Imaging vulnerable features of atherosclerotic plaque by MRI

MRI shows great promise as a noninvasive tool to investigate atherosclerotic plaques and to discriminate stable from vulnerable plaques. Published MR images of atherosclerotic plaques have generally not been validated rigorously because of inadequately performed histology, especially in the case of lipids. The major goal of this thesis work is to enhance the detection of specific plaque components that are important for plaque vulnerability (lipids and fibrous tissue) and to validate the accuracy and reproducibility of MR images by histology.; First, diffusion-weighted imaging (DWI) was developed to highlight lipid-rich regions by creating high contrast-to-noise ratio (CNR) between the lipids and all other components of the plaque. Carotid endarectomy (CEA) specimens were imaged ex vivo at high magnetic field (11.7 T) within 24 hours after surgery. Contrast-to-noise ratios (CNR) were measured and compared for the three contrast weightings (T1W, T2W and DWI); the largest CNR was obtained with DWI. The distribution of lipids (mainly cholesteryl esters; CE) was validated by histology (Oil red O and polarized light microscopy) and by image-guided spectroscopy, which detected the proton resonances characteristic of liquid CE.; Visualizing the fibrous cap in atherosclerotic plaques is a key to predicting plaque vulnerability. To enhance the contrast of the cap from other plaque components, we tested and further developed the technique of magnetization transfer contrast (MTC). In carotid plaques the magnetization transferring pulses suppressed the water signals surrounding densely packed fibrous tissue (collagen, especially type I) to create a higher magnetization transfer ratio (MTR) compared with the loose connective tissue (collagen type III, proteglycan or other fibrous tissues). A very high CNR compared to lipid-rich regions was also produced. The assignments were validated by histology (trichrome and Sirius red stain). A third key predictor of plaque instability is intraplaque hemorrhage. It remains the most challenging issue in plaque characterization by MRI. DWI may help to distinguish this component from lipids.; The methods presented (DWI and MTC) could differentiate and accurately identify plaque components ex vivo. If successfully translated in vivo, they may also permit the non-invasive identification of features of vulnerable plaque.