title: Water-fat separation with bipolar multiecho sequences.
abstract: 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 short T(2)*. 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 shifts k-space data to correct k-space echo misalignment, an image warping method that utilizes a low-resolution field map to remove field-inhomogeneity-induced misregistration, and a k-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.
authors.affiliation: Department of Radiology, Stanford University, Stanford, CA 94305, USA. wenmiao@stanford.edu
authors.forename: Wenmiao
authors.initials: W
authors.lastname: Lu
authors.forename: Huanzhou
authors.initials: H
authors.lastname: Yu
authors.forename: Ann
authors.initials: A
authors.lastname: Shimakawa
authors.forename: Marcus
authors.initials: M
authors.lastname: Alley
authors.forename: Scott B
authors.initials: SB
authors.lastname: Reeder
authors.forename: Brian A
authors.initials: BA
authors.lastname: Hargreaves
mesh_terms: D005223:Fats; D006801:Humans; D007668:Kidney / Q000033:anatomy & histology; D007717:Knee / Q000033:anatomy & histology; D008099:Liver / Q000033:anatomy & histology; D008279:Magnetic Resonance Imaging / Q000379:methods*; D019047:Phantoms, Imaging; D014867:Water
chemical_list: D005223:Fats; D014867:Water