Highly accelerated submillimeter resolution 3D GRASE with controlled T2 blurring in T2 -weighted functional MRI at 7 Tesla: A feasibility study

Abstract

Purpose: To achieve highly accelerated submillimeter resolution $T_2$ -weighted functional MRI at 7T by developing a three-dimensional gradient and spin echo imaging (GRASE) with inner-volume selection and variable flip angles (VFA).

Methods: GRASE imaging has disadvantages in that (a) k-space modulation causes $T_2$ blurring by limiting the number of slices and (b) a VFA scheme results in partial success with substantial SNR loss. In this work, accelerated GRASE with controlled $T_2$ blurring is developed to improve a point spread function (PSF) and temporal signal-to-noise ratio (tSNR) with a large number of slices. To this end, the VFA scheme is designed by minimizing a trade-off between SNR and blurring for functional sensitivity, and a new GRASE-optimized random encoding, which takes into account the complex signal decays of $T_2$ and $T_2^{\ast }$ weightings, is proposed by achieving incoherent aliasing for constrained reconstruction. Numerical and experimental studies were performed to validate the effectiveness of the proposed method over regular and VFA GRASE (R- and V-GRASE).

Results: The proposed method, while achieving 0.8 mm isotropic resolution, functional MRI compared to R- and V-GRASE improves the spatial extent of the excited volume up to 36 slices with 52%-68% full width at half maximum (FWHM) reduction in PSF but approximately 2- to 3-fold mean tSNR improvement, thus resulting in higher BOLD activations.

Conclusions: We successfully demonstrated the feasibility of the proposed method in $T_2$ -weighted functional MRI. The proposed method is especially promising for cortical layer-specific functional MRI.

Keywords: GRASE imaging; functional MRI; random sampling; variable flip angles.