Published Online
on December 8, 2005

A more recent version of this article appeared on January 1, 2006

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Submitted on July 26, 2005
Revised on October 21, 2005
Accepted on October 25, 2005

Validation of Automatically Classified Magnetic Resonance Images for Carotid Plaque Compositional Analysis

Sharon E. Clarke MD, PhD; Vadim Beletsky MD; Robert R. Hammond MD; Robert A. Hegele MD; and Brian K. Rutt PhD^*

From the Imaging Research Laboratories (S.E.C., V.B., B.K.R.) and Blackburn Cardiovascular Genetics Laboratory and Vascular Biology Research Group (R.A.H.), Robarts Research Institute; Department of Medical Biophysics (S.E.C., B.K.R.), University of Western Ontario; and Department of Clinical and Neurological Sciences (V.B., R.R.H.), London Health Sciences Centre, London, Ontario, Canada.

^* To whom correspondence should be addressed. E-mail: brian.rutt{at}imaging.robarts.ca.

Background and Purpose--MRI may be used for noninvasive assessment of atherosclerotic lesions; however, MRI evaluation of plaque composition requires validation against an accepted reference standard, such as the American Heart Association (AHA) lesion grade, defined by histopathological examination.

Methods--Forty-eight carotid endarterectomy specimen cross-sections had AHA lesion grade determined histopathologically and were concurrently imaged using combinations of 8 MRI contrast weightings in vitro. A maximum likelihood classification algorithm generated MRI "maps" of plaque components, and an AHA lesion grade was assigned correspondingly. Additional analyses compared classification accuracy obtained with a commonly used set of magnetic resonance contrast weightings [proton density (PDw), T1 (T1w), and partial T2 (T2w)] to accuracy obtained with the combination of PDw, T1w, and diffusion-weighted (Dw) contrast.

Results--For the 8-contrast combination, the sensitivities for fibrous tissue, necrotic core, calcification, and hemorrhage detection were 83%, 67%, 86%, and 77%, respectively. The corresponding specificities were 81%, 78%, 99%, and 97%. Good agreement (79%) between magnetic resonance and histopathology for AHA classification was achieved. For the PDw, T1w, and Dw combination, the overall classification accuracy was insignificantly different at 78%, whereas the overall classification accuracy using PDw, T1w, and partial T2w contrast weightings was significantly lower at 67%.

Conclusions--This study provides proof-of-principle that the composition of atherosclerotic plaques determined by automated classification of high-resolution ex vivo MRI accurately reflects lesion composition defined by histopathological examination.

Key words: atherosclerosis • carotid arteries • MRI

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