  Balanced steady-state free precession (bSSFP) can be used as an alternative
to gradient-echo (GE) EPI for BOLD functional MRI when image distortions and
signal drop-outs are severe such as at ultra-high field. However, 3D-bSSFP
acquisitions have distinct drawbacks on either human or animal MR systems. On
clinical scanners, 3D imaging is suboptimal for localized fMRI applications,
and also results in distortions, blurring, and increased sensitivity to motion
or physiological noise. On pre-clinical systems, 3D acquisitions have low
temporal resolution due to limited acceleration options, while single slice
offers insufficient coverage. The aim of the present study was to implement a
multi-slice bSSFP acquisition with Cartesian read-out to obtain non-distorted
BOLD fMRI activation maps in the human and rat brain at ultra-high field. We
show that the bSSFP signal characteristics are preserved in a new
pseudo-steady-state. In the human brain at 7 Tesla, we demonstrate that both
task- and resting-state fMRI can be performed with 2D-bSSFP, with a temporal
SNR that matches that of 3D-bSSFP, resulting in - at least - equal performance.
In the rat brain at 14 Tesla, we show that the multi-slice bSSFP protocol has
similar sensitivity to gradient-echo EPI for task fMRI, while benefitting from
much reduced distortions and drop-outs. The advantages of passband bSSFP at 14
Tesla in comparison with GE-EPI are expected to be even more marked for mouse
fMRI.
