calculating risk of ropture of an aaa- hsr 2014 congress
TRANSCRIPT
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Azienda Sanitaria Firenze 1
Stefano Michelagnoli MD, Emiliano Chisci, MD, Neri Alamanni, Phy.
Department of Surgery, Vascular and Endovascular Surgery Unit, “San Giovanni di Dio” Hospital, Florence, Italy.
Calculating risk of rupture of an AAA Milano December, 11 - 2014
www.anefix.com
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Rationale..
We would like to find out a tool to grade the
individual patient risk of rupture of an AAA....
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the estimated rupture rate for small AAAs is up as much 6% per annum
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AAA Rupture Risk Indicators
Anatomic Indicators
(Diameter)
Structural Indicators
(PWS, RPI)
Haemodynamic Indicators
(Aneurysm Formation Index(AFI),Gradient Oscillating
Number (GON), Relative Residence Time (RRT),
Oscillatory Shear Index (OSI) etc.)
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Fillinger MF, Marra SP, Raghavan ML, Kennedy FE. Prediction of rupture risk in abdominal aortic aneurysm during observation: wall stress versus diameter. J
Vasc Surg 2003;37:724–732.
Vande Geest JP, Di Martino ES, Bohra A, et al. A biomechanics-based rupture potential index for abdominal aortic aneurysm risk assessment: demonstrative application. Ann N Y
Acad Sci 2006;1085:11–21.
Gasser TC, Auer M, Labruto F, et al. Biomechanical Rupture Risk Assessment of Abdominal Aortic Aneurysm: Model Complexity versus Predicability of Finite Element
Simulations. Eur J Vasc Endovasc Surg 2010;40:176-185.
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RPI: rupture potential index
The most used risk factor
PWS = Peak Stress in AAA wall
RPI is a Safety Factor i.e.:
RPI= PWS / Wall Strength
Varies between 0 and 1: 0 = No Risk, 1 = Risk of imminent rupture
PWS depends on
AAA shape,
Wall strength: = 71.9 − 37.9 × (ILT^1/2 − 0.81) − 15.6×(NORD −
2.46) − 21.3 × HIST+19.3 × SEX (1)
Where ILT= Thickness in cm of Intraluminal Thrombus; NORD= Ratio between
AAA max diameter and non-aneurysmatic aorta diameter; HIST=0.5 in case
of positive familiary history for AAA,-0.5 otherwise; SEX=0.5 for male patient,
-0,5 for female patient
4 (1) Van de Geest JP, Wang DHJ, Wisniewski SR, Makaroun SM, Vorp DA. Towards A Noninvasive Method for Determination of
Patient-Specific Wall Strength Distribution in Abdominal Aortic Aneurysms. Annals of Biomedical Engineering, Vol. 34, No. 7,
July 2006 ( C 2006) pp. 1098–1106
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RPI how to calculate it
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CT Scan
Image Segmentation
Geometric Model creation
F.E. Model creation
Model solution and index calculation
Good CT scan required
Expensive proprietary
software required
Dedicated hardware
required
Engineering skill
required to assess the
quality of the analysis
Off-line analysis:
(1 hr from CT to results)
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Another Approach: based upon haemodynamic
indicators from 2d/3d models
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CDU
Geometry recognition
CFD, FSI mesh
Input (jpeg or Dicom)
Wss
Shear Rate
OSI
Risk factors
calculation,
evolutivity or CT
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2D instead 3D models: rationale
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CT scans required, or even a good US image
Only Open Source used (Gmsh or Netgen for meshing, CSC
Elmer for analysis)
Standard hardware required
Very short analysis time: from image to results (10 mins)
Cheap and affordable approach
Implementable even during a US routinary check
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Haemodinamic risk indicators (WSS, OSI)
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Wall Shear Stress (WSS) is the frictional force per unit area acting on the
inner vessel wall as a result of a flow of viscous blood.
Malek AM, Alper SA, Izumo S “Hemodynamic Shear Stress and Its Role in Atherosclerosis”
AMA. 1999;282(21):2035-2042.
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OSI
Oscillatory Shear Index (OSI) is used to identify regions on the vessel
wall subjected to highly oscillating WSS values during the cardiac
cycle. These regions are usually associated with bifurcating flows and
vortex formation that are strictly related to atherosclerotic plaque
formation and fibrointimal hyperplasia.
• Low OSI values occur where flow disruption is minimal
• High OSI values (with a maximum of 0.5) highlight sites where the
instantaneous WSS deviates from the main flow direction in a large
fraction of the cardiac cycle,inducing perturbed endothelial alignment.
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Methods
CTA scans were coupled with computational fluid dynamics (CFD) evaluation performed using open source software
ElmerSolver, CSC Institute of Technology, Finland.
