MRI and MRA
of the
Carotid Arteries
Robert L. Greenman
Department of Radiology
Beth Israel Deaconess Medical Center
MRI of the CAROTID
ARTERIES
• Review Pathogenesis/Progression
• Intima-Media Thickness (IMT) Studies
• Plaque Constituents
– Morphology
• Stable vs Unstable (Vulnerable)
• Survey of Methods and Results
– Most Published Results - 1.5T
• Recent 3T Carotid MRI Studies
MRI of CAROTID ARTERIES
• Stroke (1999 Statistics)
– Worldwide
• 4.4 Million Deaths/Year
• 5,000 Disabilities/Million Persons
– United States
• 750,000 Strokes/Year
• 1/3 Stroke Patients Die
• 1/2 of Survivors are Disabled
Gorelick, PB, et al Statement from National Stroke
Assoc. JAMA 1999; 281:1112-1120
MRI of CAROTID ARTERIES
• Acute Ischemic Attack
– Disruption of Atherosclerotic Plaques
– Cause of many Embolic Strokes
ATHEROSCLEROSIS
• Intimal Disease
• Inflammatory Disease
• Response of the Intima to Injury
Ross, R. “Atherosclerosis - An Inflammatory Disease”.
N Engl J Med 1999; 340:115-126
Davies, MJ, Woolf, N. “Atherosclerosis: what it is and
why does it occur?. Br Heart J; 1993: 69;S 3-11
ARTERIAL PLAQUE
COMPOSITION
• Major Components
– Lipids
• Lipid-Containing “Foam Cells” (Macrophages)
• Macrophages
– Connective Tissue
• Matrix Proteins - Collagen
• Strengthens, Holds Plaque Together
– Other Components
• Calcification
ATHEROGENESIS
1. Endothelial Injury/Dysfunction
•
Lipids are a Major Cause of Injury
2. Adhesion and Migration of Luekocytes
3. Immune/Inflammatory Response
4. Migration of Lipids (Normal and Oxidized)
5. Uptake of Lipid by Macrophages
6. Smooth Muscle Proliferation
ATHEROGENESIS
Endothelial Injury/Dysfunction
•
Causes
– Elevated and Modified LDL
– Free Radicals
• Cigarette Smoking
• Hypertension
• Diabetes
– Genetic Alterations
– Infectious Microorganisms
ATHEROGENESIS
Endothelial Injury/Dysfunction
Immune/Inflammatory
Response
•Increased Adhesiveness
•Increased Permeability
•Procoagulant Properties
•Release of Growth Factors
•Thickening of Artery Wall and “Remodeling”
ATHEROGENESIS
Plaque Formation
•Modified Lipids Migrate Into
Intima
•Ingested by Macrophages
•Uptake is Unregulated
•“Foam Cells”
•Smooth Muscle
Proliferation and Migration
•Release of Growth Factors
ATHEROGENESIS
Plaque Formation
•Foam Cells Burst - Necrotic Lipid Core
•Smooth Muscle Proliferation
•Release of Growth Factors
ATHEROGENESIS
Unstable (Vulnerable) Plaque
•Necrotic Lipid Core Grows Large
•Fibrous Cap Wears Thin - Ruptures
•Thrombus
CAROTID PLAQUES
Unstable (Vulnerable) Plaque
1. Large Core
– Necrotic Lipid
– Intraplaque Hemorrage
2. Unstable Fibrous Cap
CAROTID IMAGING
ULTRASOUND
Modality of Choice: 2D Ultrasound
• B-mode Ultrasound
– Noninvasive
– Safe
– Inexpensive
• Measurements
– Intima-Media Thickness (IMT)
– Vessel Geometry
– Lumen Diameter
– Distensibility
CAROTID IMT
MRI vs US
• Difficulties with Carotid US
– Relative position of jawbone WRT bifurcation
– Vessel tortuosity
– Calcification
• Large Intra- and Interobserver Variability
CAROTID IMAGING
MRI vs US
Little MRA - Mostly MRI
• Soft Tissue Contrast
– Lipid, Smooth Muscle, Fibrous Tissue
• MR Signal Independent of Angle
• Flow Sensitive
– Simultaneous Information on:
• Vessel Lumen
• Vessel Wall
FSE Black Blood Imaging
RR
2 x RR
Slice - Se le ct ive
Ad iab at ic 1 8 0
Pulse
TI
Re ct angula r
Non-Se le ct ive
1 8 0 Pulse
Aquisition
Double Inversion Black Blood Sequence
1 e 
TI  T1 ln

 2 
 TR
T1
null
T2-W FSE (Dark Blood)
CAROTID IMT
MEASUREMENTS
• Longitudinal Studies
– Study Progression of Atherosclerosis
• Risk Factor for Stroke ??
