Editorial Slides VP Watch – September 25, 2002 – Volume 2, Issue 38

Multicontrast MRI for Classification of Human Atherosclerosis

Provided by:

David Saloner, PhDVA Medical Center, University of California San Francisco

There has been a steady improvement in the quality of MR methods for assessing atherosclerosis 1-3

A number of ex vivo studies have validated the abilities of MRI using histology as the gold standard particularly in studies of carotid disease where a surgical specimen is available

MR Imaging of Atherosclerosis

A powerful feature of MRI is the ability to obtain images with different weighting of the physiological composition of the soft tissue in the atheroma 4-5

Recent studies have shown that major components of plaque can be characterized in vivo and that this determination is aided by acquiring several sequences with different contrast weighting

In Vivo MRI of Plaque

MRI is able to assess both the lumenal contours (using bright blood MRA methods) and the composition of the atheroma (using black blood spin echo methods)

MRI abilities

Fig.1 Contrast-enhanced MR angiogram of carotid bifurcation showing multiple projections of the 3D data and reformations in the common, most stenotic ICA, and distal ICA.

Most investigators report good success in imaging atheroma using double inversion black blood methods which effectively null all signal from flowing blood 6

High sensitivity coils are important for obtaining acceptable image quality 7

MRI abilities

A classification scale has been developed by the AHA to categorize atherosclerotic lesions into different classes of complexity 8

Reported this week in VP Watch: Cai et al performed a blinded study to determine whether readers who categorized lesions into different categories on the basis of multi contrast in vivo MRI were in agreement with classification performed on those lesions following endarterectomy surgery 9

Atherosclerosis Classification Scales

Sixty consecutive patients scheduled for endarterectomy surgery were recruited to the study.

Patients were imaged within one week prior to surgery.

MR imaging was performed from 2cm proximal to the flow divider to 2cm distal to the flow divider.

Patients and Imaging

Imaging Sequences

Four image contrasts were obtained: 3D TOF; T1 -weighted double IR 2D Fast Spin Echo; and a shared echo PD-weighted and T2-weighted Fast Spin Echo.

Fat suppression was used.

Typical acquired resolution was 0.5mm x 0.5mm x 2mm.

Endarterectomy specimens were sectioned and stained with Mallory’s trichrome and H&E.

MR images were reviewed and classified by a blinded reader as were the histological slices.

Comparisons were made of 4 - 6 levels per artery.

Comparison of in vivo MRI and histology

• MRI correctly classified lesion types in 80.2% of cases

Results

Lesion type Sensitivity Specificity

Type I-II 67% 100%

Type III 81% 98%

Type IV-V 84% 90%

Type VI 82% 91%

Type VII 80% 94%

Type VIII 56% 100%

Conclusion:

High resolution MRI has good abilities in characterizing intermediate to advanced atherosclerotic lesions.

This capability is of high importance for the conduct of longitudinal studies designed to determine the natural progression of atherosclerosis or the response of atherosclerosis to pharmacologic interventions.

Comments Multi center studies are needed to

demonstrate the generalizability of these findings.

Better methods are needed to improve spatial resolution - in particular 3D methods for providing reduced partial voluming and multiplanar reformatting abilities.

Comments: Blood products have variable signal

appearances and methods for identifiying these should be pursued.

More time-efficient methods would be helpful.

The application of extravascular and intravascular contrast agents should provide additional information on plaque composition.

Questions:

• Is MRI/A able to assess the lumenal surface, the plaque composition, or both?

• What is an important advantage of studying the carotid territory in defining the utility of MRI methods?

• What are typical spatial resolution capabilities of MRI?

References

1. Toussaint, J.F., et al., Magnetic resonance images lipid, fibrous, calcified,

hemorrhagic, and thrombotic components of human atherosclerosis in vivo. Circulation, 1996. 94(5): p. 932-8

2. Yuan, C., et al., In vivo accuracy of multispectral magnetic resonance imaging for identifying lipid-rich necrotic cores and intraplaque hemorrhage in advanced human carotid plaques. Circulation, 2001. 104(17): p. 2051-6

3. Coombs, B.D., et al., Structure of plaque at carotid bifurcation: high-resolution MRI with histological correlation. Stroke, 2001. 32(11): p. 2516-21

4. Shinnar, M., et al., The diagnostic accuracy of ex vivo MRI for human atherosclerotic plaque characterization. Arterioscler Thromb Vasc Biol, 1999. 19(11): p. 2756-61.

5. Fayad, Z.A., The assessment of the vulnerable atherosclerotic plaque using MR imaging: a brief review. Int J Cardiovasc Imaging, 2001. 17(3): p. 165-77.

6. Simonetti, O.P., et al., "Black blood" T2-weighted inversion-recovery MR imaging of the heart. Radiology, 1996. 199(1): p. 49-57.

7. Hayes, C.E., C.M. Mathis, and C. Yuan, Surface coil phased arrays for high-resolution imaging of the carotid arteries. J Magn Reson Imaging, 1996. 6(1): p. 109-12.

8. Stary, H.C., et al., A definition of advanced types of atherosclerotic lesions and a histological classification of atherosclerosis. A report from the Committee on Vascular Lesions of the Council on Arteriosclerosis, American Heart Association. Arterioscler Thromb Vasc Biol, 1995. 15(9): p. 1512-31

9. Cai, J-M, Hatsukami, T, Ferguson, M et al Classification of Human Carotid Atherosclerotic Lesions With In-Vivo Multicontrast Magnetic Resonance imaging. Circulation, 2002. 106: p 1368-73