  Purpose: To improve image quality and accelerate the acquisition of 3D MRF.
Methods: Building on the multi-axis spiral-projection MRF technique, a subspace
reconstruction with locally low rank (LLR) constraint and a modified
spiral-projection spatiotemporal encoding scheme termed
tiny-golden-angle-shuffling (TGAS) were implemented for rapid whole-brain
high-resolution quantitative mapping. The LLR regularization parameter and the
number of subspace bases were tuned using retrospective in-vivo data and
simulated examinations, respectively. B0 inhomogeneity correction using
multi-frequency interpolation was incorporated into the subspace reconstruction
to further improve the image quality by mitigating blurring caused by
off-resonance effect. Results: The proposed MRF acquisition and reconstruction
framework can produce provide high quality 1-mm isotropic whole-brain
quantitative maps in a total acquisition time of 1 minute 55 seconds, with
higher-quality results than ones obtained from the previous approach in 6
minutes. The comparison of quantitative results indicates that neither the
subspace reconstruction nor the TGAS trajectory induce bias for T1 and T2
mapping. High quality whole-brain MRF data were also obtained at 0.66-mm
isotropic resolution in 4 minutes using the proposed technique, where the
increased resolution was shown to improve visualization of subtle brain
structures. Conclusion: The proposed TGAS-SPI-MRF with optimized
spiral-projection trajectory and subspace reconstruction can enable
high-resolution quantitative mapping with faster acquisition speed.
