Case Studies
Iowa City, IA - June 2001
Stress Distribution in a Human Abdominal Aortic Aneurysm.
View
AVI movie (1.8 MB)
M. L. Raghavan, an Assistant Professor in Biomedical
Engineering at the University
of Iowa, uses Tecplot extensively because it provides the broad range
of functionality and ease-of-use that he requires to quickly analyse his
FEA (finite-element analysis) results. His animations and plots play an
important role in presenting to groups such as the National
Institute of Health and the Whitaker
Foundation.
While working at the School
of Engineering at Dartmouth
College as a research associate, Dr. Raghavan, in collaboration with
Dr. Kennedy and Dr. M. F. Fillinger, researched the biomechanics of aneurysm
disease. The goal of their research was to make aneurysm surgical planning
more safe and effective.
In the clinical management of patients with aneurysm disease, it is important
to know whether a particular aneurysm is at a high or low risk of rupture.
By visualising the stress distribution, the surgeon can get a clearer
idea of the aneurysm's rupture risk, the most susceptible region, whether
or not to perform the surgery, and then plan their surgical intervention
accordingly.
Typically, they would perform a AAA (abdominal aortic aneurysm) resection,
wherein they would cut open the abdomen, cut open the AAA and then stitch
a synthetic tube (called vascular graft) in the place of the aneurysm.
The vascular graft allows the blood to flow through the tube rather than
the aneurysmal aorta. Since the surgery is risky (patients have died from
the procudure), the surgeon needs to balance the risk of rupture with
the risk of surgery before deciding to perform the AAA resection.
Our Image of the Month represents the distribution of von Mises Stress
(an aggregate index of the three principal components of stress) on the
wall of an abdominal aortic aneurysm. Von Mises Stress is considered the
best indicator of rupture, though it is open to debate and a topic of
research.
 Main
image with a custom color map applied. This color map follows well-accepted
mapping conventions in the medical imaging community.
The geometry of the aneurysm was reconstructed from CT scan
images of the subject's abdomen. The 3-D reconstruction is done through
manually-assisted automated digitisation. The image processing and 3-D
reconstruction were performed by Medical
Media Systems, Inc. in West Lebanon, NH in collaboration with Dr.
Steve Peiper.
Dr. Raghavan takes the image-processed 3-D data and creates a computational
mesh that represents the AAA surface using custom mesh refinement algorithms.
The mesh is then used to perform a finite element stress analysis using
ABAQUS
software. Input used for the FEA is a homogenous hyper-elastic, isotropic
and incompressible material model. Material properties of the model were
determined from previously published experimental work that Dr. Raghavan
did as a graduate student under Dr. David Vorp at University of Pittsburgh.
The boundary conditions are peak internal pressure of the patient as measured
in a clinic.
The von Mises Stress output from ABAQUS is converted to a Tecplot input
data file using a program written by Dr. Raghavan. A Tecplot macro, also
written by Dr. Raghavan, reads the Tecplot data file, sets the plot attributes
and style, animates the 3-D rotation of the AAA, and exports an AVI file
of the resulting 3-D stress distribution.
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