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1. Anatomical Basis for MRI Assessment
of Human Atherosclerotic Disease:
MRI Dimensions of Post-Mortem
Carotid Artery Pairs from 38 Elderly
Individuals
W. Insull Jr., G. J. Adams,
C. B. Bordelon Jr., J. D. Morrisett
Baylor College of Medicine, Houston TX
Contact Info: William Insull Jr., M.D. winsull@bcm.tmc.edu
Joel D. Morrisett, Ph.D. morriset@bcm.tmc.edu

2. Abstract
Carotid artery atherosclerosis is a significant cause of strokes and can be
evaluated non-invasively by MRI. The purpose of this study was to determine
by MRI and EBCT the anatomical pathology of atherosclerosis in elderly
individuals most likely to have fullest expression of atherosclerosis. Pairs of
carotid artery segments around the bifurcation were obtained from 38
individuals in the anatomical laboratory after pressure perfusion fixation. They
were examined by high resolution MRI using a 1.5T clinical MR system
equipped with a phased array coil and by clinical EBCT. We measured the
location of the atherosclerotic plaque along the artery, the volumes of the artery
wall and lumen, and estimated the plaque volume. The replicate variation for
intra- and interscan volumes (coefficients of variation) measured the minimum
variances for the clinical MRI scan procedure excluding patient-dependent
sources of variance. Both MRI and EBCT showed close bilateral symmetry of
lesions for dimensions and calcium content, respectively. Carotid plaques
appeared to be single lesions with maximum development at the bifurcation
diminishing progressively to terminate about 15 mm distal and proximal to the
bifurcation. We identified the implications of these anatomical observations for
the concepts of development of atherosclerosis in single carotid lesions and
noted potential clinical applications.

3. Background
• Carotid atherosclerosis provides a potentially fruitful site for the study of
atherosclerosis development, diagnosis, and treatment.
• The carotid artery bifurcation is highly susceptible to atherosclerosis.
• The growth and development of carotid lesions is unlimited throughout all ages since
lesion size is not restricted by the limits of anatomical geometry of the artery.
• Advanced lesions of carotid atherosclerosis occur in populations with high prevalence
of atherosclerosis, stages IV to VII by the AHA classification 1995.
• All earlier stages of atherosclerotic lesions occur simultaneously with the most
advanced lesions, generally distributed radially and sequentially directly adjacent to
and contiguous to these most advanced lesions.
• The microscopic pathological characteristics of carotid atherosclerosis are similar to
atherosclerosis at other clinically significant arterial sites, coronary arteries,
peripheral arteries and aorta.
• Inter-individual variation in extent of carotid AS is broad within and among
populations, similar to wide variance for coronary and aortic disease.
• The symmetrical pairing of carotid arteries provides an opportunity to evaluate the
variance of atherosclerosis lesions between similar sites within each individual.
• Carotid arteries are readily accessible to non-invasive imaging by a variety of
techniques, including B mode ultrasound, computed tomography and magnetic
resonance imaging.

4. Purpose
Sample Acquisition:
• Cadaveric carotid arteries
were used as the model.
• Fifty perfusion-fixed
carotid pairs were excised
from human cadavers
aged 74±13 (48-98) years.
• Of the 50 sample pairs, 38
contained the entire
plaque and were suitable
for rigorous analysis.
3D Reconstruction of a
Carotid Artery from
multiple 3mm thick MRI
slices.
Cadaveric Carotid Artery
Specimen
To describe the anatomical, pathological, and dimensional
characteristics of carotid atherosclerosis in elderly individuals
using high resolution MRI 1.5T and EBCT.

5. Cadaveric Carotid Arteries
Useful properties of the cadaveric carotid artery samples:
a) Tissue was pressure perfusion fixed before excision, which
preserves the tissue and maintains in vivo geometry.
b) Most of the periadventitial tissue has been removed, which
reduces specimen bulk while retaining essential anatomical
features.
CCA
BIF
ICA
ECA
Single Cadaveric Carotid Artery Sample
The common carotid
artery (CCA), bifurcation
(BIF), external carotid
artery (ECA), and
internal carotid artery
(ICA) are clearly seen.

6. Properties of Cadaveric Carotid Arteries
Essential properties of the cadaveric carotid artery samples:
• Contain all three layers of the arterial wall (intima, media,
adventitia), and some perivascular soft tissue.
• Contain a range of lesion types.
• Give reproducible images over >1 year.
• Provide stable reference for:
• intra-laboratory standardization
• inter-laboratory standardization for multicenter clinical
trials
Other useful attributes:
• Enables comparison of left and right carotids from an
individual.
• Can be analyzed using independent techniques other than
MRI (e.g. histology, µCT, FTIR spectroscopy).

