Wei-Ching Lo1, Kawin Setsompop2,3,4,5, Congyu Liao2, Susie Yi Huang2,3,4,5, John Conklin2,3,4, Stephen F. Cauley2,3,4, Wei Liu6, Bryan Clifford1, Steffen Bollmann1, Xiaozhi Cao2,7, Zijing Zhang2,8, Daniel Polak2,3,9,10, Daniel Nicolas Splitthoff9, Thorsten Feiweier9, Qiyuan Tian2,3,4, Jaejin Cho2, John E. Kirsch2,3,4, Shivraman Giri1, Otto Rapalino3,4, Pamela W. Schaefer3,4, Larry L. Wald2, and Berkin Bilgic2
1Siemens Medical Solutions, Boston, MA, United States, 2Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Boston, MA, United States, 3Department of Radiology, Massachusetts General Hospital, Boston, MA, United States, 4Harvard Medical School, Boston, MA, United States, 5Harvard-MIT Division of Health Sciences and Technology, Massachusetts Institute of Technology, Boston, MA, United States, 6Siemens Shenzhen Magnetic Resonance Ltd., Shenzhen, China, 7Center for Brain Imaging Science and Technology, Biomedical Engineering, Zhejiang University, Hangzhou, Zhejiang, China, 8State Key Laboratory of Modern Optical Instrumentation, College of Optical Science and Engineering, Zhejiang University, Hangzhou, Zhejiang, China, 9Siemens Healtcare GmbH, Erlangen, Germany, 10Department of Physics and Astronomy, Heidelberg University, Heidelberg, Germany
Synopsis
We utilize
Wave-CAIPI and BUDA techniques to develop a rapid 2-minute protocol that
produces high in-plane resolution and distortion-free axial images for
comprehensive evaluation of the brain. The protocol includes 3D T1-weighted Wave-CAIPI
MPRAGE, 3D dark-fluid T2-weighted Wave-CAIPI SPACE-FLAIR, 2D T2*-weighted gradient
echo BUDA, 2D T2-weighted and diffusion-weighted spin-echo BUDA. The results of
the optimized protocol demonstrate comparable image quality, tissue contrast,
and spatial resolution to standard clinical scans while keeping the total scan
time to less than 2 minutes. The advanced acquisition and reconstruction framework
presented here offers a path toward increasing clinical acceptance of ultrafast
brain examinations.
INTRODUCTION
The long acquisition
times for routine brain MRI examinations can present a significant barrier to
evaluating particular clinical populations such as pediatric and uncooperative
patients. Echo planar imaging (EPI) has been widely used in many clinical protocols
to reduce imaging time and decrease motion artifacts. However, EPI is
particularly prone to geometric distortions that may limit diagnostic accuracy.
Alternatively, the fast MRI techniques Wave-CAIPIRINHA (Wave-CAIPI) [1, 2] for
3D examinations and Blip Up-Down Acquisition (BUDA) [3] for EPI examinations, provide
robust acquisition and reconstruction frameworks that achieve distortion-free,
high-resolution MRI for different applications while shortening scan time. Each
of these imaging approaches offers unique advantages and drawbacks, as
illustrated in Figure 1. Building on these strategies, we develop a highly
efficient comprehensive 2-minute protocol that combines the advantages of the
Wave-CAIPI and BUDA frameworks to achieve axial thick-slice 2D-like MRI images
with high in-plane resolution and without distortion to match the image quality
typically represented in the majority of routine brain MRI examinations. This protocol
has the potential to provide comparable image quality and image contrast to the
standard exam for the evaluation of intracranial pathology.METHODS
This study was
approved by the IRB and was HIPAA compliant. As a proof of principle, MRI data
was acquired on a single healthy volunteer on a 3T MRI scanner (MAGNETOM
Prisma, Siemens Healthcare, Erlangen, Germany) with a 32-channel multi-array
receiver coils.
