Spherical echo-planar time-resolved imaging (sEPTI) for rapid 3D quantitative T 2 * and susceptibility imaging

Abstract

Purpose: To develop a 3D spherical EPTI (sEPTI) acquisition and a comprehensive reconstruction pipeline for rapid high-quality whole-brain submillimeter $T_2^{*}$ and QSM quantification.

Methods: For the sEPTI acquisition, spherical k-space coverage is utilized with variable echo-spacing and maximum k_x ramp-sampling to improve efficiency and signal incoherency compared to existing EPTI approaches. For reconstruction, an iterative rank-shrinking B₀ estimation and odd-even high-order phase correction algorithms were incorporated into the reconstruction to better mitigate artifacts from field imperfections. A physics-informed unrolled network was utilized to boost the SNR, where 1-mm and 0.75-mm isotropic whole-brain imaging were performed in 45 and 90 s at 3 T, respectively. These protocols were validated through simulations, phantom, and in vivo experiments. Ten healthy subjects were recruited to provide sufficient data for the unrolled network. The entire pipeline was validated on additional five healthy subjects where different EPTI sampling approaches were compared. Two additional pediatric patients with epilepsy were recruited to demonstrate the generalizability of the unrolled reconstruction.

Results: sEPTI achieved 1.4 $\times$ faster imaging with improved image quality and quantitative map precision compared to existing EPTI approaches. The B₀ update and the phase correction provide improved reconstruction performance with lower artifacts. The unrolled network boosted the SNR, achieving high-quality $T_2^{*}$ and QSM quantification with single average data. High-quality reconstruction was also obtained in the pediatric patients using this network.

Conclusion: sEPTI achieved whole-brain distortion-free multi-echo imaging and $T_2^{*}$ and QSM quantification at 0.75 mm in 90 s which has the potential to be useful for wide clinical applications.

Keywords: eddy current phase correction; field inhomogeneity estimation; spherical EPTI (sEPTI); submillimeter neuroimaging; ultrafast T 2 * $$ {\mathrm{T}}_2^{\ast } $$ mapping and quantitative susceptibility mapping (QSM).