MR Imaging for the Evaluation of Carotid Atherosclerosis

David Saloner, PhDProfessor of Radiology

VA Medical CenterUniversity of California San Francisco

What is the risk factor associated with carotid disease?

a b c dIdentical diameter stenosis but different

geometries and/or composition

Standard of truth - excised specimen

High Resolution MRI of Resected Plaque

• Provides perspective on wealth of information available from MR imaging of geometry and composition

• (200µm)3 resolution (not currently obtainable in vivo)

Longitudinal MRI

Transverse MRI

Carotid specimen

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10

20

30

40

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60

circular crescentic elliptic lobular

freq

uenc

yCross-section through maximal stenosis for 9 specimens

Stenosis shape

Fibrous cap over necrotic core

Histology MRIMRI shows good contrast between fibrous cap and necrotic core as confirmed on histology

High Resolution MRI In Vivo• Can we obtain comparable data in vivo that will

provide measures of 3d descriptors?Will these permit us to follow the progression of geometry (X-sectional area, plaque bulk), and compositional features (lipidic core, fibrous cap)?

• Can we identify the features of the plaque that correlate with rapid progression in a prospective fashion?

• Can we identify the features of the plaque that confer neurological risk?

High resolution TOF-MRA using high sensitivity phase-array carotid coil

0.7mm x 0.7mm x1.0mm

Tacq = 6min

time

carotid jugular

Inject 30cc

Ttr

TdAcquisition time

Intravenous injection of GdDTPA - contrast enhanced MRA (CE-MRA) -can be used to address some shortcomings of conventional MRA.

CE-MRA 3D TOF

CE-MRA (25 s) has little motion blurring compared to 3D TOF (10 min)

CE-MRA 3D TOF

CE-MRA (TE=1.5 ms)has reduced sensitivity to complex flow cf 3D TOF (TE=6.5ms)

CE-MRA

3D TOF

Caveat: CE-MRA not always better.Tight stenosis seen as flow void on CE-MRA

Dedicated equipment provides improvements - phased-array carotid coil gives increased SNR hence better resolution

Conventional coil Phased-array coil

High-resolution 3D imaging of lumenal contours

High resoln. MRA of lumenal contours in vivo

• Provides good delineation of vascular contours with reasonable resolution - about 0.5 mm3 voxel

• 3D depiction of contours

• Little or no invasiveness

• Provides potential to evaluate cross-sectional area, and surface irregularities

Native imagewith coil

Adjusted image

Phased-array coils with postprocessing

Calcification

Calcification also noted on MRI

High resoln. MRI of vessel wall in vivo• Provides good delineation of plaque components

with reasonable resolution - about 0.5 mm3 voxel

• Sequential 2D slices providing 3D overview

• Non invasive

• Provides potential to evaluate plaque inhomogeneity - perhaps lipid core, fibrous cap, calcification, intraplaque hemorrhage

Dr. David Saloner, VASF, and Dr. F. Scott Pereles, Northwestern

High ResolutionCarotid Imaging

3D CE-MRA 3D TrueFISP 2D T2-TSENew rapid methods - new hardware

Future of MRA/I• Improved hardware and software

• Use of 3D black blood studies, plaque enhancement following contrast injection

• Implementation in longitudinal studies of disease progression/regression

• Establish true determinants of neurological risk