High‐resolution fMRI using multislice partial k‐space GR‐EPI with cubic voxels

The premises of this work are: 1) the limit of spatial resolution in fMRI is determined by anatomy of the microcirculation; 2) because of cortical gray matter tortuosity, fMRI experiments should (in principle) be carried out using cubic voxels; and 3) the noise in fMRI experiments is dominated by low‐frequency BOLD fluctuations that are a consequence of spontaneous neuronal events and are pixel‐wise dependent. A new model is proposed for fMRI contrast which predicts that the contrast‐to‐noise ratio (CNR) tends to be independent of voxel dimensions (in the absence of partial voluming of activated tissue), TE, and scanner bandwidth. These predictions have been tested at 3 T, and results support the model. Scatter plots of fMRI signal intensities and low‐frequency fluctuations for activated pixels in a finger‐tapping paradigm demonstrated a linear relationship between signal and noise that was independent of TE. The R2 value was about 0.9 across eight subjects studied. The CNR tended to be constant across pixels within a subject but varied across subjects: CNR = 3.2 ± 1.0. fMRI statistics at 20‐ and 40‐ms TE values were indistinguishable, and TE values as short as 10 ms were used successfully. Robust fMRI data were obtained across all subjects using 1 × 1 × 1 mm3 cubic voxels with 10 contiguous slices, although 1.5 × 1.5 × 1.5 mm3 was found to be optimum. Magn Reson Med 46:114–125, 2001. © 2001 Wiley‐Liss, Inc.