Proceedings of the meeting - Department of Physics - University of ...

Faster Dixon Fat-Water Imaging With Slice Multiplexed PulsesO1Kuan J LeeAcademic Radiology, University of Sheffield, Sheffield, UKIntroductionPhase sensitive imaging for fat-water separation was first proposed byDixon (1): for each slice, two images are acquired, one with fat andwater “in-phase”, and the other π “out-of-phase”. However, theoriginal method could fail if there were significant fieldinhomogeneities which introduced additional phase shifts. Toovercome this, Glover (2) suggested acquiring an additional 2π image.Unfortunately this leads to a threefold increase in minimum imagingtime.Recently we introduced a new method for multislice imaging usingthe “multiplex pulse” (3). The multiplex pulse combines severalcomponent pulses into a single pulse which has the duration of asingle component pulse. Each component pulse simultaneously selectsa different slice which is rephased with a different slice gradientrefocusing lobe, thereby avoiding aliasing. The aim of this work is toshow that multiplex pulses can be used to speed up Dixon fat-waterimaging.MethodsA 4 ms three-slice multiplex pulse (Fig 1), the slices of which arerephased by +0.15, -0.5 and -0.85 of the slice select lobe, wasdesigned as described in Ref (4). This was used in a spin-echosequence with a non-slice selective refocusing pulse; the schematic(not to scale) is shown in Fig 2. First, images were collected atvarying TE with fat and water phantoms in order to determine the inphaseecho time for each slice. Relative to the 0.5-rephased slice, the0.15 pulse’s slice was measured to be in phase 3.1 ms earlier and the0.85 pulse’s slice was in phase 1.9 ms later (data not shown). Thetiming of the readout gradients was adjusted to take this into accounti.e. all echoes to have the same relative fat-water shift.RF/SignalSSPEFEwas a non-minimum power delayed focus DBURP1 pulse). This delayis actually helpful because the time interval can be used to switch theread gradient. Interestingly, it indicates that the relative in-phase echotimes may be adjusted e.g. simply by shortening or lengthening theexisting pulse. This may be useful when adapting a sequence for useon scanners of different field strengths. Adjustment will also benecessary for other variants of Dixon imaging requiring different fatwatershifts e.g. the IDEAL method (-π/6, π/2, 7π/6) (5), or the twopoint POP (0, 135°) (6).The proof-of-principle sequence presented here uses one non-sliceselective refocusing pulse. The next step will be to implementmultiplex fat-water imaging with FSE sequences. The sequence willbe similar to GRASE (7), where extra gradient echoes are collected inbetween refocusing pulses. However, with multiplex pulses, the extragradient echoes correspond to extra slices, instead of extra k-spacelines within the same slice.It is hoped also to implement this method for fat suppression at lowfields (