Water–fat separation with bipolar multiecho sequences

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Multiecho sequences provide an efficient means to acquire multiple echoes in a single repetition, which has found applications in spectroscopy, relaxometry, and water–fat separation. By replacing the fly-back gradients in unipolar multiecho sequences with alternating readout gradients, bipolar multiecho sequences greatly reduce both echo-spacing and repetition interval. This offers many attractive advantages, such as shorter scan times, higher SNR efficiency, more robust field map estimation, reduced motion-induced artifacts, and less sensitivity to shortTSymbol. However, the alternating readout gradients cause several technical problems, including delay effects and image misregistrations, which prevent direct application of existing water–fat separation methods. This work presents solutions to address these problems, including a post-processing method that shiftsk-space data to correctk-space echo misalignment, an image warping method that utilizes a low-resolution field map to remove field-inhomogeneity-induced misregistration, and ak-space water–fat separation method that eliminates chemical-shift-induced artifacts in separated water and fat images. In addition, a noise amplification factor, which characterizes the noise present in separated images, is proposed to serve as a useful guideline for choosing imaging parameters or regularization parameters in the case of ill-conditioned separation. The proposed methods are validated both in phantoms and in vivo to enable reliable and SNR efficient water–fat separation with bipolar multiecho sequences. Magn Reson Med 60:198–209, 2008. © 2008 Wiley-Liss, Inc.

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