Variable flip angle 3D-GRASE for high resolution fMRI at 7 tesla.

Purpose: To evaluate the use of variable flip angle refocusing pulse trains in single-shot three-dimensional gradient and spin-echo (3D-GRASE) to reduce blurring and increase the spatial coverage for high spatial resolution T2 -weighted functional MRI at 7 Tesla.

Methods: Variable flip angle refocusing schemes in 3D-GRASE were calculated based on extended phase graph theory. The blurring along the slice (partition) direction was evaluated in simulations, as well as phantom and in vivo experiments. Furthermore, temporal stability and functional sensitivity at 0.8 mm isotropic resolution were assessed.

Results: Variable flip angle refocusing schemes yielded significantly reduced blurring compared with conventional refocusing schemes, with the full width at half maximum being approximately 30-40% narrower. Simultaneously, spatial coverage could be increased by 80%. The temporal signal-to-noise ratio was slightly reduced, but functional sensitivity was largely maintained due to increased functional contrast in the variable flip angle acquisitions. Signal-to-noise ratio and functional sensitivity were reduced more strongly in areas with insufficient radiofrequency transmission indicating higher sensitivity to experimental imperfections.

Conclusion: Variable flip angle refocusing schemes increase usability of 3D-GRASE for high-resolution functional MRI by reducing blurring and allowing increased spatial coverage. Magn Reson Med 76:897-904, 2016. © 2015 Wiley Periodicals, Inc.