2. INTRODUCTION
EVAR is the preferred treatment for infra-renal
abdominal aortic aneurysms when appropriate patient
selection is practiced.
Reduced short term mortality, hospital stay and ICU
care compared to that of open surgical repair.
Repeat intervention owing to endograft related
complications is necessary in approximately 15% of
patients and hence requires long-term postoperative
surveillance.
3. WHAT IS ENDOLEAK?
Endoleak is defined as a blood flow external to the
stent-graft and inside the aneurysm sac.
4. Static biphasic CT angiography is the current standard
method for pre- and postoperative imaging evaluation
of abdominal aortic aneurysms.
The recent introduction of time-resolved dynamic CT
angiography with repetitive bidirectional table
movement allows the assessment of temporal
enhancement patterns in the endograft, aneurysm sac,
and adjacent aortic branches with a high temporal
resolution.
5. AIM
To determine the time course of enhancement patterns
in the aorta and endoleaks at dynamic CT angiography
as well as their effect on the endoleak detection rate in
patients who have undergone abdominal aortic EVAR.
6. MATERIALS AND METHODS
Retrospective study.
All of the consecutive patients who were scheduled for
periodic CT follow-up of the abdominal aorta after EVAR
between February 2010 and November underwent dynamic
CT angiography.
Dynamic CT angiograms with complete coverage of the
entire endograft, followed by a static venous phase and
adequate arterial contrast with a minimal attenuation of
150 HU in the abdominal aorta during at least one phase of
dynamic CT angiography.
Exclusion criteria were incomplete image data or low
arterial contrast (attenuation <150 HU).
7. CTA protocol
All of the dynamic CT angiograms were obtained with
a 128-row CT scanner (Somatom Definition Flash;
Siemens Healthcare.
After localizer scans were obtained and 80 mL of
nonionic iodinated contrast material Iomeprol 400 mg
was injected at a flow rate of 4 mL/sec through an 18-
gauge antecubital intravenous line.
8. 10 unidirectional scan phases
of 2.5 seconds with a temporal
resolution of 5 seconds and a
field of view of 283 mm on the
long axis. The post threshold
delays were 2, 7, 12, 17, 22, 27,
32, 37, 42, and 47 seconds. A
delay of 7 seconds after the
threshold represented the
arterial phase of a biphasic
study.
Static venous phase scanning
that covered the entire
abdominal aorta with a delay
of 100 seconds after reaching
threshold.
9. IMAGE ANALYSIS
Image analysis was performed by using a commercially
available diagnostic workstation (Easyvision R11.4.1;
Philips Medical Systems.
Analysis of all of the acquisition phases was carried out
in random order without any image annotations in
consensus by two experienced radiologists with 10 and
3 years of experience who were blinded to the clinical
data and to the results of the CT examination.
10. Enhancement was calculated by subtracting the mean
attenuation measured in a region of interest from the
attenuation of the distal inferior vena cava in the first
available scan phase to exclude the attenuation of
unenhanced blood.
The degree of contrast material enhancement at each time
point was determined in the aortic lumen 1 cm cranial to
the stent-graft and in all of the detectable endoleaks larger
than 2 mm in the same position throughout all of the scan
phases.
The size of the endoleak region of interest was determined
by using the scan phase with the greatest endoleak extent
in a transverse section, and the size of this region of
interest was kept constant throughout all of the scan
phases. The distribution of the contrast material from the
endoleaks within the aneurysm sac was not assessed.
11. the overall image quality of each scan series was rated
on a five-point ordinal scale, taking into account image
noise and beam-hardening artifacts, as follows:
0 = very poor, 1 =poor, 2 = reduced, 3 = good, and 4 =
very good.
The ratings of all images obtained in the dynamic part
of dynamic CT angiography were combined, whereas
the static scan series of dynamic CT angiography was
rated separately.
12. STATISTICAL ANALYSIS
Enhancement measurements in the aorta and endoleaks
were compared by using paired t tests.
The null hypothesis (ie, that the use of dynamic CT
angiography results in an endoleak detection rate that is
similar to that at the time point of conventional biphasic
CT) was tested by using a t distribution and the Pearson x2
test.
The image quality ratings were compared by using the
Wilcoxon signed rank test.
The profile plot for aortic and endoleak enhancement was
generated on the basis of the estimated marginal means by
using analysis of variance with repeated measurements
13. RESULTS
Total: 71 patients. Men 66. mean age 72.2. women 5.
mean age 74.2.
