Flow Suppressed Spin Echo or Fast Spin Echo [FSE Techniques

T2 or T1 weighted SE images were the first to be used to identify individual plaque components [75, 76]. SE or FSE techniques are very effective in generating images with excellent soft tissue contrast. These techniques are the backbones for both plaque tissue characterization and morphological evaluation. The use of these techniques, however, requires effective flow suppression, thus the term "black-blood" (BB) techniques [77]. These BB techniques are ideal for plaque imaging because the conspicuity of the vessel wall is increased when adjacent to a hypointense lumen, and the echo (TE) and repetition (TR) times can be varied to optimize visualization of specific plaque constituents. The major disadvantages of black-blood SE techniques are the relatively long scan times, and that these sequences are based on two-dimensional data acquisition with slice thicknesses that vary between 2 and 5 mm.

Common flow-suppression techniques employed with black-blood imaging involve (1) the use of presaturation radio frequency bands applied along the direction of arterial blood flow with a SE sequence or (2) a doubleinversion recovery (DIR) sequence [78]. When presaturation techniques are used, which are less effective than DIR with slowly flowing blood, the complex flow in the carotid bulb [79] often results in artifacts created by unsuppressed flow. Artifacts may be misinterpreted as representing signal from a diseased vessel wall and lead to an overestimation of the size of the atherosclerotic lesion [63]. On the other hand, DIR sequences tend to provide excellent flow suppression, as shown in Figure 7.6. These images typically provide the most accurate quantitative measurements of disease burden and are used to identify soft cores in vivo [73].

The main drawback of DIR is a long scan time because DIR requires sequential slice acquisition. A recently proposed dual-slice DIR technique [80] utilizes one nonselective and two slice-selective inversions, followed by consecutive excitation and readout for corresponding slices. Within this approach, however, the further increase in the number of slices acquired per one TR-interval may be difficult due to the long delay between the readout of signal from last slices and the moment of nulling the signal from blood. Yarnykh et al. introduced an alternative multi-slice DIR technique based on the short inversion time (TI) and the simultaneous inversion of an entire imaged volume [81]. In this technique, within the double-inversion pulse pair, the slice-selective

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