Jordan Chad1, Andrew Curtis1, Qing-San Xiang2,3, and Sofia Chavez1,4
1Centre for Addiction and Mental Health, Toronto, ON, Canada, 2Department of Radiology, University of British Columbia, Vancouver, BC, Canada, 3Department of Physics and Astronomy, University of British Columbia, Vancouver, BC, Canada, 4Department of Psychiatry, University of Toronto, Toronto, ON, Canada
Synopsis
A
phase-based method, PLACE, is embedded into a stock DTI sequence to
enable geometric distortion correction without the need for extra
scans. We show that geometric distortion correction via this embedded PLACE DTI sequence performs like
the more traditional B0-map based correction schemes when there is no motion between scans. Furthermore, when motion
occurs, embedded PLACE yields superior results. This embedded PLACE DTI is thus more efficient and less
sensitive to motion than traditional B0 mapping methods.Purpose
DTI
relies
on
EPI
acquisitions
which are
plagued by geometric distortions along the phase-encode (PE)
direction
1. A
common
correction, such as FUGUE
2 in FSL (FMRIB Software Library),
employs a B0 field map, requiring extra non-EPI scans to generate the
map. This has the disadvantage of requiring more scan time and
it is motion sensitive beause if the
subject undergoes head motion between the B0 mapping scans and the
DTI acquisition, B0 maps cannot properly align with the distorted
images for effective correction. A more general phase-based method of
distortion correction is called
PLACE
(Phase
Labeling for Additional Coordinate Encoding)
3. PLACE relies on the phase difference between EPI-based images
that differ only by an extra PE blip and, unlike with B0 maps, no
phase unwrapping is required. PLACE has previously been applied to
DTI using extra EPI scans with varying PLACE blips
4,5 or by
acquiring two DTI datasets (with and without blip) and extracting the
PLACE information from the non-diffusion weighted images (nDWI)
6.
In this work, we modify the DTI acquisition and embed PLACE blips, in
alternating fashion, across the set of nDWIs
of
the DTI data set. As such, the distortions can be reverted without
additional scan time. We demonstrate that PLACE can be embedded
successfully in a typical brain DTI scan despite the complications of
multi-channel
SENSE-like
acceleration and partial k-space. We show that when motion is present
between B0 mapping and DTI scans, this embedded PLACE yields superior
distortion corrections compared to those obtained with conventional
B0 map-based correction schemes.
Methods
The
stock pulse sequence for DTI is slightly modified to include a PLACE blip: the amplitude of the pre-winder gradient is adjusted by 2∆ky
(where ∆ky is the PE EPI blip). This embedded
PLACE blip is incorporated for even-numbered nDWI. This modification does not
change the sequence timing nor does if affect the magnitude nDWIs. Complex-valued images were reconstructed
off-line using Orchestra SDK (GE research tool, Waukesha, WI) and
phase information from the PLACE embedded
nDWI
data was
used to perform the distortion correction on all images in the DTI
data set6.
To
test the sequence, three volunteers were scanned with consent as per
institutional REB. All acquisitions were performed on a 3T-MR750 GE
scanner (General Electric, Waukesha, WI) using an eight-channel
receive-only head coil. A standard DTI protocol was used: NnDWI
= 5, NDWI
= 23, and b = 1000 s/mm2.
Other scanning parameters were TE=82ms, TR=6.8s, FOV=25.6cm,
Nx=Ny=128,
slice thickness = 4mm, 45 slices, partial k-space, ASSET=2. To
demonstrate the effects of motion, B0 maps were obtained once, from
two gradient echo scans taken prior to the DTI acquisitions (TE1/TE2=6.5ms/8.5ms). Then, two PLACE-embedded DTI data sets were
acquired with the volunteer undergoing realistic head motion in
between scans. Motion metrics were obtained using FSL's avscale7. For full brain coverage at 4mm isotropic resolution and TR=1s,
this added 2.5min of scan time. Undistorted, reference T2-weighted FSE images were also
acquired at each head position (T2_DE_FSExL, TE1/TE2 =
11.152ms/89.216ms, TR=2.5s).
To
assess the accuracy of the distortion correction, FSL's FLIRT7 was used with 6 degrees of freedom, to coregister the head positions to an anatomical reference. Edges from a coregistered average nDWI, before and after distortion correction, were overlayed on the
FSE T2W reference image.
Results and Discussion
In
the absence of motion, the embedded PLACE, requiring no extra scans
nor phase unwrapping, is able to correct geometric distortions as
well as FUGUE. In the case of motion occurring between the B0 mapping
and DTI scans, distortion correction by FUGUE is worsened, in all cases, as the B0
map no longer applies to the new head position, despite coregistration to account for the motion. In contrast, embedded PLACE is not affected, i.e., geometric distortions are
corrected. Table 1 lists metrics for the motion occurring
between the
B0-map
scans
and the second DTI acquisition. Figure 1 displays representative results where edges of the
distortion corrected nDWIs are overlayed on the FSE reference. Zoomed regions are highlighted below each image. For volunteer 1, edges match best for PLACE at the posterior brain border and gyri. For volunteer 2, the edges of the ventricles are better matched with PLACE. Quantitative analysis of the geometric distortion is underway using white matter segmentation and FA maps, for a
more rigorous comparison. Also, more subjects and motion are being
tested.
Conclusion
Embedding
PLACE blips on nDWIS within the DTI set allows for effective
distortion correction of the images. This method requires no extra scans and it is more robust
to motion than using extra scans for B0 mapping.
Acknowledgements
This project was funded in part by funds from the James S. McDonnell foundation awarded to Nancy J. Lobaugh.References
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