OP = W − F
½ [IP − OP] = ½ [(W+F) − (W−F)] = ½ [2F] = F → Fat only image
Today, Dixon-type pulse sequences are offered as a standard fat-suppression technique by nearly every manufacturer, finding clinical utility especially for abdominal imaging at 3.0T. The current GE offerings (IDEAL, Flex) are described above. Siemens has both 2- and 3-point techniques under the generic name DIXON; Philips offers multi-point" Dixon ("mDixon"); Hitachi and Canon have 2-point sequences called "FatSep" and "WFOP" (Water-Fat Opposed Phase) respectively.
Regardless of the details, current Dixon-type sequences all produce four sets of images as shown below: water only, fat only, in-phase, and out-of-phase. The fat-only images offer the potential for fat-quantification. A minor disadvantage is an increase in minimum TR value (required to allow time for collection of the multiple echoes).
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Newer processing techniques for the Dixon method may reduce or eliminate phase-induced fat-water swaps in the future. Seed-growing algorithms have been developed that connect high-confidence areas of water or fat signal with neighboring areas which may falsely converge to the wrong substance.
Dimitrov I. White Paper. mDIXON Quant delivers robust, high quality fat quantification. Philips NetForum Community, 2017. (Brief description of a 6-echo, 7-peak fat-modeling method offered by Philips for fat quantification and T2*-mapping, useful in liver imaging)
Dixon WT. Simple proton spectroscopic imaging. Radiology 1984; 153:189-194.
Glover G. Multipoint Dixon technique for water and fat proton and susceptibility imaging, J Magn Reson Imaging 1991;1:521‐530.
Ma J. Dixon techniques for water and fat imaging. J Magn Reson Imaging 2008; 28:543-558.
Reeder SB, Pineda AR, Wen Z, et al. Iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL): application with fast spin-echo imaging. Magn Reson Med 2005;54:625-635.
What is meant by in-phase vs out-of-phase imaging?