Mean BMI: 28.2.
Ten types of endografts were used.
18. OVERALL IMAGE QUALITY
The mean overall image quality of the images obtained
in dynamic and static CT angiography phases was at
least 3, indicating an overall good image quality.
Radiation Exposure
The total average dose-length product at dynamic CT
angiography was 1344 mGy which consisted of an
average dose-length product of 784.5 mGy for the
dynamic phases and 560.1 mGy for the venous phase.
19. DISCUSSION
Imaging after EVAR is essential.
CT is considered to be the method of choice for these
follow-up examinations.
Contrast-enhanced US cannot replace CT angiography in
the post-EVAR follow-up with regard to graft anchoring
integrity, aneurysm morphology, or visceral vessel patency.
At most centres, CT is usually performed at least during
the contrast-enhanced arterial and venous phases.
Dynamic CT angiography with a large z-axis field of view,
such as is needed for coverage of the abdominal aorta, is
currently only available with two 128-sec-tion scanners
(Somatom Definition AS+ and Somatom Definition Flash,
Siemens Healthcare)
20. The maximum endoleak
enhancement was reached at
22 seconds after the bolus-
tracking threshold, during the
phase when the bolus of
contrast material had already
passed the aorta.
The highest endoleak
detection rate was achieved
later, at 27 seconds after the
bolus-tracking threshold,
during the phase when the mean
peak enhancement of the aorta
and the endoleak had already
passed and maximum contrast
could be achieved between the
remaining endoleak
enhancement and the rapidly de-
enhancing aorta.
For the commonly used biphasic
static CT angiography, we
conclude that the arterial phase
is acquired too early and the
venous phase too late to identify
the maximum number of
endoleaks
21. Scan phases 3 and 6, at 12 and 27 seconds after the
bolus-tracking threshold, respectively, are the most
useful scan phases in patients have undergone EVAR.
The first shows the highest aortic enhancement and
should be used to evaluate the aorta and its branches,
detect early endoleaks, and assess the endograft
anchoring and position.
The second of the two phases is used to detect
endoleaks.
A late venous phase (with a delay of 300 seconds) can
help detect low-flow endoleaks.
22. The percentage of endoleaks after EVAR is generally
reported to be 20%–30%.
The endoleak rate of approximately 45% found with
dynamic CT angiography in this series.
Most of the detected endoleaks were type II leaks and
the current knowledge regarding their management
differs widely.
This prevalence might explain some of the type V
endoleaks observed in bi-phasic CT angiography,
where sac enlargement is detected but no contrast
material can be detected outside the graft.
23. RADIATION DOSE
Dynamic CT angiography had a 10% higher radiation
dose and the venous phase had a 20% lower radiation
dose.
Exclusion of the dynamic CT angiography scan phases
with low diagnostic value (phases 1, 2, 9, and 10 at 2, 7,
42, and 47 seconds after the bolus-tracking threshold,
respectively), reduction of the tube voltage to 80 Kv
would reduce the dose by more than 20% compared
with that in our current dynamic CT angiography
protocol.
Restricting the venous phase acquisition to the region
of interest would result in an overall dose reduction of
approximately 65%.
24. With use of this reduced dose protocol, the dynamic
information of the enhancement patterns would not
be lost because the protocol would still include the
phases with the highest endoleak enhancement and
highest endoleak detection rates.
If the scanner used does not allow a dynamic
examination and the post-EVAR follow-up is
performed as biphasic static CT angiography, an
additional scan phase over the period of 22–32 seconds
after the bolus-tracking threshold should be added.
25. LIMITATIONS
Dynamic CT angiography was performed with a
reduced tube voltage of 80 kV. This choice resulted in
noisier images and helps explain the difference in
overall image quality between the dynamic and static
acquisitions.
the use of the lower kilovolt peak increased the image
contrast because the selected tube voltage was closer
to the limits of iodine absorption.
26. CONCLUSION
Dynamic CT angiography during post-EVAR follow-up
revealed that the peak enhancement of endoleaks is
significantly different than that of the aorta and is not
adequately evaluated with conventional biphasic CT
angiography protocols.
Use of dynamic CT angiography is associated with an
increased endoleak detection rate compared with the
detection rates at the time points of conventional
biphasic CT.
