Ding Xia^{1,2}, Zidan Yu^{1,2,3}, Riccardo Lattanzi^{1,2,3}, and Martijn A Cloos^{1,2}

^{1}Center for Advanced Imaging Innovation and Research, Department of Radiology, New York University School of Medicine, New York, NY, United States, ^{2}Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, NY, United States, ^{3}Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, NY, United States

We
evaluated the ability of three reported MR fingerprinting methods to
mitigate
the effect
of B_{1}^{+} inhomogeneity
at 7T. Results from each method were compared with gold
standard results. All methods provided relatively accurate T1 quantification. We
show that T2 cannot be accurately quantified at 7T without accounting for B_{1}^{+}
in the MR fingerprinting dictionary. The Inversion-Recovery-FISP-FLASH
(IRFF) method provided the most accurate T2 values. We conclude that the use of
both FISP and FLASH segments best encodes B_{1}^{+} into the
fingerprint.

Experiments were performed on a 7T MRI system (Magnetom, Siemens) with a 1-channel transmit/32-channel receive head coil (Nova Medical). The phantom consisted of 7 glass tubes with different T1/T2 values immersed in saline solution. The three MRF sequences were tested, with 6 and 10 radial spokes per time-point, for a 5mm slice (256x256, 0.5x0.5mm

Parameter maps were reconstructed with an algorithm

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Figure 1: a.
adiabatic
inversion (IR) followed by a 2D golden-angle radial readout, b. flip angle train of MRF-FISP, c. TR train of MRF-FISP, d.
Sawtooth-B1
flip angle train, e. Zoom in of "sawtooth" profile of flip angles, f. flip angle train of IRFF

Figure 2: Results (T1, T2 and B_{1}^{+ }maps) from all methods with
6
radial spokes per timepoint,
including
MRF-FISP
w/o B1,
MRF-FISP
w/ B1,
MRF-FISP
ext B1,
Sawtooth-B1
and IRFF. Plus the T1/T2 maps estimated via
gold standard (GS).

Figure 3:
Median and standard deviation of each ROI from T1/T2 maps
estimated via three MRF methods and gold standard (GS).

Figure 4: T2 and B_{1}^{+} maps of Sawtooth-B1
and IRFF with
10 radial spokes per timepoint, and T2 maps of gold standard (GS).