Chemical shift encoding‐based water–fat separation methods

Abstract

The suppression of signal from fat constitutes a basic requirement in many applications of magnetic resonance imaging. To date, this is predominantly achieved during data acquisition, using fat saturation, inversion recovery, or water excitation methods. Postponing the separation of signal from water and fat until image reconstruction holds the promise of resolving some of the problems associated with these methods, such as failure in the presence of field inhomogeneities or contrast agents. In this article, methods are reviewed that rely on the difference in chemical shift between the hydrogen atoms in water and fat to perform such a retrospective separation. The basic principle underlying these so-called Dixon methods is introduced, and some fundamental implementations of the required chemical shift encoding in the acquisition and the subsequent water-fat separation in the reconstruction are described. Practical issues, such as the selection of key parameters and the appearance of typical artifacts, are illustrated, and a broad range of applications is demonstrated, including abdominal, cardiovascular, and musculoskeletal imaging. Finally, advantages and disadvantages of these Dixon methods are summarized, and emerging opportunities arising from the availability of information on the amount and distribution of fat are discussed.

Current artefacts in cardiac and chest magnetic resonance imaging: tips and tricks

“…However, artefacts may also be created by using the Dixon technique (Figures 9 and 10). 7 Truncation artefact (Gibbs phenomenon/ ringing artefact) These artefacts occur near sharp image boundaries because of undersampling of high spatial frequencies, and they are observed both in phase-and frequency-encoding directions. They appear .…”

Water-fat separation incorporating spatial smoothing is robust to noise

“…In the original paper, two spin echoes were used, and the fat signal was modeled as a single peak. Today, spoiled gradient echo sequences are often used, and many methods have been developed that can make use of arbitrarily many echoes (multi-echo methods) [2][3][4][5][6][7][8][9][10][11][12][13]. Additionally, a multi-peak fat spectra [3][4][5][6][7][8][9][10][11][14][15][16][17] may be employed for improved signal separation.…”

“…The enforcement of spatial smoothness has been performed using region growing [2][3][4]14,[18][19][20][21][22], which operates locally. One problem with region growing approaches is that when swaps do occur, they typically propagate well beyond the voxels where they originate [5]. A potentially more robust alternative to region growing are graph cuts [6][7][8][9][10][11]15,16], which can optimize entire slices or whole volumes at once.…”

FSE T2-weighted two-point Dixon technique for fat suppression in the lumbar spine: comparison with SPAIR technique

“…By strategically acquiring images at specific echo time (TE) values, two separate images can be acquired in which water and fat signals are located in-phase and out-of-phase, respectively. By addition and subtraction of the two images, Dixon method provides both water-only and fat-only images (5,7,10,11). As FSE T2-weighted two-point Dixon sequence (T2 Dixon) generates both T2 and fat-suppressed T2-weighted images at a single acquisition, it could reduce the total scan time compared with the conventional techniques.…”