Assessing the spatial precision of SE and GE-BOLD contrast at 7 Tesla

Spin echo (SE) EPI offers an alternative to standard gradient echo (GE) EPI for functional MRI. SE-EPI offers improved spatial specificity, since signal changes originate from the microvasculature, but its lower functional sensitivity has limited the usage of this sequence in fMRI experiments. Differential fMRI paradigms, in which two closely matched stimulus conditions are used, can suppress the contribution from veins, thus also offering improved spatial specificity compared to conventional block or event-related designs with long "rest" periods. In this study, we employed a differential fMRI paradigm to stimulate bands of primary visual cortex with pre-defined widths by using visual stimuli comprised of complementary rings of contrast-reversing checkerboard patterns (8 Hz). This paradigm was used to investigate the spatial specificity of GE and SE-BOLD contrast at 7T. Results show that the contrast-to-noise ratio (CNR) is larger for GE-EPI data than for the SE-EPI data for band widths in the range 1.7-6.6 mm, however as the width of the band decreases the CNR for GE and SE sequences converges. These results suggest that when using a differential mapping paradigm, GE-BOLD contrast is better for studying functional features that are larger than ~1.5 mm in size.