Real time shimming over multiple regions across the DTI volume with motion correction using a multislab navigated DTI sequence

A Alhamud^{1}, Ernesta M. Meintjes^{1}, and AndrĂ© J.W. van der Kouwe^{2}

The DvNav sequence acquires two navigators with different echo times immediately after each DWI volume to reconstruct in real time two different field maps: one for the navigator volume and one for the DTI volume. For accurate motion estimation the navigator field of view (FOV) must cover the whole object being imaged. The field map of the DTI volume is generated by mapping the location of the DTI volume onto the navigator field map. Here we create a field map for groups of slices (slabs) by mapping the position of the slab onto the navigator field map. We considered slabs of different thicknesses as illustrated in Figure 1. Slab 1 includes all DTI slices (for example 73 slices). For each successive slab, two DTI slices on either end are removed. The n-th slab therefore includes the (75-2n) central DTI slices, where n=1-35. Since the thickness of each partition (slice) in the navigator is 8 mm, the thickness of the slab to be shimmed in the DTI volume has to be greater than 8 mm so that multiple field map points are available for fitting in the through-plane direction. Following the shim calculation for all slabs, the sequence receives and stores these parameters in addition to the motion estimates. The DvNavSlab sequence applies for each slice the shim values of the smallest slab that it belongs to.

All scans were performed on a Siemens Allegra 3T scanner (Siemens Healthcare, Erlangen, Germany). Two healthy subjects (25 and 30 years) were scanned with a T1W and different diffusion sequences. These include the standard twice-refocused 2D diffusion pulse sequence (Std) (Reese et al., 2003), DvNavW (W refers to shimming across the whole DTI volume) and DvNavSlab. The standard sequence was acquired without subject motion (NoMo), while in the other two scans the subjects were instructed to move (Mo) during the acquisitions. The following parameters were used for each diffusion acquisition: TR = 11163 ms, TE = 86 ms, 73 slices, matrix size 112 x 112, in-plane FOV 224 x 224 mm2, slice thickness 2 mm, 30 non-collinear diffusion gradient directions, diffusion weighting = 1000 s mm-2, eight b=0 scans. Retrospective motion correction and co-registration to the T1W image were performed using FLIRT in FSL (FMRIB Software Library; http://www.fmrib.ox.ac.uk/fsl). The DTI tensor was calculated using dtifit and a white matter (WM) mask was calculated from the T1W image using Fast segmentation.

A. Alhamud, Aaron T. Hess, Paul A. Taylor, Ernesta M. Meintjes, and André J.W. van der Kouwe. Updating Shim Dynamically During Diffusion Tensor Imaging Acquisition; In: Proceedings of the 23rd Annual Meeting of ISMRM 2014; Milan, Italy.

Reese TG, Heid O, Weisskoff RM, Wedeen VJ.Reduction of eddy-current-induced distortion in diffusion MRI using a twice-refocused spin echo.Magn Reson Med. 2003;49(1):177-82.

Illustration
of how slabs are defined and slab-specific shim parameters are determined by
mapping the location of each slab onto the navigator field map

Comparison
of zero (frequency drift, ΔF in Hz) and first order (Gx, Gy, Gz in µT/m) shim
parameters for a scan with motion for two different slabs throughout 38 DWI
volumes. The arrows indicate instances where motions occur. ΔF values were
scaled by a factor of 10 on the plots

FA
maps (0.2<FA<1) from (a) standard, (b) DvNavW, and (c) DvNavSlab
acquisitions overlaid on T1W WM mask (yellow). In a, b and c yellow denotes the
misalignment between each diffusion scan and the T1 image. DvNavSlab_Mo shows
less geometric distortion especially in the frontal region of the brain

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)

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