Three‐point dixon method enables whole‐body water and fat imaging of obese subjects

Abstract

Dixon imaging techniques derive chemical shift-separated water and fat images, enabling the quantification of fat content and forming an alternative to fat suppression. Whole-body Dixon imaging is of interest in studies of obesity and the metabolic syndrome, and possibly in oncology. A three-point Dixon method is proposed where two solutions are found analytically in each voxel. The true solution is identified by a multiseed three-dimensional region-growing scheme with a dynamic path, allowing confident regions to be solved before unconfident regions, such as background noise. 2p-Phase unwrapping is not required. Whole-body datasets (256 3 184 3 252 voxels) were collected from 39 subjects (body mass index 19.8-45.4 kg/m 2 ), in a mean scan time of 5 min 15 sec. Water and fat images were reconstructed offline, using the proposed method and two reference methods. The resulting images were subjectively graded on a four-grade scale by two radiologists, blinded to the method used. The proposed method was found superior to the reference methods. It exclusively received the two highest grades, implying that only mild reconstruction failures were found. The computation time for a whole-body dataset was 1 min 51.5 sec 6 3.0 sec. It was concluded that wholebody water and fat imaging is feasible even for obese subjects, using the proposed method. Magn Reson Med 63:1659-1668, 2010. V C 2010 Wiley-Liss, Inc.Key words: three-point Dixon; whole-body MRI; water and fat separation; chemical shift imaging; fat suppression By utilizing the property of chemical shift, MR signals arising from different chemical species can be separated. Dixon techniques use a spectrum model to derive chemical shift-separated water and fat images from multiple source images acquired at different echo times (points). Dixon water-only images form an alternative to fat suppression techniques, such as short tau inversion recovery and spectral inversion recovery, that is potentially insensitive to magnetic field inhomogeneity.Due to the ability of fat quantification (1), Dixon imaging is of interest in studies of obesity and the metabolic syndrome. Whole-body Dixon imaging allows accurate investigation of adipose tissue distribution (2) and may also be useful in oncology (3).Dixon's original two-point method (4) assumed that the phase difference in the source images was caused by chemical shift only and did not account for other sources of phase distortion, such as amplitude of static field (B 0 ) inhomogeneity. However, the requirement of a homogeneous magnetic field can be relaxed by including B 0 inhomogeneity in the signal model. This has been done by acquiring an additional image, giving a three-point method (5-7), or by modification of the two-point method (8).Two main challenges for Dixon methods are the ambiguity of identifying water and fat, and 2p phase unwrapping of the B 0 field map. However, by acquiring the source images with constant echo spacing, these problems can be separated. In fact, phase unwrapping is not necessary for water and fat ...

Fat quantification in skeletal muscle using multigradient-echo imaging: Comparison of fat and water references

“…A first guess for off resonance was used to avoid fat/water ambiguity and was obtained using the procedure described in (35) for in vivo data, and using the phase differences between adjacent echoes for phantom data.…”

Water-fat separation incorporating spatial smoothing is robust to noise

“…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].…”

Usefulness of the fast spin-echo three-point Dixon (mDixon) image of the knee joint on 3.0-T MRI: comparison with conventional fast spin-echoT2weighted image

“…1 Regarding FS, the two-point Dixon method is limited by B 0 field heterogeneity and fat-water swapping and needs a relatively long sequence time. 2 The three-point Dixon (mDixon) technique using a three-phase difference (0, p and 2p) between fat and water signals allows a flexible echo time and sequence design and compensates for B 0 field heterogeneity.…”