Whole brain, high resolution multiband spin-echo EPI fMRI at 7T: A comparison with gradient-echo EPI using a color-word Stroop task

Abstract

A whole brain, multiband spin-echo (SE) echo planar imaging (EPI) sequence employing a high spatial (1.5 mm isotropic) and temporal (TR of 2 s) resolution was implemented at 7 Tesla. Its overall performance (tSNR, sensitivity and CNR) was assessed and compared to a geometrically matched gradient-echo (GE) EPI multiband sequence (TR of 1.4 s) using a colour-word Stroop task. PINS RF pulses were used for refocusing to reduce RF amplitude requirements and SAR, summed and phaseoptimized standard pulses were used for excitation enabling a transverse or oblique slice orientation. The distortions were minimized with the use of parallel imaging in the phase encoding direction and a post-acquisition distortion correction. In general, GE-EPI shows higher efficiency and higher CNR in most brain areas except in some parts of the visual cortex and superior frontal pole at both the group and individual-subject levels. Gradient-echo EPI was able to detect robust activation near the air/tissue interfaces such as the orbito-frontal and subcortical regions due to reduced intra-voxel dephasing because of the thin slices used and high in-plane resolution.

How to choose the right MR sequence for your research question at 7 T and above?

“…This was made possible using low SAR PINS pulses to acquire whole brain spin echo data at 7T at high spatial resolution (~ 1.5mm isotropic) and adequate temporal resolution (TR=1.9s) (Koopmans et al, 2012). However in a head-to-head comparison of spin echo versus gradient echo contrast in an activation study, the results were disappointing in the sense that gradient echo was consistently superior, even in regions where strong through-slice inhomogeneity gradients were to be expected (Boyacioglu et al, 2014). Results obtained at 9.4 T have also cast doubt on the hypothesis that only dynamic averaging in tissue compartments are contributing to the spinecho BOLD contrast (Budde et al, 2014) as the signal contribution from the macrovasculature was reduced rather than eliminated in spin-echo fMRI.…”

A Hitchhiker's Guide to Functional Magnetic Resonance Imaging

“…When compared to spin-echo EPI, gradient echo acquisitions have higher BOLD sensitivity, imaging speed and versatility, and have been used in the majority of fMRI studies. On the other hand, spin-echo sequences have been proposed as a viable alternative when the goal is to obtain increased functional localization in the capillary bed (especially at high fields) and when specific regions of interest (ROIs) are less superficial regions such as for example the ventromedial frontal and anterior inferior temporal cortex are the primary focus of the study (Norris, 2012; Boyacioğlu et al, 2014; Halai et al, 2014; Chiacchiaretta and Ferretti, 2015). …”

Uses, misuses, new uses and fundamental limitations of magnetic resonance imaging in cognitive science

“…Gradient-echo (GE) BOLD changes are maximal at the cortical surface, and may even be detected in pial veins several millimetres downstream from the active grey matter. To minimize this problem, some researchers advocate spin-echo BOLD, but its sensitivity is much lower than GE-BOLD, even at 7 T [101,102]; and even with spin-echo acquisition, much BOLD signal arises from principal intracortical veins [103,104]. Because most of the signal arises from larger venules and surface veins, the effective GE-BOLD resolution in the plane of the cortex cannot be better than the spacing of principal intracortical veins (about 0.7 mm) [10,105,106].…”