• Risk Factor for Cardiovascular Disease ??
CAROTID IMT
MEASUREMENTS
• IMT and Brain Infarction
– Touboul, et al. Circulation 102:313-318,
2000
CAROTID IMT
MEASUREMENTS
IMT of Common Carotid Artery
• Some Studies Suggest IMT Related to
– Cardiovascular Risk Factors
– Prevalence of Atherosclerosis of
• Peripheral
• Coronary
• Femoral
CAROTID IMT
MEASUREMENTS
Increased IMT Associated with
• Age
– Howard, et al. 1993
• Hypertension
– Zanchetti, et al. 1998
• Diabetes
– Kawamori, et al. 1992
• Hyperlipidemia
– Poli, et al. 1988
• Increased IMT Associated with CAD
– Crouse, et al. Circulation: 92:1141-1147, 1995
CAROTID IMT
MEASUREMENTS
Study: Risk Factors that Predict Stroke/MI
• Iglesias, et al.
– (Subset “Rotterdam Study”)
– 374 Subjects - Stroke or MI
– 1496 Controls
– Mean Follow-up: 4.2 years
• Results/Conclusions
– Significant association between Carotid IMT
and Stroke and MI
– Predictive Value Low When Combined With
Other Clinical Observations
Iglesias, et al. Stroke: 32:1532-1538, 2001
CAROTID IMT
MRI vs US
• MRI Studies of IMT Measurement
– Crowe, et al. JMRI: 21:282-289, 2005
– 2D US vs 3D MRI (Black-Blood TSE)
– 10 Healthy Subjects, 5 Hypertensive
Patients
• Results
– Bland-Altman Analysis
• Mean Diff. MRI & US - 1.2%
– Significant Difference in IMT (P<0.05)
Between Hypertensive &NonHypertensive for Both Methods
CAROTID IMT
MRI vs US
• Wall Thickness Measurements
• 3D-TSE MRI vs US
Crowe, et al, 2005 JMRI;21:282-289
CAROTID IMT
MRI vs US
• Wall Thickness Measurements
• 3D-TSE MRI vs US
Crowe, et al, 2005 JMRI;21:282-289
CAROTID IMT
MRI vs US
• Correlation Study
• DIR Black Blood MRI vs US
• 17 Patients
– Intermediate/High Framingham
Cardiovascular Risk Score
• Results
– Significant Correlation
– R = 0.72, p < 0.05
•Mani, et al. J Cardiovasc Magn R 8:529-534 2006
CAROTID CE-MRA
• Identifies Luminal Narrowing
–100% Sensitivity
–92% Specificity
–Compare to Conventional MRA
» Wutke, et al. Stroke 33:1522-1529 2002
• Not Plaque Size
• Vessel May Remodel
• Can Overestimate Extent of
Stenosis
Image of the entire vascular region from the aortic arch to the intracranial vessels
Wutke, R. et al. Stroke 2002;33:1522-1529
Copyright ©2002 American Heart Association
MRI of CAROTID PLAQUES
• Non-Invasive
• MRI Signal Intensities
– Based on Tissue Biochemical Environment
– Soft Tissue Contrast
– Lipid vs Smooth Muscle, Fibrous Tissue
• Vessel wall/Lumen Contrast
• Flexibility in Achieving Desired Contrast
MRI of CAROTID PLAQUES
• Dark Blood Sequences
– T2-Weighted
– T1-Weighted
– PD- Weighting (Proton Density)
• Bright Blood Sequences
– 2D and 3D Time-of-Flight (TOF)
• Others
– Magnetization Transfer (MT)
– Diffusion Sensitive
MRI of CAROTID PLAQUES
• MRI Contrast of Atherosclerotic Plaque
Yuan, C, et al. Radiology 2001; 221:285-299
MRI of CAROTID PLAQUES
• MRI Contrast Mechanisms
• Identification of:
– Lipid Core
– Calcium Deposits
– Fibrous Connective Tissue
– Intraplaque Hemorrhage
1.5T MRI STUDIES
• Multiple Weightings – T2W, T1W, PDW
• Shinnar, et al. Arterioscler Thromb Vasc Biol
1999;19:2756-2761
• Yuan, et al. Radiology 2001; 221:285-299
• Fibrous Cap Thickness Measurement
• Hatsukami, et al. Circulation 2000;102:959-964
• Contrast Enhanced
• Yuan, et al. J Magn Reson Imaging 2002; 15:62-67
MRI of CAROTID PLAQUES
• T2-Weighting
– Delineates Lipid Core and Thrombus
Yuan, C, et al. Radiology 2001; 221:285-299
MRI of CAROTID PLAQUES
• Multiple-Weighting Study
Mitsumori, L, et al. JMRI 2003 17:410-420
MRI of CAROTID PLAQUES
• Multiple-Weighting Study
• Lipid Core/Fibrous Cap
Mitsumori, L, et al. JMRI 2003 17:410-420
MRI of CAROTID PLAQUES
• Multiple-Weighting Study
• Pitfall of DIR-BB FSE studies
Mitsumori, L, et al. JMRI 2003 17:410-420
MRI of CAROTID PLAQUES
• Multiple-Weighting Study
• Calcification
Mitsumori, L, et al. JMRI 2003 17:410-420
MRI of CAROTID PLAQUES
• 3D TOF (Bright Blood)
– Bright Lumen/Dark Fibrous Tissue
– Identify Unstable Fibrous Caps in vivo
• Imaging Parameters:
– TR = 23 ms
– TE = 3.8 ms
– 2 Signal Averages
– Scan Time = 2 - 4 Min.
