. This technique might be useful to evaluate the
carotid artery after angioplasty with stenting and especially to detect
restenosis in the long-term follow-up after treatment.
Acknowledgments
We thank Olivier Godefroy for his helpful comments, Eric D'haese for
photographic reproductions, Corinne Rose and Melanie Cnockaert for their
assistance in preparing the manuscript, and the technical staff of the CT
department for their support.
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Introduction
Subjects and Methods
