Xiaoxuan He1, Naoharu Kobayashi1, Myung Kyun Woo1, Edward J. Auerbach1, Xiaoping Wu1, and Gregory J. Metzger1
1Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN, United States
Synopsis
In this simulation study, we explored the feasibility of incorporating
parallel transmission to achieve a 3D inner volume MR fingerprinting with an aim
to mitigate field inhomogeneities at UHF and reduce field of view (rFOV) for high
resolution acquisitions and improved parameter estimation. Our preliminary results showed uniform
and consistent contrast within the rFOV. We are currently working on the
implementation of the method for further experimental validations.
Introduction
Magnetic resonance fingerprinting (MRF) (1)
has been introduced as a rapid and quantitative MR imaging technique with many potential
clinical and research applications. While most studies focus on standard
clinical field strengths, the SNR advantages at ultra-high field (UHF) promise
to benefit MRF by supporting higher spatial resolutions. However, to mitigate
the B1+ inhomogeneity at UHF more complicated dictionary modeling is typically
required which includes the modeling of spatial varying transmit B1+ fields (2-4).
For body imaging such as prostate, a longer scan is also required due to the
need to encode the entire plane of excitation which limits acquisition
efficiency. In this study, we explore the possibility of using tailored 3D
selective parallel transmission (pTx) excitation pulses to address transmit B1+
issues as well as enable reduced field of view (rFOV) imaging which can reduce
scan time with the promise of improved parameter estimation at UHF. The
feasibility of the proposed methods is demonstrated through a simulation study,
with ongoing efforts to implement the methods to experimentally validate.Methods
- 3D RF Pulse Design
A 3D spherical spiral-in multi-shell k-space trajectory (5-7)
was designed to provide a 3D excitation pulse with a nominal field of
excitation (FOX) of 60 mm and a maximum gradient slew rate of 162 T/m/s
resulting in a 10.32 ms pulse. A flat B0 map and simulated B1+ maps at 7T from
an 8-channel transmit loop head coil was used for the pulse design following the
spatial domain method (8).
The RF pulses, gradients and k-space trajectory are shown in Figure 1, with the
simulated excitation profile of the pulse shown in Figure 2.
- MRF Sequence Design
We implemented the original TrueFISP based MR fingerprinting
design (1)
in the simulation. As proof of principle, 960 time points were used to simulate
the spin evolution. The flip angle and repetition time (TR) evolution are shown
in Figure 3. Due to the length of the 3D pulse, the resulting TR increased ranging
from 17.2 to 21.2 ms versus 10.5 to 14 ms in the original MRF paper (1).
However, when acquiring with an rFOV, the read-out duration can remain the same
for a higher resolution or reduced when maintaining the same resolution.
- Simulation Method
A 2mm isotropic numerical brain phantom (9)
with a field of view of 217x181x181mm3 was used in simulation. Rather
than imaging the nominal FOX of 60 mm, a slightly larger rFOV of 72 mm was spatially
encoded considering the transition band of the designed RF pulses. Relaxation
during RF excitation was simulated to account for the non-analytical nature of
the tailored RF pulses. Instead of using a specific 3D read-out trajectory, we
simplified the simulation by fully sampling the zoomed FOV with a different
orientation each time frame, allowing the outer volume signal resulting from
the imperfections of the RF pulses to be aliased into the FOX in a less
coherent manner. The read-out acquisition matrix was 36x36x36 with 2mm
isotropic resolution and images were reconstructed using NUFFT (10).
Dictionary matching was done using a conventional dot-product approach (1).Results
The reconstructed parametric maps including proton density,
off-resonance, T1 and T2 are shown in Figure 4.a – 4.d, respectively. The
quantification error maps of T1 and T2 are shown in Figure 4.e and 4.f. In
general, the maps show good and consistent contrast in gray matter, white
matter and CSF without aliasing artifacts. The uniformity within these tissues
also suggest effective mitigation of B1+ inhomogeneities. Quantification deviations
were mainly observed at the edge of the imaged rFOV, namely the transition band
of the pulse where the excitation profile degraded. Additionally, residual
errors in parameter estimation most likely come from the limited length of fingerprints
and discretization errors, both of which should resolve in actual measurements.Discussion
Our preliminary simulation results demonstrated the
potential of using pTx to achieve a reduced FOV for MR fingerprinting. While parallel
transmission has been primarily used to mitigate B0 and B1+ inhomogeneity at
UHF, it may also help reduce data acquisition and dictionary size for MRF and
thus improve the robustness of parameter estimation. With a reduced FOV,
additional acceleration factor or improved spatial resolution may possibly be
achieved. Despite the imperfections of the tailored pulses, the
pseudo-randomized acquisition and pattern matching reconstruction as used by
MRF may effectively mitigate such imperfections and therefore find useful
application at UHF especially for body imaging. However, additional strategies are
needed to tackle with lipid in the skull and torso, especially when considering
that the receive sensitivity profile may further enhance the contamination by
the outer volume signal, requiring more pulse optimization strategies to
improve the excitation fidelity and overall performance.Conclusion
We have demonstrated the potential of 3D inner volume MR fingerprinting
with parallel transmission. The proposed method is currently being implemented for
experimental validations in the brain and body.Acknowledgements
Apart from grant support from NIBIB P41 EB027061, the
authors would like to thank Seng-Wei Chieh for constructive discussion.
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