7. Imaging Protocol
Samples in EBCT Scanner
Ex Vivo Imaging Apparatus 1.5T GE Clinical MRI Scanner
An Imatron EBCT scanner and AccuImage
software were used to obtain calcification
scores for each sample.
A 1.5T GE Horizon LX clinical MRI
equipped with Pathway phased array coils
was used to acquire PDW, T1W, and T2W
images with an in-plane spatial resolution of
0.195 mm and a slice thickness of 3mm.

8. Carotid Artery Volume Quantitation using MRI
Measurement Algorithm:
• A semiautomatic active contour algorithm was used
to define the boundaries of the lumen and the outer
wall of the artery.
• The generalized gradient vector field force was used
as the external force for the active contour algorithm.
• The area of each contour was measured and
multiplied by the slice thickness to obtain the volume.
T2W MRI Image
Internal Lumen
External Lumen
Artery Wall
1. Initial Contours 2. Final Contours 3. Measurements

9. Plaque Volume Estimation and Assumptions
Estimation:
• Estimate normal wall thickness as the average minimum wall thickness within
each branch of the carotid tree.
• Estimate normal wall volume by sweeping a thickness contour around the
outer wall.
• Plaque Volume = Total Wall Volume – Normal Wall Volume.
• Percent Stenosis = Plaque Volume / Estimated Normal Lumen.
Assumptions:
• A normal, non-diseased wall is represented by the wall with the minimal
thickness.
• Normal wall thickness remains constant around the artery wall.
• Normal wall thickness remains the same within each branch of the carotid.
Internal Lumen
External Lumen
Artery Wall
2. Measurements1. T2W MRI Image
Normal Wall
Plaque
3. Plaque Estimate

10. Comparison of Volumes within Artery Pairs
Computation and Comparison of Aggregate Volumes:
• For comparison purposes, slices were indexed by their distance from the
bifurcation.
• The bifurcation was defined as the first MRI slice in which both the internal
and external lumen were visible as two separate orifices.
• Aggregate volumes for nine contiguous slices bounding the bifurcation were
computed for each sample.
1. Locate Bifurcation 2. Align Slices by Offset 3. Compute Volumes
MRI of
Left
Carotid
Artery
MRI of
Right
Carotid
Artery
Slice 10
Slice 11
Offset 0 on Left
Offset 0 on Right
Left Aggregate Volume
Right Aggregate Volume

11. Multiple Contrast Imaging
PDW
T1W
T2W
Contiguous
3mm thick
MRI slices.
I/E2 I/E1 Bifurc. C1 C2
• Tissues were imaged using multiple contrast weightings, including proton density,
T1, and T2-weightings.
• The different weightings provide differential contrast among principal tissue
components (e.g. necrotic core, fibrous cap, calcification, thrombus) within the
atherosclerotic plaque.
• Cai et al. have demonstrated that it is possible to differentiate between different
tissue types using multi-contrast MR imaging in vivo.

12. LuLu
HH
RCRC
LiLi
FCFC
SSSS
Correlation of Histology and MR Imaging
• Cross-sectional MR images closely match histology slices.

13. Carotid Volume Distributions
Slice volume averages were computed for slices at the same
offset in the left and right carotid arteries for the 38 pairs.
Average Left
Slice Volumes at
Offset 0
Sample 1 Offset 0 Left
+
Sample 2 Offset 0 Left
Average Right
Slice Volumes at
Offset 0
Sample 1 Offset 0 Right
+
Sample 2 Offset 0 Right
= =vs.

14. 0
20
40
60
80
100
120
140
160
C10 C8 C6 C4 C2 B I2/E2 I4/E4 I6/E6
Slice
Volume(mm
3
)
Common
Internal
External
Lumen Volume Distributions
in the Left and Right Carotids
Slice volume profiles measured using the semiautomated active contour algorithm. Distance
between slices is 3mm.
Dotted lines are left carotid volumes.
Solid lines are right carotid volumes.

15. 0
50
100
150
200
250
300
350
400
C10 C8 C6 C4 C2 B I2/E2 I4/E4 I6/E6
Slice
Volume(mm
3
)
Common
Internal
External
Total Artery Volume Distributions
in the Left and Right Carotids
Slice volume profiles measured using the semiautomated active contour algorithm. Distance
between slices is 3mm.
Dotted lines are left carotid volumes.
Solid lines are right carotid volumes.

