Yi-Hang Tung1, Frank Godenschweger1, Myung-Ho In2, Alessandro Sciarra3, and Oliver Speck1
1Biomedical Magnetic Resonance, Otto-von-Guericke University Magdeburg, Magdeburg, Germany, 2Department of Radiology, Mayo Clinic, Rochester, MN, United States, 3Medicine and Digitalization, Otto-von-Guericke University Magdeburg, Magdeburg, Germany
Synopsis
VAT-DIADEM
is able to accelerate the distortion-free T2 weighting and diffusion weighting
imaging sequence DIADEM at ultra-high field. The VAT gradient is turned on
during imaging read-out in the slice selection direction. To further increase the
acquisition efficiency, we developed a blipped version of the VAT gradient and applied
simultaneous multi-slice imaging, based on blipped-CAIPI. The result is distortion
reduction in EPI or acceleration in DIADEM and the increased volume coverage
without increasing TR.
INTRODUCTION
A novel multi-shot EPI technique,
termed DIADEM (Distortion-free Imaging: A Double Encoding Method)1,2 was proposed recently for distortion-free imaging.
The
DIADEM uses double encoding including the inherent EPI phase-encoding
(EPI-PE) and additional spin-warp phase-encoding (SW-PE) to
form a 3D data I(s,y,x). With this SW-PE, it can produce distortion-free and
blurring-free T2-weighted and diffusion-weighted images. However, the
extra loop of the SW-PE is time consuming. A common acceleration approach
for DIADEM is the use of a reduced field of view (rFoV) in the
SW-PE by skipping the step of SW-PE3.
The unfolding process can be successful if the reduced
FoV in the SW-PE direction still covers the entire range of geometric
distortions. Otherwise, folding-over artifacts will appear in the calculated
DIADEM
image. A recent study4
demonstrated that further FoV reduction was achievable with view-angle-tilting
(VAT) method by reducing the level of distortion in the EPI-PE
direction.
In this study, we propose the VAT-DIADEM
combined with the simultaneous multi slice (SMS)5 acquisition to achieve higher acceleration
factor in the acquisition and less folding-over ghost in the SMS-DIADEM
data.METHODS
All
the phantom scans were performed on a 7T scanner (Siemens Healthineers,
Erlangen, Germany) using a 32-channel head coil (Nova Medical, Wilmington MA,
USA). A SMS factor of 2 and no in-plane parallel imaging were applied to
maximize the level of distortion and minimize the unfolding bias, which
resulted in an echo-spacing of 1.0 ms. Other imaging
parameters were: TR/TE = 1000.0/80.0 ms, 1.0×1.0×4.0
mm3 resolution with 300% distant factor, and the reduced
resolution factor of 4 in EPI-PE. Figure 1 shows the sequence diagram with (a) SMS-DIADEM (b) SMS-VAT-DIADEM. The VAT gradient amplitude is set to the same
as in the blipped-CAIPI gradient amplitude. As the SMS-DIADEM sequence is
unfolded by single-band EPI reference, SMS-VAT-DIADEM is similarly
unfolded by VAT-EPI.RESULTS and DISCUSSION
Figure 2 shows the unfolded SMS-EPI
and DIADEM images with blipped-CAIPI gradients and blipped VAT gradients for
the same acceleration (i.e. rFoV) factor of 4. From Figure 2 (e,f) SMS-EPI and (g,h) SMS-VAT-EPI it can be
seen that unfolded EPI images show less distortion, but increased image blurring
for VAT gradient. From Figure 2 (a,b)
SMS-DIADEM and (c,d)
SMS-VAT-DIADEM show that fold-over artifacts only appears in the SMS-DIADEM for
an
acceleration factor of 4. The highest fold-over-free rFoV
acceleration factor is 2 for SMS-DIADEM and 4 for SMS-VAT-DIADEM. The value is
consistent with previous VAT studies4,6.
Although
signal-to-noise ratio for SMS-VAT-DIADEM is around 40% of SMS-DIADEM, the
differences could be minimized with a high VAT gradient amplitude and the
use of a high in-plane parallel imaging factor4. Note that single-band images are in good
consistence with multi-band images (data not shown).CONCLUSION
In this
work, SMS is additionally added into the VAT-DIADEM to
further accelerate the DIADEM acquisition. Combining both the VAT and SMS
gradients on the slice axis allows rapid DIADEM
acquisition
not only in the in-plane (SW-PE) but also in the though-plane direction.Acknowledgements
This
project is supported by DFG-grant SP 632/4-2.
References
- In
MH, et al., NeuroImage. 2017; 148:20-30
2
- In
MH, et al., JMRI. 2019
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M et.al., MRM 2004
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YH et al., ISMRM 2018, p.1024
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- Tung
YH et al., ESMRMB 2019