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Results: 143 pts
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2D
3D
rAAA rAAA rAAA
Distribution of the 2D-3D CFD parameters OSI (A), TAWSS (B) and RRT (C) vs. the presence of non aneurysmatic aorta, AAA, or ruptured AAA.
2D OSI
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Results: 143 pts
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2D
3D
rAAA rAAA rAAA
Distribution of the 2D-3D CFD parameters OSI (A), TAWSS (B) and RRT (C) vs. the presence of non aneurysmatic aorta, AAA, or ruptured AAA.
2D OSI
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AAA Rupture RISK SCORE
multivariate analysis
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B Constant 1.746 OSI
<0.3 -0.804
0.3-0.48 -0.406
>0.48 0
AAA diameter <5.5 cm -0.552
5.5-7 cm -0.047
>7 cm 0 Diabetes -0.255
The Risk Score for AAA rupture
AAA I
LOW RISK - RR <0.5% per
annum
<2.3
AAA II
MODERATE RISK: RR 1% per
annum
2.3-6.5
AAA III
HIGH RISK: RR >50% per
annum
>6.5
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AAA rupture risk score
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CLINICAL RELEVANCE
For small AAAs: Low risk patients (class AAA I) could be followed-up as reported by current guidelines (AAAs 3.0-4.0 cm in size require yearly ultrasound surveillance, with half-yearly follow-up if enlargement is noted to be greater than 10% per year; AAAs 4.0-5.5 cm in size require 6 monthly abdominal ultrasound).
Moderate AAA risk patients (AAA II) could be followed at 3-month intervals.
High AAA risk patients (AAA III) could be offered to undergo a repair operation even if the 5.5 cm threshold has not been reached yet according to our scoring system.
For large AAAs: A delayed repair for AAA>5.5 cm with a reduced risk of rupture in old patients with many comorbidities (AAA I class according to our study), could be hypothesized. Therefore, a
SCREENING: Another possible role of the score herein proposed is its possible combination with an AAA screening programme. In fact, our ongoing research is to combine 2D CDU images to CFD evaluation and give a quick risk of AAA rupture to each screened patient in order to offer an accurate follow-up or treatment.
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WSS at the systole
WSS at the diastole
Velocity
An example
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An example An example
A ruptured AAA
OSI
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WSS at the systole
WSS at the diastole
Velocity
An example
A ruptured AAA
OSI
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A future prospective:
from ultrasound
38 patients with coupled 2D images and CT scan
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2D vs 3D geometries
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US Image
2D Model
3D Model
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Results from structural analysis of the
3D model
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Von Mises
Stress
RPI (or
PWRR) >
100%
Patient at
High Risk
(Wall
Strength
5.83 E5 Pa)
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2D Risk evaluation: OSI Index
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OSI Index
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2D Risk evaluation: OSI Index
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OSI Index distribution
calculated from a 2d model
is not incoherent with Von
Mises as calculated from a
3D model
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Remarks
The use of 2d model, deduced from a US scan, allows
a risk assessment in this case comparable with the
one produced by a 3D analysis
In order to understand if the 2d approach is a feasible
approach we compared the results got from 18 patients
with 2d OSI indicator and 3D RPI indicator (8 pts with
no AAA and 10 with ruptured AAA)
If the use of 2d models is comparable with the use of
3d model, the healthy patients and the ruptured AAA
should be selected in the same way and should give
the same risk evaluation
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Comparing 2d OSI & 3D RPI
The RPI based risk indicator is:
If AAA diam > 4.5 cm && RPI > 50%
Risk presence
Else
No Risk
The OSI based risk indicator is:
If AAA diam > 4.5 cm && OSI > 0.3
Risk presence
Else
No Risk
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Comparison
