Examination of carotid pre-bifurcation expansion to predict boundary layer separation

Team 3Ashley, John, Minh, Jeff, Aaron

Problem Statement

• The purpose of this study is to develop a new procedure or index for predicting coronary artery disease in patients by using fluid mechanics and bimolecular principals along with ultrasound imaging techniques.

Background

• Carotid has been subjected to extensive studies to try to find factors that can predict atherosclerosis• Atherosclerotic risk is currently determined using

Carotid Intima Media Thickness (CIMT) • Measurement of arterial wall thickness,

thicker=higher risk

• This and most other correlations fail to include measurements which can predict boundary layer separation

Background

• Most predictors don’t take into account fluid mechanical effects which can lead to low wall shear stress (WSS) and wall thickening causing plaque buildup

• Geometry from beginning of expansion to the bifurcation point is most likely to initiate boundary layer separation

Figure 1. The black rectangle encompasses the region of interest

for this diffuser model.

Background

Background• Relating this pre-bifurcation expansion to a diffuser, we

recognize:• The change in pressure and• Bifurcation angle can lead to boundary layer separation

• Boundary layer separation corresponds to low wall shear stress

• Areas of low wall shear stress have been shown to correlate to the buildup of atherosclerotic plaque.

Hypothesis

• Increased atherosclerotic risk corresponds to pressure changes and large bifurcation angles in the pre-bifurcation expansion.

Procedure• Region of Interest:

Procedure: Developing an Index• Using MRI images from literature

with pre-assigned risk levels created an index that assessed risk based on:• Pressure Change• Bifurcation angle

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Procedure: Applying the Index

• Test the system on 6 subjects

• Four measurements taken using ultrasound• D1 – Diameter at beginning of expansion

• D2 – Diameter at widest point in bifurcation

• L – Length between diameters• Velocity of the blood in the carotid at D1

• Calculate pressure change and bifurcation angle

Procedure: Applying the Index

Procedure: Repeatability• Testing repeatability:• 4 measurements on one artery• 5 different “technicians”

Results

Discussion• Creating the Index• For the geometries from literature, no velocity measurements

were known, so pressure change for all risk levels were calculated with the same velocity• So little trend is visible with respect to the pressure change• In reality, these velocities would be different

• The accuracy of the index requires a longer term study• Our patients were young and healthy, and since our index is truly

predictive, we would need to retest the same patients later in life to see if the predictions were correct.

Statistics: RepeatabilityTechnician D1 D2 L V1 Theta ΔP 1 0.556 1.223 0.98 71.2 0.597595 2426.45 2 0.575 0.965 1.06 72.4 0.352553 2290.50 3 0.533 1.180 1.27 71.1 0.471178 2422.39 4 0.540 1.120 1.18 72.0 0.456845 2451.93

Data taken from same patient by multiple technicians

Conclusion• Boundary layer separation can be mathematically related to

pressure difference and bifurcation angle.• Our proposed index cannot be confirmed without long-term

studies• The relation between pressure difference and risk requires

more data to accurately determine• Trends evident in sample data warrant further investigation

Limitations• 2D geometry capability• Probe depth• Grainy image of ultrasound• Young age group• Bernoulli’s assumptions• Multiple “technician” error

Future Work• 3D CFD fluid flow • 3D scanned

geometry from patients

Lifetime monitoring• Development of

atherosclerosis over time

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Questions?