Steady-state functional MRI using spoiled small-tip fast recovery imaging

Abstract

Purpose: To determine whether a recently proposed steadystate magnetic resonance imaging (MRI) sequence, "small-tip fast recovery" (STFR), can be used for functional brain imaging. Compared to existing functional MRI (fMRI) based on T2∗-contrast and long echo time, STFR has the potential for high-resolution imaging with reduced B0 artifacts such as geometric distortions, blurring, or local signal dropout. Methods: We used Monte Carlo Bloch simulations to calculate the voxel-averaged steady-state signal during rest and activation, for blood oxygen level dependent (BOLD) and STFR. STFR relies on a tailored "tip-up" radiofrequency pulse to align the spins with the longitudinal axis after each data readout segment, and here we performed proof-of-concept in vivo STFR fMRI experiments using a tip-up pulse tailored to a two-dimensional region-of-interest in motor cortex. Experiments were performed on multiple subjects to test reliability of the functional activation maps. Results: Bloch simulations predict a detectable functional signal that depends mainly on intravoxel dephasing, and only weakly on spin diffusion. STFR produces similar activation maps and signal change as BOLD in finger-tapping experiments, and shows reliability comparable to BOLD. Conclusion: STFR can produce functional contrast (even with short TE), and is a potential alternative to long-TE (T2∗) fMRI. The functional contrast arises primarily from the interaction between T2∗-like dephasing and the tailored tip-up pulse, and not from spin diffusion.