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.
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.
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?