16. 0
50
100
150
200
250
C10 C8 C6 C4 C2 B I2/E2 I4/E4 I6/E6
Slice
Volume(mm
3
)
Common
Internal
External
Total Wall Volume Distributions
in the Left and Right Carotids
Total wall slice volume profiles calculated as total artery volume minus lumen volume.
Distance between slices is 3mm.
Dotted lines are left carotid volumes.
Solid lines are right carotid volumes.

17. 0
20
40
60
80
100
120
140
C10 C8 C6 C4 C2 B I2/E2 I4/E4 I6/E6
Slice
Volume(mm
3
)
Common
Internal
External
Normal Wall Volume Distributions
in the Left and Right Carotids
Slice volume profiles estimated using the automated plaque estimation algorithm. Distance
between slices is 3mm.
Dotted lines are left carotid volumes.
Solid lines are right carotid volumes.

18. 0
10
20
30
40
50
60
70
80
C10 C8 C6 C4 C2 B I2/E2 I4/E4 I6/E6
Slice
Volume(mm
3
)
Common
Internal
External
Dotted lines are left carotid volumes.
Solid lines are right carotid volumes.
Plaque Volume Distributions
in the Left and Right Carotids
Slice volume profiles estimated using the automated plaque estimation algorithm. Distance
between slices is 3mm. Plaque volume is concentrated near the bifurcation, with 80% of the
plaque within 9mm of the bifurcation in the internal carotid, 12 mm within the external
carotid, and 18mm in the common carotid.

19. 0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
50%
C10 C8 C6 C4 C2 B I2/E2 I4/E4 I6/E6
Slice
Volume(mm
3
)
Common
Internal
(Circles)
External
(Triangles)
Percent Stenosis Distributions
in the Left and Right Carotids
Slice volume profiles estimated using the automated plaque estimation algorithm. Distance
between slices is 3mm. Percent stenosis was calculated by dividing the estimated plaque
volume by the estimated original lumen.
Dotted lines are left carotid volumes.
Solid lines are right carotid volumes.

20. Wall Thickness Distributions
• Wall thickness distributions were calculated for each MR slice
in the carotid artery.
• Averages for each branch were computed by taking the mean
of the average wall thickness of each slice within that branch.
• Maximums for each branch were computed by finding the
largest maximum wall thickness for each slice within that
branch.
Maximum
Wall
Thickness

21. Scatter Plots of the Average and Max Wall Thickness
in mm. of the Internal, External and Common
Carotid Artery Branches from MRI
0 1 2 3
0
1
2
3
Left Internal
RightInternal
0 1 2 3
0
1
2
3
Left External
RightExternal
0 1 2 3
0
1
2
3
Left Common
RightCommon
r=0.63r=0.75r=0.48
0.0 2.5 5.0 7.5 10.0
0.0
2.5
5.0
7.5
10.0
Left Common
RightCommon
0.0 2.5 5.0 7.5
0.0
2.5
5.0
7.5
Left Internal
RightInternal
0.0 2.5 5.0 7.5
0.0
2.5
5.0
7.5
Left External
RightExternal
r=0.57r=0.23 r=0.52
Average Wall Thicknesses
Maximum Wall Thicknesses

22. Scatter Plots of the
Average Wall Thickness v. the Maximum Wall Thickness
in mm. of the Internal, External and Common
Carotid Artery Branches from MRI
0 1 2 3
0.0
2.5
5.0
7.5
Left Internal Avg
LeftInternalMax
0 1 2 3
0.0
2.5
5.0
7.5
Right Internal Avg
RightInternalMax
0 1 2 3
0
1
2
3
4
5
Left External Avg
LeftExternalMax
0 1 2 3
0.0
2.5
5.0
7.5
Right External Avg
RightExternalMax
0 1 2 3
0.0
2.5
5.0
7.5
10.0
Left Common Avg
LeftCommonMax
0 1 2 3
0.0
2.5
5.0
7.5
10.0
Right Common Avg
RightCommonMax
r=0.73
r=0.85
r=0.74
r=0.71
r=0.79
r=0.53

23. Aggregate Volumes Statistics
Aggregate volumes were computed for nine contiguous slices
bounding the bifurcation for each sample in the 38 pairs.
vs.
Left Aggregate Volume Right Aggregate Volume

