Sagittal T1-weighted image, from which the functional bSSFP imaging slices parallel with the calcarine sulcus were prescribed.
Magnified T1-weighted image showing the selection of region of interest near the occipital lobe (white square; 17.2 mm in width and within the targeted shimming area), from which the nonactivated voxels (absolute value of T score <2.39) were included for temporal noise analysis.
Analysis results from 15 sets of bSSFP fMRI with frequency shifts of −8.3 Hz (a), 0 Hz (b), and 8.3 Hz (c), respectively, to provide a broader coverage of the blood oxygenation sensitivity. For image sets with same frequency setting, activation pattern agreed nicely although spatial resolution differed. Note that with 64 or 128 matrix size, where the in-plane pixel width of 3.4 or 1.7 mm is close to that used in conventional fMRI based on echo-planar imaging, the activation pattern is rather coarse compared with the overlaying T1-weighted image. As the spatial resolution increases, the activated areas become better and delicately aligned with the gray matter borders.
Activation-time curves in three fMRI experiments with OFF-ON checkerboard visual stimulation. The signals were normalized with respect to the baseline signals as measured from the primary visual cortex area. All experiments demonstrated the trend of increased activation signals at higher spatial resolution, likely reflecting the high specificity of activation pixels via a reduction of partial volume effects.
The trends of increase in the number of activation voxels and decrease of activation volume, respectively, at higher spatial resolution. Error bars represent standard deviations from the four experiments.
(a) Temporal signal-to-noise ratio (tSNR) calculated for the voxels in the occipital lobe shows that tSNR decreases as spatial resolution increases. (b) Percentage functional signal changes calculated at 0 Hz offset frequency for the four experiments shows increased functional signal changes as image resolution becomes higher, despite of the tSNR reductions. Differences in functional signal changes are statistically significant with p < 0.05 between any two matrix sizes, except between 384 and 512 matrix sizes (student t-test). (c) The reduction in tSNR and increase in functional signal changes at higher spatial resolution result in nearly constant functional contrast-to-noise ratio.
Another demonstration of spatial resolution effects on fMRI, with experimental results obtained using alternating unilateral visual field stimulus. Shown in the figures are the functional activation maps from the experiment trial at an offset frequency of 7 Hz. Activation maps at voxel size of 0.75 × 0.75 × 3 mm3 and 3 × 3 × 3 mm3 are overlaid on T1-weighted images in (a) and (b), respectively. Both results reveal the mapping of left-visual field and right-visual field nicely to the contralateral right and left hemispheres, respectively. Note that the activation signals at the ipsilateral side of the visual stimulus align nicely with gray matter sulci shown on the T1-weighted image in (a), which would have possibly been mistaken as noise at low resolution (b) (white arrows).
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