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Pt. ID Age Sex Diam [cm.] Wall Strength [Pa] Von Mises Stres [Pa] RPI 3D OSI 2D Risk RPI Risk OSI
Pz1 77 F 1.938 1.05075E+06 1.73000e+05 16.46% 0.494 NO NO
Pz2 68 M 2.153 1.23186E+06 1.34726E+05 11.00% 0.106 NO NO
Pz3 77 F 2.243 1.031726E+06 1.95000E+05 18.90% 0.133 NO NO
Pz4 68 M 2.566 1.199602E+06 3.72000E+05 31.01% 0.140 NO NO
Pz53 69 F 2.681 1.020478E+06 1.77000E+05 17.34% 0.114 NO NO
Pz211 63 M 3.379 1.193518E+06 2.10213E+05 17.61% 0.166 NO NO
Pz213 81 M 3.001 1.216672E+06 5.76500E+05 47.38% 0.083 NO NO
Pz214 75 F 2.906 1.301486E+06 3.28100E+05 31.81% 0.135 NO NO
Pz128 63 M 7.502 1.91266E+05 1.640000E+06 857.45% 0.473 YES YES
Pz130 83 M 6.150 5.41050E+05 3.051000E+06 563.90% 0.497 YES YES
Pz131 82 M 6.470 5.16090E+05 7.08900E+06 137.36% 0.422 YES YES
Pz132 77 F 7.069 1.00000E+05 1.348000E+06 1348.00% 0.479 YES YES
Pz135 85 F 6.761 4.11399E+05 5.80000E+05 140.98% 0.497 YES YES
Pz136 81 M 9.424 1.00000E+05 1.380000E+06 1380.00% 0.5 YES YES
Pz137 90 M 8.515 1.00000E+05 6.70000E+05 331.11 0.499 YES YES
Pz143 65 M 4.570 6.57015E+05 1.360000E+06 207.00% 0.310 YES YES
Pz145 74 M 9.377 2.89383E+05 1.194000E+06 412.60% 0.499 YES YES
Pz216 84 M 6.749 3.52413E+05 7.91000E+05 224.45% 0.494 YES YES
Von Mises Stress is calculated using 3d models from CT Scan, with non linear HyperElastic Solver (Code Aster)
Wall Strentgh is calculated according to (1)
OSI index is calculated using 2d models created from 2d sections of a CT scan, using Elmer CSC CFD solver
(1) Vande Geest JP, Wang DHJ, Wisniewski SR, Makaroun SM, Vorp DA. Towards A Noninvasive Method for Determination of Patient-Specific Wall
Strength Distribution in Abdominal Aortic Aneurysms. Annals of Biomedical Engineering, Vol. 34, No. 7, July 2006 ( C 2006) pp. 1098–1106
Identical risk evaluation for 2d and 3d models
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Comparison
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Pt. ID Age Sex Diam [cm.] Wall Strength [Pa] Von Mises Stres [Pa] RPI 3D OSI 2D Risk RPI Risk OSI
Pz1 77 F 1.938 1.05075E+06 1.73000e+05 16.46% 0.494 NO NO
Pz2 68 M 2.153 1.23186E+06 1.34726E+05 11.00% 0.106 NO NO
Pz3 77 F 2.243 1.031726E+06 1.95000E+05 18.90% 0.133 NO NO
Pz4 68 M 2.566 1.199602E+06 3.72000E+05 31.01% 0.140 NO NO
Pz53 69 F 2.681 1.020478E+06 1.77000E+05 17.34% 0.114 NO NO
Pz211 63 M 3.379 1.193518E+06 2.10213E+05 17.61% 0.166 NO NO
Pz213 81 M 3.001 1.216672E+06 5.76500E+05 47.38% 0.083 NO NO
Pz214 75 F 2.906 1.301486E+06 3.28100E+05 31.81% 0.135 NO NO
Pz128 63 M 7.502 1.91266E+05 1.640000E+06 857.45% 0.473 YES YES
Pz130 83 M 6.150 5.41050E+05 3.051000E+06 563.90% 0.497 YES YES
Pz131 82 M 6.470 5.16090E+05 7.08900E+06 137.36% 0.422 YES YES
Pz132 77 F 7.069 1.00000E+05 1.348000E+06 1348.00% 0.479 YES YES
Pz135 85 F 6.761 4.11399E+05 5.80000E+05 140.98% 0.497 YES YES
Pz136 81 M 9.424 1.00000E+05 1.380000E+06 1380.00% 0.5 YES YES
Pz137 90 M 8.515 1.00000E+05 6.70000E+05 331.11 0.499 YES YES
Pz143 65 M 4.570 6.57015E+05 1.360000E+06 207.00% 0.310 YES YES
Pz145 74 M 9.377 2.89383E+05 1.194000E+06 412.60% 0.499 YES YES
Pz216 84 M 6.749 3.52413E+05 7.91000E+05 224.45% 0.494 YES YES
Von Mises Stress is calculated using 3d models from CT Scan, with non linear HyperElastic Solver (Code Aster)
Wall Strentgh is calculated according to (1)
OSI index is calculated using 2d models created from 2d sections of a CT scan, using Elmer CSC CFD solver
(1) Vande Geest JP, Wang DHJ, Wisniewski SR, Makaroun SM, Vorp DA. Towards A Noninvasive Method for Determination of Patient-Specific Wall
Strength Distribution in Abdominal Aortic Aneurysms. Annals of Biomedical Engineering, Vol. 34, No. 7, July 2006 ( C 2006) pp. 1098–1106
Identical risk evaluation for 2d and 3d models
95 % coincidence in
risk evaluation using
RPI & OSI
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Conclusions
The use of 2d models seems a promising alternative
approach to the conventional RPI calculations based upon
3d models derived from CT scan
In a near future it seems to be possible to use 2D
ultrasound images for calculating aneurysm rupture risk.
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A potential selective treatment and follow-up could be offered to patients with either a small or a large AAA on the basis of the individual risk of AAA rupture.