Yuan Zheng1, Lele Zhao2, Zhongqi Zhang2, Yu Ding1, and Jian Xu1
1UIH America, Inc., Houston, TX, United States, 2United Imaging Healthcare, Shanghai, China
Synopsis
GRE
sequences benefits from increased SNR at high field and do not suffer from banding artifacts
and SAR issues as bSSFP sequences, which makes it a good alternative for some
applications that typically use bSSFP at 1.5 T or 3T. We have implemented the autocalibrated multiband GRE cardiac cine
sequence at 5 T. The autocalibrated multiband acquisition increases the
scan efficiency by reducing the scan time and simplifying the
multiband imaging workflow. Iterative reconstruction with sparsity
constraints was used which can also accommodate in-plane undersampling.
We have evaluated this application on volunteers and achieved cardiac cine images with satisfactory quality.
Introduction
Cardiac
cine MRI is an important clinical tool for assessing cardiac diseases1.
Usually balanced steady-state free precession (bSSFP) sequences are used for SNR
and image contrast considerations on 1.5 T and 3 T scanners. However, at even higher
field, bSSFP images suffer from severe banding artifacts due to worse B0
homogeneity2; The specific absorption rate (SAR) can also be
problematic due to increased RF frequency3. On the other hand,
the spoiled gradient-echo sequence (GRE) is free of banding artifact and typically
uses smaller flip angles which deposit much less RF energy; Besides, the SNR
of GRE sequences also increase with the field strength. Therefore, GRE
sequences can be a practical alternative for cardiac cine imaging at high
field.
Conventional
cine only scans one slice at a time, leading to prolonged scan time which not
only causes patient discomfort but is also challenging for patients having
difficulties in repeated breath-holding. Autocalibrated multiband (MB) imaging4-6
is a promising acceleration technique to tackle this problem. MB imaging
leverages coil array sensitivity variations and scans multiple slices
simultaneously with minimal SNR penalty. A specially designed phase modulation
pattern also enables auto-calibration which eliminates the need for separate
scans for single-band reference images.
In
this study, we implemented the autocalibrated multiband GRE cine sequence. An iterative
reconstruction procedure exploiting spatial and temporal sparsity was used to achieve
best image quality. The application was evaluated on a 5 T scanner on
volunteers.Theory
The
autocalibrated multiband GRE cine sequence is demonstrated in Fig.1 using MB =
2 as an example. Phase modulation is achieved by adjusting the RF excitation
phase. Taking the first slice as reference, the temporal phase modulation
adjusts the phase of the second slice as follows: 1, the phase advances by π
for adjacent phase-encoding (PE) steps; 2, the global phase offset alternates
between 0 and π for adjacent frames, so that the two slices are in-phase in odd
frames and out-of-phase in even frames. Such phase modulation not only produces
controlled aliasing for better preserving SNR7, but also allows self-calibration,
i.e., extracting low temporal resolution single-band reference images from the
MB data themselves by linear combination of the frames (Fig. 2).
Coil
sensitivity maps are subsequently calculated from the reference images. Cine
images are then reconstructed by minimizing the following cost function:
argmax in
which x1 and x2 are the two cine image series of the two
slices, s1 and s2 are coil sensitivity maps, p1
and p2 are phase modulations, y is the acquired multi-slice k-space
data, D represents the k-space sampling operator, F represents
the Fourier transform operator, and T represents the temporal TV
operator which is the sparsifying transform for l1
regularization, λ is an adjustable parameter of the regularization strength.Methods
The
proposed auto-calibrated multiband GRE cine application was tested on a healthy
volunteer on a 5 T scanner (United Imaging Healthcare, Shanghai, China) with an
8-channel RF transmit system. A total of 48 receive channels were used (a
24-channel cardiac coil and a spine coil with 24 channels selected). Imaging
parameters include: FOV = 400 × 300 mm, matrix = 256 × 154, thickness = 7 mm, MB
factor = 2, gap = 35 mm, bandwidth = 1200 Hz/pixel, FA = 10°, TR = 3.7 ms,
TE = 1.8 ms, views per frame=15, number of heart beats = 11. All images were
collected in a single breathhold. Results
Fig.
3 zoomed in on the heart and shows 8 frames from systole to diastole. Heart
structures were well delineated, and heart motion was well preserved. No
obvious artifacts were observed except a dark band near the back, which is
likely due to B1 inhomogeneity commonly found on high-field systems.Discussion and Conclusion
We have implemented the autocalibrated
multiband GRE Cine sequence at 5 T. This application benefits from increased
SNR at high field and does not suffer from issues related to bSSFP. The
autocalibrated multiband acquisition reduces the scan time and simplifies the
multiband imaging workflow with no need for extra calibration scans. An
iterative reconstruction procedure with sparsity constraints was used which can also
easily accommodate in-plane undersampling. This application was evaluated on
volunteers and achieved cine images that satisfactorily describes cardiac morphology and motion. Overall, the
autocalibrated multiband GRE cine application provides a useful alternative for
bSSFP cine at 5 T.Acknowledgements
No acknowledgement found.References
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