24. 0 500 1000 1500 2000
0
500
1000
1500
2000
Left Lumen
RightLumen
Scatter Plots of Carotid Artery
Aggregate Volumes from MRI
0 1000 2000 3000 4000
0
1000
2000
3000
4000
5000
Left Total Artery
RightTotalArtery
0 1000 2000 3000
0
1000
2000
3000
Left Total Wall
RightTotalWall0 500 1000 1500 2000
0
500
1000
1500
2000
Left Normal Wall
RightNormalWall
0 500 1000 1500
0
500
1000
1500
Left Plaque
RightPlaque
0 25 50 75
0
25
50
75
Left Percent Stenosis
RightPercentStenosis

25. Average Carotid Artery Aggregate Volumes from MRI
0
500
1000
1500
2000
2500
3000
3500
Left
Lumen
Volume
Right
Lumen
Volume
Left Total
Artery
Volume
Right Total
Artery
Volume
Left Total
Wall
Volume
Right Total
Wall
Volume
Left
Normal
Wall
Volume
Right
Normal
Wall
Volume
Left
Plaque
Volume
Right
Plaque
Volume
• Average volumes in mm3 for left and right carotid volumes from MRI.
• None of the left v. right volumes were significantly different.
• Error bars are standard deviations.

26. Scatter Plots of Carotid Artery Calcium Volume Scores
from EBCT
0 500 1000 1500 2000
0
500
1000
1500
2000
Left Calcium Volume Score
RightCalciumVolumeScore
0 1000 2000 3000
0
1000
2000
3000
Left Agatston Score
RightAgatstonScore
• Two separate scores were computed from the same images.
• The Agatston Score is calculated based on calcification area times a scale factor.
• The Volume Score uses the isotropic interpolation to calculate the volume of
calcification.
• The two calcification scores are highly correlated with one another (r=0.997).
• The scores are not normally distributed within the population of individuals.

27. Average Carotid Artery Calcification Scores from EBCT
Two separate scores were computed from the same images. The Agatston Score is calculated
based on calcification area times a scale factor, whereas the Volume Score uses the isotropic
interpolation to calculate the volume of calcification.
Left and Right Carotid Artery EBCT Calcification Scores
from 38 Sample Pairs
Error Bars are Standard Deviations
Agatston Score Volume Score
-200
0
200
400
600
800
1000
Left Agatston
Score
Right Agatston
Score
Left Calcium
Volume Score
Right Calcium
Volume Score

28. Left v. Right Carotid Volume (MRI) and
Agatston Score (EBCT) Concordance Correlations
• Lin’s concordance correlation coefficients of left and right carotid volumes
from MR and left and right calcification scores from EBCT.
• Lin’s concordance correlation coefficient measures the agreement between a
pair of variables.
• Error bars are 95% confidence intervals.
0.54
0.63
0.71
0.64
0.58
0.51
0.95 0.94
0.0
0.2
0.4
0.6
0.8
1.0
Lumen
Volume
Total Artery
Volume
Total Wall
Volume
Normal
Wall
Volume
Plaque
Volume
Percent
Stenosis
Agatston
Score
Volume
Score
ConcordanceCorrelationCoefficient

29. Correlations between Agatston Score (EBCT) and
Aggregate Volumes (MRI)
Correlations of Agatston Score vs Aggregate Volumes (N=76)
Error Bars are 95% Confidence Intervals
-0.17
0.27
0.50 0.46 0.44
0.53
-0.60
-0.40
-0.20
0.00
0.20
0.40
0.60
0.80
1.00
Lumen
Volume
Total Artery
Volume
Total Wall
Volume
Normal Wall
Volume
Plaque
Volume
Percent
Stenosis
Aggregate Volume
CorrelationCoefficient

30. Reproducibility Statistics
• To test the reproducibility of MRI imaging of the models, two image sets,
consisting of two full acquisitions (PDW, T1W, T2W) on four models, were
acquired.
• Between the two sets the holder was removed from the magnet, the coils were
removed from the holder, and the temperature of the water bath was re-
equilibrated.
• Arterial volumes of each model were quantified using the semiautomated
algorithm.
• The reproducibility of the measured volumes from the different models is
quantified using the coefficient of variation (mean±SD), which is expressed as a
percentage.
Lumen Total Wall Normal Wall Plaque
Within Sets
COV (N=8)
0.33±0.24 0.70±0.30 1.01±1.11 2.48±2.14
Between Sets
COV (N=4)
2.19±2.00 2.63±1.75 3.96±2.16 4.47±1.56

