  Purpose: To demonstrate an ultrashort echo time magnetic resonance
fingerprinting (UTE-MRF) method that can simultaneously quantify tissue
relaxometries for muscle and bone in musculoskeletal systems and tissue
components in brain and therefore can synthesize pseudo-CT images.
  Methods: A FISP-MRF sequence with half pulse excitation and half spoke radial
acquisition was designed to sample fast T2 decay signals. Sinusoidal echo time
(TE) pattern was applied to enhance MRF sensitivity for tissues with short and
ultrashort T2 values. The performance of UTE-MRF was evaluated via simulations,
phantoms, and in vivo experiments.
  Results: A minimal TE of 0.05 ms was achieved in UTE-MRF. Simulations
indicated that extension of TE sampling increased T2 quantification accuracy in
cortical bone and tendon, and had little impact on long T2 muscle
quantifications. For a rubber phantom, an average T1/T2 of 162/1.07 ms from
UTE-MRF were compared well with gold standard T2 of 190 ms from IR-UTE and T2*
of 1.03 ms from UTE sequence. For a long T2 agarose phantom, the linear
regression slope between UTE-MRF and gold standard was 1.07 (R2=0.991) for T1
and 1.04 (R2=0.994) for T2. In vivo experiments showed the detection of
cortical bone and Achilles tendon, where the averaged T2 was respectively 1.0
ms and 15 ms. Scalp images were in good agreement with CT.
  Conclusion: UTE-MRF with sinusoidal TE variations shows its capability to
produce pseudo-CT images and simultaneously output T1, T2, proton density, and
B0 maps for tissues with long T2 and short/ultrashort T2 in the brain and
musculoskeletal system.
