Rapid Geometry-Corrected Echo-Planar Diffusion Imaging at Ultrahigh Field: Fusing View Angle Tilting and Point-Spread Function Mapping.

Purpose: Severe geometric distortions induced by tissue susceptibility, water-fat chemical shift, and eddy currents pose a substantial obstacle in single-shot EPI, especially for high-resolution imaging at ultrahigh field. View angle tilting (VAT)-EPI can mitigate in-plane distortion. However, the accompanied strong image blurring prevented its widespread applications. On the other hand, point-spread function mapping (PSF)-EPI can correct distortion and blurring accurately but requires prolonged scan time. We present fused VAT-PSF-EPI and possibilities for acceleration.

Methods: MR signal equations were explicitly derived to quantify image blurring in VAT-EPI and the maximum acceleration capacity in VAT-PSF-EPI. To validate the theoretical prediction, phantom measurements with varying in-plane parallel imaging factors, slice thicknesses, and RF pulses were conducted at 7 Tesla. In addition, in vivo human brain scans were acquired with T and diffusion weighting to assess distortion and blurring correction.

Results: VAT can effectively suppress distortion, and the introduced image blurring is corrected through PSF encoding. Up to fourfold acceleration (only 5 shots) in VAT-PSF-EPI was achieved compared with standard PSF-EPI without VAT. VAT-induced signal loss was mitigated by adjusting the sequence parameters and EPI resolution. In vivo T -weighted EPI data with 1.4 mm resolution demonstrate immunity to water-fat chemical shift-induced distortion. Very high-spatial resolution diffusion-weighted EPI (0.7 × 0.7 × 2.8 mm and 1.2 mm ) demonstrates the immunity to eddy current-induced distortion.

Conclusion: VAT-PSF-EPI is a novel spin-echo EPI-based sequence for fast high-resolution diffusion imaging at ultrahigh field.