31. Summary of Results
The anatomical characteristics of the carotid plaques have been described by the
average values of the study subjects:
• Carotid plaque is a single continuous lesion extending from the common carotid into
the internal and external branches, without evidence of discontinuities of structure
that indicate multiple plaques at the carotid site.
• Lesions are located at the region of the bifurcation, presumably in the area of the
carotid bulb, within 15mm proximally and distally from the flow divider. Lesions’
longitudinal development within the artery wall appears to be equal along the
common carotid and the internal and external branches.
• Lesion bulk is greater in the internal carotid that in the internal carotid. Volumes
within the external carotid are 30 to 50% smaller per slice.
• The sum of the plaque volumes in the internal and external carotid in the slice most
proximal to the bifurcation is equal to the plaque volume in the common carotid slice
most proximal to the bifurcation.
• The maximum volume of plaque is at the region of the bifurcation. This is probably
the region with the most advanced plaque.
• Proximal and distal to the region of maximal development, the volume per slice
decreases rapidly in a curvilinear fashion.

32. Discussion of Results
1. Studying elderly patients has the inherent advantage of studying individuals with
the fullest development of atherosclerosis due to prolonged exposure to all risk
factors. The development of carotid atherosclerosis, raised lesions to the naked
eye, is progressive with age and in high risk populations is a companied by
increasing occurrence of complicated and calcified lesions. Solberg et al
2. The analysis of variance of MRI measurements of a single cadaveric carotid artery
provides an estimate of the minimum variance achievable with the MRI scan
procedure alone. This procedure excludes all sources of variance related to
patients, as within scan movements, and interscan differences of positioning and
movement. This provides as basis for systematic analysis of variance of MRI
clinical scans that are essential for estimating sample sizes for studies of treatment
effects on the dimensions of the plaque volume.
3. The comparison of pairs of carotids within individual patients shows the strong
bilateral symmetry of the disease. This supports the use of analysis of a single
artery as an estimate of the burden of atherosclerosis among similar arteries. It also
starts to define the inter-arterial variance of atherosclerosis within an individual,
the least difference as it occurs between bilaterally symmetrical arteries.

33. Discussion of Results
The analysis provides measurements on the development of atherosclerosis within a
single plaque. The results support the following statements:
• The plaque region with the greatest volume, at the carotid bifurcation, probably has
the plaques most advanced stage of development, and are probably the oldest
region of the plaque.
• The plaque regions with the smallest detectable volumes, at the proximal and distal
edges of the plaque, probably have the earliest stage of development, and are the
youngest region of the plaque.
• Since plaques appear to grow centrifugally along radial vectors, similar young
regions probably occur at the lateral edges of the plaque.
• These observations support the concept of the plaques’ centrifugal growth and
development along radii from the initial site.
• Whether the grades of lesions described by the AHA classification are arranged
along the radial vectors of growth requires further study.

34. Discussion of Results
The analysis of the anatomical dimensions of carotid plaques from MRI images has
substantial implications for clinical use of MRI:
• Diagnosis of atherosclerosis by detection of arterial lesions.
• Staging of the development of carotid atherosclerosis. Staging procedures require
initially the quantitative measurements of the lesion location and the lesions’
physical dimensions. While plaques’ degree of surface involvement surface, and
compositional heterogeneity have been used customarily for staging plaque
development, the volume of the plaque can now be used to further define the stage
of development.
• Rationale for selection of therapy based on the characteristics of the plaque, it’s
location, dimensions and composition.
• Monitoring therapy developing criteria for significant therapeutic effect, as
reduction in plaque volume.
• Adjusting therapy if treatment effects are not satisfactory
• Guidance to endarterectomy surgeon for dissection to excise lesions.

35. References and Acknowledgements
References
• Adams GJ, Simoni DM, Bordelon
CB, et al. Stroke. 2002;33:2575-
2580.
• Cai J-M, Hatsukami TS, Ferguson
MS, et al. Circulation.
2002;106:1368-1373.
• Karmonik C, Eldrige C, Vick GW, et
al. Am J Cardiol. 2001;88:78E.
• Solberg LA, McGarry PA, Moossy J,
et al. Ann N Y Acad Sci.
1968;149:956-973.
• Stary HC, Chandler AB, Dinsmore
RE, et al. Circulation.
1995;92:1355-1374.
• Zarins CK, Giddens DP, Bharadvaj
BK, et al. Circ. Res. 1983;53:502-
514.
Acknowledgements
• Funding was provided by grants to Dr.
Morrisett from the Welch Foundation
(Q1325) and the National Heart, Lung
and Blood Institute of the NIH
(HL07812 and HL63090.
• Gareth Adams was supported in part by
a training fellowship from the Keck
Center for Computational and Structural
Biology of the Gulf Coast Consortia
(NLM 5T5LM07093).
• EBCT calcification scoring was
performed by Darlene Simoni, RT.