MRI of CAROTID PLAQUES
• 3D TOF (Bright Blood)
• Spatial Resolution
– Slice Thickness = 2 mm
– Acquisition Matrix:
• Size = 256 x 256
• Voxel Size = 0.5 x 0.5 x 2 mm
– Zero Filled:
• Matrix = 512 x 512
• Zero Filled Voxel Size = 0.25 x 0.25 x 2 mm
MRI of CAROTID PLAQUES
• 3D TOF (Bright Blood)
• Unstable Fibrous Cap Detection
Yuan, C, et al. Radiology 2001; 221:285-299
MRI of Neovasculature
• Neovasculature in Plaques Associated with
– Infiltration of Inflammatory Cells
– Plaque Destabilization
– Involved In Recruitment of Leukocytes
– Inflammatory Cells Present at Rupture Sites
• Measurement of Neovasculature may
Identify Vulnerable Plaques
• Contrast-Enhanced MRI
MRI of Neovasculature
Kerwin, et al. Circulation 107:851-856 2003
1.5T to 3.0T Comparisons
• All Studies Evaluated Black-Blood
• Anumula, et al, Acad Radiol;2006 12:1521-1526
• Compared Multi-coil Arrays
• Univ. Pennsylvania (FW Wehrli)
• Yarnykh, et al, JMRI;2006 23:691-698
• Compared Multicontrast
• Univ. Washington (C. Yuan)
• Koktzoglou, et al,JMRI;2006 23:699-705
• Compared Multi-slice
• Northwestern & Mt Sinai (L Debiao, ZA Fayad)
• Greenman, et al. MRI 2007 In Press
• Vessel Wall Sharpness with Field Strength and Spatial Resolution
1.5T to 3.0T Comparisons
• Conclusions of studies
• 3.0T Improves
• SNR
• CNR
• SNR can be traded for higher spatial resolution
Purpose
• Apply Edge Detection to Evaluate effect of
• Spatial Resolution
• Static Field Strength
• Carotid Imaging Methods:
• 2D-TOF (GRE)
• 2D Black-Blood (Double-IR FSE)
Edge Detection
• Have Been Used to Evaluate and Compare Vessel Edge
Definition
• Coronary Arteries (Bright Blood)
• Compare Results of Acquisition Schemes
• Deriche Algorithm
• First-order derivative
• Create derivative or “edge” images
Vessel sharpness: “The average edge value along the calculated
vessel border” … “Higher edge values correspond to better
vessel definition.”
•Botnar, et al, Circulation 1999;99:3139-3148
•Weber, et al, JMRI 2004;20:395-402
•Deriche, R, IEEE Trans PAMI. 1990;12:78-87
(“Fast algorithms for low-level vision”)
METHODS
Compared Spatial Resolution/Field Strength:
• 0.27 mm X 0.27 mm X 2.0 mm
• 1.5T and 3.0T; n = 12 (1.5T-H, 3.0T-H)
• 0.55 mm x 0.55 mm x 2.0 mm (Zero-Filled 2X)
• 1.5T only; n= 5 (1.5T-L)
• 2D DIR Black Blood FSE
• 2D Gradient Echo Time-of-Flight
2D-TOF
Results
DIR-BB
Results
Gradient Value Comparison
Carotid Edge Values
MRI of CAROTID PLAQUES
• Identify Stable/Unstable Plaques
• Follow Progression of Plaque
Development
• Monitor Therapies
CONCLUSIONS
• 3.0 Tesla MRI Offers Improved Signal-toNoise Ratio – Resolution
• IMT Measurements
• May Be Able To Identify Unstable Plaques
• Improved Accuracy: Measurement Cap Thickness
• May Improve Monitoring of Interventional
Therapies