Zhilang Qiu1,2, Sen Jia1,2, Shi Su1, Yanjie Zhu1, Xin Liu1, Hairong Zheng1, Leslie Ying3, Haifeng Wang1, and Dong Liang1
1Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen, China, 2Shenzhen College of Advanced Technology, University of Chinese Academy of Sciences, Shenzhen, China, 3University at Buffalo, The State University of New York, Buffalo, NY, United States
Synopsis
Whole-brain intracranial vessel wall imaging (VWI) requires long scan time for high resolution and large coverage. In this study, we applied a novel three-dimensional parallel imaging technique, Wave-CAIPI, to accelerate whole-brain intracranial VWI. The highly accelerated (11×) VWI takes 3.5 minutes, at an isotropic resolution of 0.6 mm. Compared to conventional two-dimensional parallel imaging technique (2D CAIPI), Wave-CAIPI is able to achieve higher SNR and better imaging quality on vessel wall depictions.
Introduction
Dark-blood
MR vessel wall imaging (VWI) is a useful tool in detecting non-stenotic
atherosclerosis undetected by bright-blood MR angiography (MRA).1 Intracranial
VWI is technically more challenging than carotid VWI,2 since
smaller size and deeper location require high resolution and large spatial
coverage which lead to long acquisition time. Whole-brain intracranial VWI has
been expedited to include the detection of distal branches.3 Though
accelerated by partial Fourier, parallel imaging (GRAPPA) and elliptical
scanning simultaneously, the acquisition time was still over 7 minutes in
recent report.4 A more efficient parallel acquisition technique (2D
CAIPI)5 has been demonstrated to achieve better imaging quality than
GRAPPA in high-resolution 3D imaging using SPACE sequence.6 However,
higher acceleration factor for further scan time reduction is still challenging
due to signal-to-noise ratio (SNR) loss in parallel imaging.
The recently proposed Wave-CAIPI7 is a novel parallel imaging
technique for highly accelerated 3D imaging with improved SNR than 2D
CAIPI, by utilizing the coil sensitivity variations in three dimensions. This
study aims to apply this technique for highly accelerated (11×) whole-brain
intracranial VWI at an isotropic resolution of 0.6 mm, reducing the scan time
to 3.5 minutes. It was compared with that accelerated by 2D CAIPI at the same
settings in healthy volunteers, in terms of SNR and the quality of vessel wall
delineation.Methods
Improved T1-weighted SPACE8
was designed for better cerebrospinal fluid (CSF) suppression in VWI by
applying flip-down pulse and modifying the refocusing pulse series. The
Wave-CAIPI technique was implemented and applied based on this sequence. Fig. 1
illustrates the sequence diagram. In this work, a slight modification (truncation)
was made to the original wave gradients, for keeping zero moment nulling and
avoiding the overlaps between the wave gradient and the encoding gradients
along Gz. With the modification, the Wave-CAIPI technique can be successfully
applied in SPACE sequence for VWI.
IRB approved study
was performed on a 3T Siemens Tim Trio MRI system with a commercial 32-channel
head coil. A young healthy volunteer was recruited with informed consent being
obtained. Two whole-brain intracranial VWI scans using the improved T1w-SPACE
sequence with different acceleration techniques were performed after
localization. One acceleration technique was Wave-CAIPI, and the other was 2D
CAIPI for comparison. These two scans used the common parameters as follows:
resolution = 0.6 mm, TE/TR = 8.5/1000 ms, bandwidth = 334 Hz/pixel, ETL = 48,
FOV = 230 mm (HF, head-foot) × 216 mm (AP, anterior-posterior) × 190 mm (LR,
left-right). While in Wave-CAIPI, wave gradient amplitude = 13 mT/m and the
maximum slew rate = 192 mT/m/ms, cycles = 7. The acceleration factors of
Wave-CAIPI and 2D CAIPI were both 11× (3×3 CAIPIRINHA sampling + elliptical
scanning) and both scans took 3.5 minutes. The 2D CAIPI acceleration embedded a
24×24 calibration region in the k-space center for coil sensitivity estimation,
while separate ACS data with 24×24 k-space lines was acquired immediately after
Wave-CAIPI scan to estimate the coil sensitivity maps. Four calibration scans
were also acquired to characterize the point spread function (PSF) for the
Wave-CAIPI encoding model. The separate ACS scan and the four calibration scans
in Wave-CAIPI took an extra time of about 30 seconds. All datasets were
reconstructed offline in MATLAB (Mathworks, Natick, MA, USA) on a workstation
with 40 GPU cores and 256 GB memory.Results
Fig.
2 shows two sagittal slices of the reconstructed 3D images of whole-brain
intracranial VWI, which were accelerated by the 2D CAIPI technique and the
Wave-CAIPI technique, respectively. The SNR of the reconstructed images in Wave-CAIPI
is obviously higher than 2D CAIPI. The vessel walls of the basilar artery (BA) are visible
in Wave-CAIPI reconstruction, while deteriorated by the heavy noise in 2D CAIPI
reconstruction. This can also be seen in the coronal views (Fig. 3).
Fig.
4 presents two transversal slices of the reconstructed 3D images, showing the
vessel walls of the internal carotid arteries (ICA). The vessel walls of the
ICA are visible in Wave-CAIPI reconstruction, while suffering from severe noise
in 2D CAIPI reconstruction.
The
SNR improvement is more significant in the central part of the field of view
(FOV), where the signal sensitivity is reduced due to long distance from the
coils. The improvement in such region is valuable, as several intracranial
vessels such as basilar artery (BA) and middle cerebral artery (MCA) are deeply
located inside the brain.Discussion
In this study, a novel 3D parallel imaging
technique, Wave-CAIPI, is introduced to expedite whole-brain intracranial VWI.
The highly accelerated (11×) imaging takes 3.5 minutes, at an isotropic
resolution of 0.6 mm. Compared to 2D CAIPI, Wave-CAIPI exhibits improved SNR
and better quality of vessel wall depictions. The Wave-CAIPI accelerated
whole-brain intracranial VWI is still a little bit noisy for clinical acceptance,
due to large acceleration and Wave-CAIPI is less beneficial for applications
with high resolution and high bandwidth.9 Further improvement could
be gained by incorporating sparsity constraint.10 Additionally, recent
works have reported the advantages of joint intracranial and carotid vessel
wall imaging.11,12 Wave-CAIPI accelerated whole-brain intracranial
VWI would be extended to joint intracranial and carotid VWI in the future.Acknowledgements
Zhilang
Qiu and Sen Jia contributed equally to this work. This work was supported in
part by the grant from the National Natural Science Foundation of China
(61871373, 81801691, 81729003 and U1805261), the State Key Program of National
Natural Science Foundation of China (81830056), the National Key R&D
Program of China (2017YFC0108802), the Strategic Priority Research Program of
Chinese Academy of Sciences (XDB25000000), and the Natural Science Foundation
of Guangdong Province (2018A0303130132).References
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