Steady-state functional MRI using small-tip fast recovery ( STFR ) imaging

Introduction: The majority of functional brain MR imaging studies use T2*-weighted gradientecho sequences with single-shot readout (BOLD fMRI), which can provide high activation contrast but suffers from signal loss near air/tissue interfaces and off-resonance-induced image artifacts (distortions or blurring). Steady-state fMRI based on balanced steady-state free precession (bSSFP) uses segmented readouts and can produce excellent image quality, but is susceptible to dark “banding” artifacts in regions of high B0 inhomogeneity. Small-tip fast recovery (STFR) imaging is a recently-proposed steady-state imaging sequence that is a potential alternative to bSSFP [1,2]. STFR can provide bSSFP-like image contrast, but with reduced signal variations due to resonance offsets. STFR relies on a tailored "tip-up," or "fast recovery," RF pulse to align the spins with the longitudinal axis after each data readout segment (Fig. 1(a)). The design of the tip-up pulse is based on the acquisition of a separate off-resonance (B0) map. However, it is not yet known whether STFR is suitable for fMRI, and whether the functional contrast mechanism is the same as in passband bSSFP. Here we investigate the use of STFR for steady-state fMRI, using Monte Carlo Bloch simulations and experimental observations. Our simulations indicate that the functional contrast mechanisms for bSSFP and STFR are different: Whereas contrast in bSSFP fMRI is diffusion-driven, functional contrast in STFR fMRI arises primarily from the spectral properties of the steady-state signal profile (see Fig. 1(b)). Our simulations also predict that STFR fMRI produces enhanced activation contrast compared to bSSFP fMRI, and we present experimental evidence for this in visual cortex.