J Korean Soc Magn Reson Med.  2011 Apr;15(1):57-66. 10.13104/jksmrm.2011.15.1.57.

Water-Fat Imaging with Automatic Field Inhomogeneity Correction Using Joint Phase Magnitude Density Function at Low Field MRI

• Kim PK
• Ahn CB

• KMID: 2041228
• DOI: http://doi.org/10.13104/jksmrm.2011.15.1.57

Abstract

PURPOSE
A new inhomogeneity correction method based on two-point Dixon sequence is proposed to obtain water and fat images at 0.35T, low field magnetic resonance imaging (MRI) system.
MATERIALS AND METHODS
Joint phase-magnitude density function (JPMF) is obtained from the in-phase and out-of-phase images by the two-point Dixon method. The range of the water signal is adjusted from the JPMF, and 3D inhomogeneity map is obtained from the phase of corresponding water volume. The 3D inhomogeneity map is used to correct the inhomogeneity field iteratively.
RESULTS
The proposed water-fat imaging method was successfully applied to various organs. The proposed 3D inhomogeneity correction algorithm provides good performances in overall multi-slice images.
CONCLUSION
The proposed water-fat separation method using JPMF is robust to field inhomogeneity. Three dimensional inhomogeneity map and the iterative inhomogeneity correction algorithm improve water and fat imaging substantially.

Keyword

Water-fat imaging; Inhomogeneity correction; Low-field MRI; Joint phase-magnitude density function

MeSH Terms

Joints
Magnetic Resonance Imaging
Water
Water

Figure

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  Fig. 1 A joint phase-magnitude density function for water-fat imaging is schematically shown, where RW and RF denote water and fat ranges, respectively.

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  Fig. 2 Water-fat images and corresponding JPMF for a phantom. Magnitude in the in-phase image (a), phase in the out-of-phase image (b), and corresponding JPMF (c). Fat and water are denoted by "F" and "W", respectively in (a). A spin echo-based Dixon sequence was used with repetition time (TR) of 500 ms, and echo time (TE) of 26 ms. P1 and P2 in the JPMF denote the peak populations for water and fat.

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  Fig. 3 Two dimensional range of water in the JPMF (a), and corresponding water region (b) in the phantom image shown in Fig. 2.

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  Fig. 4 Flow chart for the proposed water-fat imaging.

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  Fig. 5 Change of JPMF and corresponding water region as the number of iterations of the inhomogeneity correction increases. Numbers of iterations are 1(a), 2(b) and 3(c), respectively.

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  Fig. 6 Water images of the phantom with the number of iterations of the inhomogeneity correction are 1 (a), 3 (b), and 5 (c), respectively. Corresponding phase maps are shown in (d), (e), and (f), respectively. Fat image of the phantom after inhomogeneity correction (iteration number of 5) is shown in (g). The intensity profiles of the water images along the broken lines are shown in (h).

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  Fig. 7 In-vivo axial knee images of a volunteer. Magnitude of the in-phase image (a), phase of the out-of-phase image (b) obtained by a spin echo sequence at 0.35 T(TR=500 ms, TE=25 ms, FOV=230 mm, spatial resolution= 0.89 mm, flip angle (FA)=90°, slice thickness= 8 mm). Water image, fat image, and corresponding JPMF are shown in (c), (d), and (e), respectively before the inhomogeneity correction. Water image, fat image, and corresponding JPMF after the proposed inhomogeneity correction are shown in (f), (g) and (h), respectively. The number of iterations for inhomogeneity correction was 5. Note the improvements in the water images especially in the regions shown by the arrows.

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  Fig. 8 In-vivo multi-slice water (top) and fat (bottom) images of knee (a), hand (b), and shoulder (c) of volunteers are shown. They are obtained by a gradient echo-based Dixon sequence at 0.35T. The repetition time was 500 ms, TE for the out-of-phase and in-phase imaging was 9.8 ms and 19.6 ms, respectively. Other imaging parameters are: FOV=250 mm, FA=60°, Thickness=5 mm, spatial Resolution=0.97 mm (a); FOV=230 mm, FA=60°, Thickness=3 mm, spatial Resolution=0.89 mm (b); FOV=250 mm, FA=60°, Thickness=5 mm, spatial Resolution=0.97 mm (c).

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