The imaging protocols included:
(i) Wave-protocol: A prototype 3D Wave-CAIPI MPRAGE
for T1-weighted images (TR/TE: 2500/4.35ms; TI: 1110ms; acceleration factor (R):
3x3, FOV: 256x232x144mm3; resolution: 1x1x4 mm3) and a
prototype 3D Wave-CAIPI SPACE-FLAIR for dark-fluid T2-weighted images (TR/TE:
5000/391ms; TI: 1800ms; R: 3x2, FOV: 224x224x144mm3; resolution:
0.88x0.88x3 mm3) were acquired. Slab-selective excitation and saturation band were used to acquire data in the axial plane. To avoid signal wrapping, 90% of slab thickness was excited for Wave-CAIPI SPACE-FLAIR (Figure 2). Wave-CAIPI SPACE-FLAIR was reformatted to 0.88x0.88x4mm3 for subsequent comparisons. An efficient sampling order for Wave-CAIPI MPRAGE was achieved by progressing along the phase encoding first followed by the partition encodings.
(ii) BUDA-protocol: A prototype 2D gradient echo (GRE) BUDA for T2*-weighted images (TR/TE: 2900/29ms; R: 4, FOV:
256x256x144mm3; resolution: 1x1x4 mm3) and a prototype 2D spin-echo
(SE) BUDA for T2-weighted and diffusion-weighted images (DWI) (TR/TE:
3000/60ms; R: 4, multiband factor (SMS): 2, FOV: 256x256x144mm3; resolution:
1x1x4 mm3, phase partial Fourier (PF): 6/8, b-value: 0, 1000 s/mm2
with 6 directions) were acquired. In addition, a fast distortion-free GRE prescan was also acquired to perform coil sensitivities estimation for BUDA. Figure 2 shows the sequence diagram of SE EPI with BUDA, where the interleaved blip-up and blip-down shots acquired the complementary subsets of the k-space [3]. Multiband RF-excitations were combined with the blipped-CAIPI technique [4] to simultaneously acquire 2 slices per EPI-shot. The reconstruction of BUDA data used in-house developed MATLAB code to obtain distortion-free images.
(iii) Standard clinical brain MRI protocol: The protocol used for comparison was adapted from the clinical protocol used at MGH and included T1-weighted dark-fluid turbo spin echo (TSE), T2-weighted dark-fluid TSE, T2*-weighted FLASH, T2-weighted TSE, and diffusion-weighted single-shot EPI sequences.RESULTS
The total scan time
was less than 2 minutes when using the proposed comprehensive distortion-free
protocols. This includes T1-weighted MPRAGE (10 seconds + 2 seconds ACS data),
T2-weighted dark-fluid SPACE-FLAIR (50.5 seconds), T2*-weighted images (8.7
seconds), T2-weighted images (9 seconds) and diffusion-weighted images (36
seconds). The image quality of T1-weighted MPRAGE and T2-weighted dark-fluid
SPACE-FLAIR images was comparable to standard clinical reference in one
volunteer (Figure 3). Figure 4 compares the image quality of distortion-free
T2*-weighted, T2-weighted images and corresponding clinical standard. The
diffusion-weighted images and clinical standard are shown in Figure 5.DISCUSSION
The imaging
parameters for Wave-CAIPI and BUDA techniques were optimized to achieve similar
spatial resolution and image contrast compared to the clinical protocol. One
saturation band was applied to the proximal end of an imaging slab in both
Wave-CAIPI MPRAGE and Wave-CAIPI SPACE-FLAIR to suppress signals close to the
end edge of the imaging volume that would cause artifacts due to wrapping.
Moreover, several techniques such as coil mixing [5] may further improve
aliasing artifacts. This study introduced the possibility of acquiring axial
thick-slice 2D-like MRI using 3D Wave-CAIPI techniques. It notes that wave
corkscrews with large signal amplitudes during phase and partition encodings
causes blurring artifacts when increasing the slice thickness. The BUDA method
provided high in-plane resolution distortion-free images. As can be seen in
Figure 5, the BUDA images had superior sharpness when compared to the clinical
reference. The BUDA image appeared to be darker in the center, and this can be
mitigated by pre-scan normalization. Additional investigation on volunteers and
patients is warranted to further validate the benefits of our approach.CONCLUSION
A highly efficient distortion-free
protocol combining the unique advantages of Wave-CAIPI and BUDA acquisition
strategies provides comparable image quality, tissue contrast, and spatial
resolution to standard clinical scans while keeping the total scan time to less
than 2 minutes.Acknowledgements
No acknowledgement found.References
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