Recent studies have shown that a combination of different contrast weightings is necessary for noninvasive characterization of plaque morphology [82, 102]. T1-, T2-, PD-, gradient echo, and other contrast schemes have all been evaluated for plaque tissue characterization. Different contrast weightings reveal different features of the plaque. For example, the MR signal intensity of hemorrhage is dependent on the structure of hemoglobin and its oxidation state . Recent hemorrhage with short T1 and long T2 shows a hyperintensive signal intensity on all contrast weighted images, and is readily identified. Calcified tissues, which have very little water and appear dark on all contrast weightings, are easily detected. Calcification on the plaque surface or cal-cific nodules extending into the lumen is difficult to detect due to low signal intensity, which is easily masked on black-blood sequence (SE or FSE T1, PD-and T2-weighted)  but they are easily detected on bright-blood sequence (TOF). In an ex vivo study , fibrocellular tissue and thrombus appeared relatively dark on PD-weighted images. Calculating relaxation parameters (such as the actual T2 of the components) is useful for the differentiation. In addition, diffusion-weighted MRI is a good technique for identifying thrombus and hemorrhage in plaque . There are, however, still significant problems in obtaining high-resolution diffusion images in vivo because of the problems of motion and low SNR of these images. For different types of lipids , MR signal patterns (from T1-, T2-, and proton density-weighted images) could be used to readily distinguish each lipid type from surrounding porcine aortic media. Table 7.1 summarizes plaque tissue contrast based on carotid lesions.
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