This is an example of using image registration to determine the boundary conditions for the human aorta.

1. DEFORMABLE IMAGE REGISTRATION
OF A HUMAN AORTA
Alexander I. Veress, Ph.D.

2. Strain Measurements of the
Thoracic Aorta
 Determining the deformations and strains together
with known loading can give an indication of the
relative health of tissue.
 What follows is the measurement of strains in the
human thoracic aorta.

3. Image Selection:
 The end-diastolic and end-systolic images were
taken from the full cardiac cycle.
 The difference image shows the extent of the
expansion of the aorta.
End-diastole End-systole
Template Target Difference

4. Model Creation
 The aorta lumen manually segmented and an outer
surface was defined (left ). The outer surface
encompassed the 1.5 mm thickness of the aorta and
some of the adjacent connective tissue. These
surfaces were meshed using predominantly
hexahedral elements (right).

5. Registration Analysis
 The registration analysis was completed.
Inner and outer Circumferential Strain
 Inner and outer surfaces align with target image (left) indicating good global
registration. The strain results show an approximately 5% average
circumferential tensile strain within 2 mm of the lumen.These results agree
well with those of a middle aged patient. (Morrison et al. JVasc Surg.,2009 April ; 49(4):
1029–1036.)

6. Results/Discussion
 These results document the amount of expansion
that this thoracic aorta undergoes over the cardiac
cycle.
 The strain results could be used to back out the
aorta material properties as long as the aortic
pressure loading was known.
 Brachial artery blood pressure (arm cuff)
measurements are very convenient and non-
invasive but overestimate (~12-18 mmHg) the
pressure loading on the aorta due to pressure wave
reflection. (Munir et al. Hypertension. 2008;51:112-118)

7. Results/Discussion Cont.
 Pre-strain would need to be accounted for in order
for the material properties to have relevance.
 If one needed to match the displacement
characteristics (compliance matching) of an implant
to the native aorta then the strain data from the
registration could be used together with a forward
model of the implant to create a design that would
have similar pressure/ displacement characteristics.