MRI and MRA of the Carotid Arteries Robert L. Greenman Department of Radiology Beth Israel Deaconess Medical Center.

MRI and MRA of the

Carotid Arteries

Robert L. GreenmanDepartment of Radiology

Beth Israel Deaconess Medical Center

• 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 the 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

MRI of CAROTID ARTERIES

Gorelick, PB, et al Statement from National Stroke

Assoc. JAMA 1999; 281:1112-1120

• Acute Ischemic Attack– Disruption of Atherosclerotic Plaques

– Cause of many Embolic Strokes

MRI of CAROTID ARTERIES

• Intimal Disease

• Inflammatory Disease

• Response of the Intima to Injury

ATHEROSCLEROSIS

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

• Major Components– Lipids

• Lipid-Containing “Foam Cells” (Macrophages)• Macrophages

– Connective Tissue• Matrix Proteins - Collagen• Strengthens, Holds Plaque Together

– Other Components• Calcification

ARTERIAL PLAQUE COMPOSITION

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

• Causes– Elevated and Modified LDL– Free Radicals

• Cigarette Smoking• Hypertension• Diabetes

– Genetic Alterations– Infectious Microorganisms

ATHEROGENESIS Endothelial Injury/Dysfunction

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

1. Large Core– Necrotic Lipid– Intraplaque Hemorrage

2. Unstable Fibrous Cap

CAROTID PLAQUES Unstable (Vulnerable) Plaque

Modality of Choice: 2D Ultrasound• B-mode Ultrasound

– Noninvasive– Safe– Inexpensive

• Measurements– Intima-Media Thickness (IMT)– Vessel Geometry– Lumen Diameter– Distensibility

CAROTID IMAGINGULTRASOUND

• Difficulties with Carotid US– Relative position of jawbone WRT bifurcation– Vessel tortuosity– Calcification

• Large Intra- and Interobserver Variability

CAROTID IMTMRI 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

CAROTID IMAGINGMRI vs US

FSE Black Blood Imaging

2 x RR

RectangularNon-Selective180 Pulse

Slice-SelectiveAdiabatic 180Pulse

Aquisition

RR

Double Inversion Black Blood Sequence

TI

€

TInull

= −T1⋅ln1+ e

− TR

T1

2

⎡

⎣ ⎢

⎤

⎦ ⎥

T2-W FSE (Dark Blood)

• Longitudinal Studies– Study Progression of Atherosclerosis

• Risk Factor for Stroke ??

• Risk Factor for Cardiovascular Disease ??

CAROTID IMTMEASUREMENTS

• IMT and Brain Infarction– Touboul, et al. Circulation 102:313-318,

2000

CAROTID IMTMEASUREMENTS

IMT of Common Carotid Artery

• Some Studies Suggest IMT Related to – Cardiovascular Risk Factors– Prevalence of Atherosclerosis of

• Peripheral• Coronary• Femoral

CAROTID IMTMEASUREMENTS

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 IMTMEASUREMENTS

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

CAROTID IMTMEASUREMENTS

Iglesias, et al. Stroke: 32:1532-1538, 2001

• 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 &Non-Hypertensive for Both Methods

CAROTID IMTMRI vs US

• Wall Thickness Measurements• 3D-TSE MRI vs US

Crowe, et al, 2005 JMRI;21:282-289

CAROTID IMTMRI vs US

• Wall Thickness Measurements• 3D-TSE MRI vs US

Crowe, et al, 2005 JMRI;21:282-289

CAROTID IMTMRI 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

CAROTID IMTMRI vs US

•Mani, et al. J Cardiovasc Magn R 8:529-534 2006

• 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

CAROTID CE-MRA

Copyright ©2002 American Heart Association

Wutke, R. et al. Stroke 2002;33:1522-1529

Image of the entire vascular region from the aortic arch to the intracranial vessels

• 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

MRI of CAROTID PLAQUES

• 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

1.5T MRI STUDIES

• 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 CAROTID PLAQUES

• 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

MRI of Neovasculature

MRI of Neovasculature

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

•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”)

Vessel sharpness: “The average edge value along the calculatedvessel border” … “Higher edge values correspond to better vessel definition.”

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

METHODS

2D-TOF Results

DIR-BB Results

Gradient Value Comparison

Carotid Edge Values

• Identify Stable/Unstable Plaques

• Follow Progression of Plaque Development

• Monitor Therapies

MRI of CAROTID PLAQUES

• 3.0 Tesla MRI Offers Improved Signal-to-Noise Ratio – Resolution

• IMT Measurements

• May Be Able To Identify Unstable Plaques• Improved Accuracy: Measurement Cap Thickness

• May Improve Monitoring of Interventional Therapies

CONCLUSIONS