Characterization of Carotid Artery Plaque Components on Magnetic Resonance Imaging Using Signal Intensity of the Phantom as a Reference

Rationale and Objectives

To evaluate the properties of plaque by the use of magnetic resonance imaging (MRI), it is necessary to use a material with stable signal intensity (eg, muscle or submandibular gland) as a reference. However, there may be differences between individuals. Therefore, we used a small phantom set on the circumference of the neck as a reference. The signal intensity ratio (SIR) methods using the phantom as a reference were reviewed for discrimination of the properties of plaque in the carotid artery.

Materials and Methods

Three phantoms (phantom 1: water; phantom 2: 5 μmol gadopentetate dimeglumine; and phantom 3: 2.5 μmol gadopentetate dimeglumine) were set around the neck. SIR was calculated for each region of interest and compared according to pathological grade.

Results

The method using a phantom as a reference reduced the standard deviations of tissue ratios to 0.16 from 0.27 in comparison with the method using muscle and showed a close correlation with pathological grade. In addition, the agreement rates with pathological grade and grades from each SIR using signal intensity of the phantom as a reference were higher than using signal intensity of the muscle as a reference to 0.86 from 0.63 for two-dimensional images and to 0.86 from 0.71 for three-dimensional images.

Conclusions

The method described here reduced error compared to the method using muscle as a reference, and the results were closely correlated with pathological grade.

Atherosclerotic carotid disease is a major cause of cerebral ischemia . It is generally accepted that mildly stenotic plaques with a thin or ruptured fibrous cap, large lipid core, and hemorrhage are more susceptible to rupture than are plaques with a thick cap, high degree of fibrosis, and calcification. Plaque lesions characterized by a lipid-rich necrotic core, by the presence of a thinned fibrous cap, or by intraplaque hemorrhage are regarded as high-risk unstable plaques that are likely to rupture and lead to cerebral ischemia . In 1995, Stary et al defined different atherosclerotic subtypes to allow pathological identification of plaques most likely to cause symptoms. The stepwise progression of atheroma, with or without symptoms, in its later accelerated phase is often associated with intraplaque hemorrhage, thus providing further evidence for the importance of this form of complex plaque in atherosclerotic disease . Therefore, it is important to evaluate not only stenosis rate but also the properties of vulnerable plaque.

Magnetic resonance imaging (MRI) provides a means of noninvasively characterizing and monitoring carotid atherosclerotic lesions over time. The sequences are optimized for vessel wall imaging of the carotid artery for two-dimensional (2D) black-blood (BB) imaging using double inversion recovery preparations and for three-dimensional (3D) BB imaging with 3D fast spin echo (FSE) with variable refocusing pulse using small flip angle . High-risk plaques (soft and intraplaque hemorrhage) show high signal intensity on fat suppression T1-weighted images (T1WI) and/or fat suppression T2-weighted images (T2WI) . To evaluate the properties of plaque, it is necessary to perform normalization relative to a material with stable signal intensity, because the signal intensity on MRI changes with the signal gain.

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Materials and methods

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Patients

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Phantoms

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Pathological Grade Classification

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MRI Protocol

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Image Analysis

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SIR(plaque/muscle)=signalintensityofplaque/signalintensityofmuscle SIR

(

plaque

/

muscle

)

=

signal

intensity

of

plaque

/

signal

intensity

of

muscle

SIR(plaque/phantom2)=signalintensityofplaque/signalintensityof5μmolGd−DTPAphantom SIR

(

plaque

/

phantom

2

)

=

signal

intensity

of

plaque

/

signal

intensity

of

5

μmol

Gd

-

DTPA

phantom

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Table 1

Grade Classification Using Signal Intensity Ratio (SIR)

2D-Spin Echo BB T1WI 3D T1WI SIR(plaque/muscle) SIR(plaque/phantom 2) SIR (plaque/muscle) SIR (plaque/phantom 2) Grade 3 SIR>1.25 SIR>0.4 SIR>2 SIR>0.3 Grade 2 1.25 > SIR>0.5 0.4 > SIR>0.3 2 > SIR>2 0.3 > SIR>0.2 Grade 1 0.5 > SIR 0.3 > SIR 1 > SIR 0.2 > SIR

2D, two-dimensional; 3D, three-dimensional; BB, black-blood; T1WI, T1-weighted imaging.

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Results

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Table 2

Agreement Rates with Grades from Each Signal Intensity Ratio (SIR) and Pathological Grade

2D-Spin Echo BB T1WI 3D T1WI SIR (plaque/phantom 2) SIR (plaque/muscle) SIR (plaque/phantom 2) SIR (plaque/muscle) 0.86 0.63 0.86 0.71

2D, two-dimensional; 3D, three-dimensional; BB, black-blood; T1WI, T1-weighted imaging.

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Discussion

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Conclusions

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