Yuan Zheng1, Lele Zhao2, Zhongqi Zhang2, Zhehao Zhang2, Xingxian Shou1, Haojie Li3, Jian Xu1, Weiguo Zhang1, Lu Huang3, and Liming Xia3
1UIH America, Houston, TX, United States, 2United Imaging Healthcare, Shanghai, China, 3Tongji Hospital, Wuhan, China
Synopsis
Simultaneous multi-slice (SMS) imaging is an
efficient MRI technique that better preserves SNR than conventional parallel
imaging (PI). However, single-band images usually need to be acquired to
provide references for slice-unaliasing, causing an untrivial scan time
overhead in some scenarios. We extended a recently reported self-calibrated SMS
imaging strategy based on RF phase modulation, and propose using gradient
temporal modulation to achieve auto-calibrated SMS bSSFP, which avoids the
bSSFP banding shift and is preferred in cardiac Cine due to its contrast
and intrinsically high SNR. In-plane PI can also be incorporated with shared
auto-calibration.
Introduction
Simultaneous Multi-Slice (SMS) imaging is a
technique that improves the time efficiency of MRI pulse sequences by exciting and
imaging multiple slices simultaneously1. However, single-band images usually need to be acquired separately to provide references for the
slice-unaliasing task2, causing an untrivial scan time overhead in some
scenarios, especially in certain cardiac imaging applications that require
breath-holding (BH). Recently an auto-calibrated SMS CAIPIRINHA imaging
technique which eliminated the need for extra reference scans was reported3.
An additional RF phase modulation was applied along the temporal dimension, enabling extraction of single-band references from the multiband (MB) data. However, RF phase modulations may
introduce artifacts in balanced steady-state free precession (bSSFP) sequences, since spins are
subjected to an additional phase each TR and the bSSFP banding artifacts would
be shifted. In this abstract, we propose using gradient modulation to achieve
the temporal phase encoding required for auto-calibrated SMS. Since TR-TR
signal evolution is not affected, the bSSFP bandings remain unchanged. In-plane
parallel imaging (PI) can also be integrated into this framework and share the
auto-calibration property. We implemented this technique in bSSFP cardiac Cine
sequences and demonstrated its advantages in clinical applications. Theory
Phase modulation of the proposed technique is
achieved by modifying the rephrasing and prephasing gradient lobes4, as shown in
Fig.1. The gradient pair is modified such that a desired phase shift is added
to the signal before ADC but cancelled out afterwards. Therefore, the net
gradient moment remains zero and the TR-TR spin evolution is unchanged.
For auto-calibrated SMS CAIPIRINHA imaging,
the gradient induced phases serve two purposes: within a frame they cause
controlled aliasing; between frames they create controlled phase offsets of the
collapsed slices, allowing extraction of single-band reference images.
The phase modulation table and image
reconstruction procedure for MB = 2 are shown in Fig.2. Taking slice 1
as reference, the linear phase modulation causes an FOV/2 shift of slice 2 with
a phase offset varying by π between even and odd frames. The reference images
can be extracted by summation and subtraction of adjacent frames. Although the
reference images are slice unaliased, they are of lower temporal resolution because
different time frames are mixed. The reference images are then used for kernel
calibration to unalias single-frame MB images using slice-GRAPPA5.
The phase modulation table and steps for
extracting single-band reference images for MB = 2 and PI = 2 are shown in
Fig.3. Frame 1, 2 samples odd PE lines, while Frame 3, 4 samples even PE
lines, etc, in a manner similar to temporal GRAPPA6. The phase offset of
slice 2 alternates by π between frames. Four frames are needed to extract the
single-band references. Taking the first four frames for example, frames
1, 2 and 3, 4 are first combined respectively to separate the collapsed slices,
leaving single-band images with in-plane ghosting. The ghosting patterns are
different depending on whether even (frame 3, 4) or odd (frame 1, 2) lines are
sampled. By combining them, ghost-free images are obtained, which can be
used as reference images for reconstructing single-frame images from the MB data, using slice-GRAPPA
and GRAPPA algorithms. Methods
The proposed auto-calibrated SMS bSSFP Cine
was tested on a healthy volunteer on a 3.0 T scanner (uMR780, United Imaging
Healthcare, Shanghai, China), with 40 receive channels (a 24-channel cardiac
coil and a 16-channel spine coil). Imaging parameters include: FOV = 360×300
mm, matrix = 224×224, thickness = 8 mm, 2 slices were imaged with gap = 40
mm, bandwidth = 1200 Hz/pixel, FA = 45°, TR/TE = 3.1/1.4 ms, frame # = 23
per heartbeat, views per frame = 13. Four scans were performed with no
acceleration, PI (2), MB (2), and MB (2) + PI (2). The number of heart beats
needed were: 34 for no acceleration; 17 for PI
(2); 17 for MB (2) and 9 for MB (2) + PI (2). The no-acceleration scan was
completed in two BHs, while all others were collected within a single BH.Results
Fig.4 compares images of the same cardiac
phase acquired with different approaches. Heart structures were well delineated in
all images. No obvious artifacts were observed, although a visible SNR drop was
observed in the MB (2) + PI (2) image,
which is expected due to undersampling of k-space and the g-factor penalty.
Fig.5 zoomed in on the heart and shows 10 consecutive frames acquired with MB
(2) and MB (2) + PI (2). Wall motion was well preserved in both Cine image series. Discussion and Conclusion
SMS is an SNR efficient MRI acceleration technique
and is preferred over conventional PI in many applications. Based on a recently
proposed auto-calibrated SMS imaging concept, we used gradient temporal
modulation to achieve auto-calibrated SMS bSSFP, which avoids the bSSFP banding
shift and is preferred in cardiac Cine due to its contrast and intrinsically high SNR. We
also showed that in-plane PI can be incorporated with shared auto-calibration. The proposed
technique was demonstrated on volunteers and achieved high-quality Cine images. Overall, our proposed
technique can be used to collect SMS bSSFP images without dedicated reference scans
and could be used to simplify the clinical scan work flow and reduce scan time.Acknowledgements
No acknowledgement found.References
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