Jenni Schulz1, Lauren J Bains1, José P Marques1, and David G Norris1
1Donders Institute for Cognitive Neuroimaging, Radboud University Nijmegen, Nijmegen, Netherlands
Synopsis
HASTE sequences
have
high RF power deposition which particularly limits their use at high
resolutions and high field strengths. In this work, TRAPS and fixed SAR Power Independent
of Number of Slices (PINS)-refocusing pulses were used to reduce the power deposition. Multiband excitation pulses were added to enable arbitrary user-defined slice
orientations, with the added benefit of accelerating the acquisition due to the
simultaneous excitation of multiple slices. Good image quality was obtained for a MB4-HASTE protocols with 3.1 and 1.1 mm resolution, and a whole-brain MB6-HASTE protocol with 1.1 mm resolution.Introduction
Ultrafast spin-echo sequences such as Turbo Spin Echo (TSE) and Half Fourier
Acquisition Single Shot Turbo Spin Echo (HASTE) have found wide-spread
application. However, these sequences
have
high RF power deposition which particularly limits their use at high
resolutions and high field strengths. Implementing fixed SAR Power Independent
of Number of Slices (PINS)-refocusing pulses
1 has been shown to
reduce the power deposition to an acceptable level in the closely related Turbo
Spin Echo (TSE) sequence
2,3.
However, when PINS pulses are used for both excitation and refocusing,
slice orientations are limited by the periodic and infinite nature of the PINS
slice profile
2. The use of a multiband (MB) pulse
4 for
excitation and a PINS pulse for refocusing retains the benefits of PINS for
reduced power deposition while enabling arbitrary user-defined slice
orientations. Multiband excitation
pulses have the added benefit of accelerating the acquisition due to the
simultaneous excitation of several slices. Blipped CAIPIRINHA, which shifts the
simultaneously acquired slices according to their z position, was included in
the sequence to improve the quality of MB reconstruction
5, and TRAPS
6
was implemented to allow further reductions in power deposition and effective
TE.
Methods
Standard MB excitation pulses
and PINS refocusing pulses were incorporated into the standard Siemens HASTE
sequence. In order to reconstruct the images on the scanner, an integrated FLASH reference
scan was incorporated into the beginning of the sequence. Images were
acquired on a Siemens 3T Prisma system from a healthy volunteer after obtaining written informed
consent.
A MB4-HASTE scan was acquired with
3.1 mm resolution in 5 seconds with a 10 s reference scan at 100% SAR
(10% SAR per slice): 40 axial slices, 20% slice gap, TR 660ms, TE 61ms, 3.1x3.1x3.0mm resolution,
64x64 matrix, partial Fourier 5/8, flip angle 162 degrees, TRAPS, multiband
factor of 4, CAIPI 2. The standard HASTE
sequence with the same parameters required 28s and ran at 63% SAR.
A second MB4-HASTE scan was acquired
with 1.1 mm resolution in 120s with a 31 s reference scan at 55% SAR ( 3%
SAR per slice): 72 axial slices, 20% slice gap, TR 3290, TE 74ms, 1.1 mm
isotropic resolution, 192x192 matrix, partial Fourier 5/8, flip angle 120
degrees, TRAPS, multiband factor of 4, CAIPI 2.
The standard HASTE sequence with the same parameters required 3 min 58s
and ran at 24% SAR.
A MB6-HASTE full-brain scan
was acquired with a multiband factor of 6 and 1mm isotropic resolution in 85
seconds with 100% SAR (20% SAR per slice): 120 axial slices, 30% slice gap, TR
4320 ms, TE 99ms, 1.1x1.1x0.8 mm resolution, 192x192 matrix, partial Fourier
5/8, flip angle 120 degrees, TRAPS, multiband factor 6, CAIPI 3. The standard HASTE sequence with the same
parameters required 8min 40 s and ran at 25% SAR.
Results
MB4-PINS HASTE shows good image quality which is
comparable to a standard HASTE acquisition in both a faster (5s acquisition
time) version and a higher resolution (1.1mm isotropic spatial resolution)
version (figure 1). MB6-PINS also shows
good image quality for a full-brain 1.1mm isotropic resolution protocol (figure
2). These images demonstrate that HASTE
scans can be reconstructed using FLASH reference data without significant artifacts
or leakage between individual slices.
Discussion and Conclusion
The slightly different contrast observed in the MB4-PINS
HASTE compared to the standard HASTE at 3 T seen in figure 1 may be due to
differing levels of magnetization transfer
2. Higher MB factors
should be attainable with further protocol optimization, allowing better
separation of aliased slices and therefore closer slice spacing. Additionally, the
“faster” and “higher resolution” protocols shown here can be further optimized
for the desired application by adjusting parameters such as MB factor,
CAIPIRINHA shift, GRAPPA factor, ETL and resolution. This would enable the MB-PINS HASTE sequence
to be used, on the one hand, for anatomical T2-imaging, and on the other hand, potentially for fMRI experiments.
Acknowledgements
No acknowledgement found.References
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