Quantitative monitoring of abdominal aortic aneurysms using intravascular and three-dimensional ultrasound

Abdominal aortic aneurysms are the 13th leading cause of death in Western world
for people aged between 60 and 85 years. Criteria for intervention (maximum
diameter exceeds 5.5 cm in males, 5.0 cm in females, or a growth rate of 0.5 cm
in 6 months) have shown to be unreliable[1][2]. Hence, a new approach for
rupture risk assessment is needed[3][4]. From a mechanical point of view the
aneurysm will rupture if the mechanical stress, induced by the blood pressure,
exceeds the local strength of the vessel wall. Therefore, the strength of the
vessel wall and the local elastic behaviour of the vessel wall, could be better
predictors for rupture risk[5][6]. In this study, the global and local elastic
behaviour of the aneurysmal abdominal aortic wall and thrombus are
characterized by the analysis of intravascular and real-time 3D ultrasound
images.

The advantage of this study, compared to other studies in the literature, is
the use of multi-perspective images obtained by the combination of
intravascular ultrasound[7] and non-invasive 3D ultrasound images[8]. Previous
research for AAA rupture risk assessment used mainly Computed Tomography
(CT)[10] and sporadically Magnetic Resonance Imaging (MRI)[11]. However, due to
the radiation exposure, the use of nephrotoxic contrast agents and the lack of
temporal resolution, CT cannot be used as a routine screening method for
patients with an abdominal aortic aneurysm. MRI has a high soft tissue
contrast, however it is expensive and has limitations regarding the resolution.
Therefore ultrasound imaging is the ideal image modality since it has a high
temporal resolution, the ability for real-time 3D imaging, and the low costs.
While nowadays researchers often still assume a uniformly distributed wall
thickness of 2 mm of the whole aorta[11], the addition of intravascular
ultrasound images will provide the ability to measure the wall thickness
locally with high precision. Using intravascular and 3D ultrasound imaging
combined, the full geometry, the global distensibility, and local elasticity of
the aortic wall and thrombus can be determined[12]. Furthermore, stress
analysis can be performed, using the same data as vital input.

Ultimately, this study is intended to accurately assess the rupture risk
potential of the aneurysm. This would support screening, diagnosis and clinical
decision making in terms of the need of endovascular aneurysm repair.

1. Darling, R et al. Autopsy study of un-operated abdominal aortic aneurysms.
The case for early resection, Circulation, 1977, Vol. 56, No. 3 pp. 161-4.
2. Conway, K et al. Prognosis of patients turned down for conventional
abdominal aortic aneurysm repair in the endovascular and sonographic era :
Szilagyi revisited, Journal of vascular surgery, 1977, Vol. 33, No. 4 pp. 752-
757.
3. Pape L et al. Aortic Diameter >5.5 cm Is Not a Good Predictor of Type A
Aortic Dissection. Observations from the International Registry of Acute Aortic
Dissection (IRAD). Circulation. 2007;116:1120-1127
4. Neri E et al. Limited role of aortic size in the genesis of acute type A
aortic dissection. European Journal of Cardio-thoracic Surgery 2005;89 857-863
5. Fillinger M et al. Prediction of rupture risk in abdominal aortic aneurysm
during observation: wall stress versus diameter., Journal of vascular surgery,
2003Vol. 37, No. 4 pp. 724-32.
6. Gasser, T et al. Biomechanical rupture risk assessment of abdominal aortic
aneurysms: model complexity versus predictability of finite element
simulations., European journal of vascular and endovascular surgery: the
official journal of the European Society for Vascular Surgery, 2010, Vol. 40,
No. 2 pp. 176-85.
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Muiswinkel, J. M., van Sambeek, M. R., ... & van Urk, H. (1999). Accurate
assessment of abdominal aortic aneurysm with intravascular ultrasound scanning:
validation with computed tomographic angiography. Journal of vascular surgery,
29(4), 631-638.
8. Petterson, N. J., van Disseldorp, E. M., van Sambeek, M. R., van de Vosse,
F. N., & Lopata, R. G. (2019). Including surrounding tissue improves
ultrasound-based 3D mechanical characterization of abdominal aortic aneurysms.
Journal of biomechanics, 85, 126-133.
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biomarkers., European journal of vascular and endovascular surgery : the
official journal of the European Society for Vascular Surgery, 2010, Vol. 39,
No. 4 pp. 410-6.
10. Speelman, L et al. Patient-specific AAA wall stress analysis: 99-percentile
versus peak stress., European journal of vascular and endovascular surgery :
the official journal of the European Society for Vascular Surgery, 2008, Vol.
36, No. 6 pp. 668-76.
11. De Putter, S et al. Patient-specific initial wall stress in abdominal
aortic aneurysms with a backward incremental method., Journal of biomechanics,
2007, Vol. 40, No. 5 pp. 1081-90
12. Karatolios K. et al. Measurement of aortic wall strain with 3D speckle
tracking. Thorac Cardiovasc Surg 2012; 60;111

Primary Objective:
Patient-specific full geometry assessment of abdominal aortic aneurysms (AAA).
The assessment includes the shape and thickness of the intraluminal thrombus
and the aortic wall.

