Tissue characterisation of atherosclerotic carotid plaques by MRI

Abstract

Carotid artery plaques with intraplaque haemorrhage or atheromatous debris have been found to be associated with an increased risk of embolic stroke. Other methods have failed to detect plaque morphology, and it is not clear whether MRI allows differentiation between prognostically and therapeutically relevant plaque types. We examined 17 carotid bifurcation plaques which had been removed in toto by MRI. For quantifying MR signal intensities (I) the contrast-to-noise ratio (CNR) was used: (ITissue-IRef)/SDRef, with normal saline (0.9%) as reference (Ref) and the standard deviation (SD) of the noise. Measurements were correlated with the histopathological appearance of "simple plaques", consisting of fibrous intimal thickening, lipid deposits and/or atheromatous tissue with cholesterol crystals, largely calcified plaques, and "complicated plaques", containing recent intramural haemorrhage or friable atheromatous debris. Significantly different mean CNR could be measured in the three plaque types on T1- and T2-weighted sequences (p < 0.00001) and using the FLASH pulse sequence with a flip angle of 15 degrees (p < 0.001). With the T1-weighted sequence simple plaques showed a CNR of 4.4 +/- 2.3, calcified plaques -4.8 +/- 2.6 and complicated plaques 15.1 +/- 4.3. Using this technique, each single plaque could be correctly classified, an unalterable prerequisite for a clinical application. To date, motion artefacts due to patient movement or insufficiently triggerable vessel pulsation in combination with relative long acquisition times (6-7 min) have limited in vivo investigations. If these problems could be overcome, MRI might become a valuable technique for studying carotid plaque morphology.

Correlation Between Carotid Intraplaque Hemorrhage and Clinical Symptoms

“…We excluded 17 that were validation studies comparing plaque imaging with histology [33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49] ; 7 that failed to provide sufficient data for each group 25,26,50 -54 ; 4 that were highly selected studies 19,[55][56][57] ; 2 that included patients with extracranial restenosis 58,59 ; and 8 that were duplicate records. 1,3,5,14,21,60 -62 The group sizes were very different in 1 study.…”

Structure of Plaque at Carotid Bifurcation: High-Resolution MRI With Histological Correlation

“…Necrotic core 23Ϯ12 (19) Values are meanϮSD, with numbers in parentheses indicating number of comparisons. CNR was calculated from MPR images according to the following formula: (S x ϪS y )/ where S x and S y are the average signals from the compared components, and is the signal standard deviation from a noise-only ROI from the slice in which the comparison was made.…”

“…To date, magnetic resonance (MR)-based studies of atherosclerotic tissue primarily have been based on spectroscopy or 2D imaging. [17][18][19][20][21][22][23] These techniques resolve tissue at a level of 1 to 2 mm in Ն1 direction, obscuring the submillimeter architecture of plaque by partial volume averaging.…”

In vivo and ex vivo measurements of the mean ADC values of lipid necrotic core and hemorrhage obtained from diffusion weighted imaging in human atherosclerotic plaques

“…Atherosclerotic plaque characterization by these sequences is generally based on the signal intensities in multi-contrast images such as T1w, T2w, and proton density. However, the in vivo quantification of high-risk plaque components by conventional MRI signal intensity suffers from low sensitivity and specificity (9,10).…”