Reconstruction Algorithms

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MR image reconstructions contain broad areas that include non-Cartesian image reconstruction (such as gridding), undersampled data reconstruction (such as parallel imaging), off-resonance correction, fat/water separation, gradient imperfections and segmentation and analysis tools. Depending on applications, not only excitation and acquisition schemes are properly designed but also efficient reconstruction techniques should be followed to achieve good image quality and provide diagnostic utility.

In body MRI, reliable fat suppression is important to show pathology or tissue morphology better and to increase dynamic ranges of images. Among different fat suppression methods, fat/water separation techniques are the least sensitive to field inhomogeneity. In addition, rapid imaging is necessary to reduce artifacts by cardiac or breath motions during a scan. To reduce scan time, using non-cartesian trajectory such as spiral and EPI, partial Fourier methods and undersampling k-space data is pretty common, which all require specific reconstructions to get artifacts-free images. Undersampled data reconstruction techniques using compressed sensing and parallel imaging are shown at the Rapid Imaging section. Here are some of our reconstruction techniques for fat/water separation.

Fat/Water Separation

Estimating field map correctly is important for reliable fat/water separation from multiecho sequences. Here is a example of fat/water separation by estimating field maps using golden section search combined with multiresolution pyramid structure [1]. Here, field map values can be estimated efficiently by coarse-to-fine propagation, exploiting smoothly varying nature of field maps.

Separated water/fat images and estimated field map of abdomen, where usually large field inhomogeneity is present. Fat/water images were acquired from a 42 second breath-hold scan with covering the entire liver (256 x144 x 28 matrix size).


Bipolar multiecho sequences replace fly-back gradients in unipolar multiecho sequences with alternating readout gradients and reduce echo spacing, thus can provide more robust field map estimation, shorter scan times, higher SNR efficiency, reduced motion-induced artifacts and less sensitivity to T2* decay. However, the alternating readout gradients cause problems including delay effects and image misregistrations. We solved these problems by using k-space water-fat separation, eliminating chemical shift-induced artifacts and correcting k-space echo misalignment [2].

Schematic diagram of bipolar multiecho GRE sequence.
Separated fat and water images of knee scan from bipolar multiecho sequence at 1.5T.

References

  1. Lu W, Yu H, Shimakawa A, Alley M, Reeder SB, and Hargreaves BA. Water-fat separation with bipolar multiecho sequences. Magn Reson Med 60(1): 198-209, 2008 Jul. PubMed HubMed [lu2008a]
  2. Lu W and Hargreaves BA. Multiresolution field map estimation using golden section search for water-fat separation. Magn Reson Med 60(1): 236-44, 2008 Jul. PubMed HubMed [lu2008b]
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