Secondary Objective(s):
IVUS-based assessment of tissue deformation for improved modelling of AAAs.
Global and local characterization of tissue deformation will describe the
elastic behaviour and wall stress of both thrombus and wall. A novel
multi-perspective ultrasound imaging platform, combining high-frequency
intravascular and non-invasive 3D ultrasound imaging allows for such
quantitative functional imaging.

This is a study for patients who will undergo an already scheduled endovascular
aneurysm repair (EVAR). When the patient agrees to participate in the study,
first, an additional non-invasive 3D ultrasound dataset is acquired of the
abdominal aortic aneurysm (AAA). 3D ultrasound is without any risks and takes
no additional time. Second, the patient will be prepared for the EVAR. This
procedure carries certain risks including damage to the blood vessel, bruising
or bleeding at the puncture side, and infection. These risks are mostly caused
from placing the guidewire, which is needed to be done for the EVAR procedure.
These risks are already present, and are not caused by the addition of the
measurements for this study. Before the stent-graft is actually placed, the
already positioned guidewire is used for an additional intravascular ultrasound
measurement of the AAA. The catheter will be pulled-back at a speed of 0.5 mm/s
using a motorized pullback device. The risks of this intravascular ultrasound
are negligible, since these measurements are performed using the same guidewire
that is already necessary for the EVAR procedure. Furthermore, this
intravascular ultrasound takes maximum 10-15 minutes. Finally, a last
non-invasive 3D ultrasound dataset is acquired at the same position. These 3D
ultrasound images include the guidewire which is necessary for registration.
The intravascular ultrasound measurement automatically obtains both DICOM and
envelope data. Envelope data is the post-processed received ultrasound data
before it is transformed into DICOM data. This envelope data does not require
additional measurements or time and will therefore not influence the procedure.
Using the envelope data, the AAA tissue deformation can be measured more
accurately and the thrombus can be segmented more accurately.

Multiple patient-specific datasets including both intravascular and 3D
ultrasound are necessary to determine the global and local elastic behaviour
and aneurysm wall characteristics. A large amount of datasets is required
because the use of deep learning techniques in post-processing.

Offline analysis will be performed on the intravascular and 3D ultrasound
datasets and consists of post-processing of the data: wall and thrombus
segmentation, motion tracking and elastography. Also a novel multi-perspective
ultrasound imaging platform, combining high-frequency intravascular ultrasound
and 3D ultrasound will be used. Furthermore, a patient-specific Finite Element
Model will be used to simulate the elastic behavior and the wall stress based
on the multi-perspective imaging data. These methods will be developed using
Matlab© and ABAQUS© software and applied to the available data to estimate the
elastic behavior mechanical properties of the AAA vessel wall and thrombus in
time. This post-processing is performed by the principal investigator, at and
in collaboration with, the Eindhoven University of Technology (TU/e). As a
secondary user, Philips and affiliates will receive the data for
post-processing.

This is a feasibility study in which we are unsure about the percentage of
patients of which we can acquire successful measurements. All patients who want
to participate in this study are included. The required number of 50 patients
is based on the total amount of patients who are scheduled for an endovascular
aneurysm repair in the Catharina Hospital Eindhoven, which is about 80 patients
every year. Therefore, it would be feasible to include approximately 50
patients in the upcoming two years.

There is no personal benefit in taking part in this study. The research adds
maximum 10-15 minutes to the total EVAR procedure. The catheter for the
intravascular ultrasound will use the guidewire that is already in position for
placing the stent-graft during the EVAR procedure. Any procedure that places a
catheter inside a blood vessel carries certain risks. These risks include
damage to the blood vessel, bruising or bleeding at the puncture side, and
infection. These risks are mostly caused from placing the guidewire, which is
already done for the EVAR procedure. The risks associated with the extra
intravascular ultrasound pullbacks itself can therefore be considered
negligible.
In summary, the patients are only asked 10-15 minutes of their time, and in
return, they will contribute to an increment in knowledge about the influence
of thrombus and the risk assessment which ultimately will replace diameter
surveillance, prevent premature rupture of AAAs, prevent overtreatment of AAAs
and reduce the risks and costs involved.