  Purpose: To rapidly obtain high isotropic-resolution T2 maps with whole-brain
coverage and high geometric fidelity.
  Methods: A T2 blip-up/down echo planar imaging (EPI) acquisition with
generalized Slice-dithered enhanced resolution (T2-BUDA-gSlider) is proposed. A
radiofrequency (RF)-encoded multi-slab spin-echo EPI acquisition with multiple
echo times (TEs) was developed to obtain high SNR efficiency with reduced
repetition time (TR). This was combined with an interleaved 2-shot EPI
acquisition using blip-up/down phase encoding. An estimated field map was
incorporated into the joint multi-shot EPI reconstruction with a structured low
rank constraint to achieve distortion-free and robust reconstruction for each
slab without navigation. A Bloch simulated subspace model was integrated into
gSlider reconstruction and utilized for T2 quantification.
  Results: In vivo results demonstrated that the T2 values estimated by the
proposed method were consistent with gold standard spin-echo acquisition.
Compared to the reference 3D fast spin echo (FSE) images, distortion caused by
off-resonance and eddy current effects were effectively mitigated.
  Conclusion: BUDA-gSlider SE-EPI acquisition and gSlider-subspace joint
reconstruction enabled distortion-free whole-brain T2 mapping in 2 min at ~1
mm3 isotropic resolution, which could bring significant benefits to related
clinical and neuroscience applications.
