TISSUE CHARACTERIZATION OF ATHEROSCLEROTIC CAROTID PLAQUES BY MRI

Carotid artery plaques with intraplaque haemorrhage or atheromatous de bris have been found to be associated with an increased risk of emboli c stroke. Other methods have failed to detect plaque morphology and it is not clear whether MRI allows differentiation between prognosticall y and therapeutically relevant plaque types. We examined 17 carotid bi furcation plaques which had been removed in tote by MRI. For quantifyi ng MR signal intensities (I) the contrast-to-noise ratio (CNR) was use d: (I-Tissue - I-Ref/SDRef, With normal saline (0.9 %) as reference (R ef) and the standard deviation (SD) of the noise. Measurements were co rrelated with the histopathological appearance of ''simple plaques'', consisting of fibrous intimal thickening, Lipid deposits and/or athero matous tissue with cholesterol crystals, largely calcified plaques, an d ''complicated plaques'', containing recent intramural haemorrhage or friable atheromatous debris. Significantly different mean CNR could b e 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-weigh ted sequence simple plaques showed a CNR of 4.4 +/- 2.3, calcified plaques -4.8 +/- 2.6 and compli cated plaques 15.1 +/- 4.3. Using this technique, each single plaque c ould be correctly classified, an unalterable prerequisite for a clinic al application. To date, motion artefacts due to patient movement or i nsufficiently triggerable vessel pulsation in combination with relativ e long acquisition times (6-7 min) have limited in vivo investigations . If these problems could be overcome, MRI might become a valuable tec hnique for studying carotid plaque morphology.