Sensitivity and specificity of high-resolution balanced steady-state free precession fMRI at high field of 9.4 T

Balanced steady-state free precession (bSSFP) is an attractive fMRI method at high fields due to minimal spatial distortion. To examine sensitivity and specificity of bSSFP fMRI at ultrahigh magnetic field of 9.4 T, we performed high-resolution pass-band high flip-angle (16°) bSSFP fMRI with four phase cycling (PC) angles at two repetition times (TR) of 10 ms and 20 ms and conventional gradient-recalled-echo (GRE) fMRI with TR of 20 ms on rat brain during forepaw stimulation. The sensitivity of bSSFP fMRI with TR of 20 ms was higher than that of GRE fMRI regardless of PC angle. Because of magnetic field inhomogeneity, fMRI foci were changed with PC angle in bSSFP fMRI, which was more prominent when TR was shorter. Within a middle cortical layer region where magnetic field inhomogeneity was relatively small, the homogeneity of bSSFP fMRI signals was higher at shorter TR. Acquisition of baseline transition-band bSSFP images helped to identify pass- and transition-band regions and to understand corresponding bSSFP fMRI signals. Fourier analysis of the multiple PC bSSFP datasets provided echoes of multiple pathways separately, and the main echo component showed lower sensitivity and better homogeneity than the free induction decay component. In summary, pass-band bSSFP techniques would have advantages over GRE-based fMRI in terms of sensitivity, and may be a good choice for fMRI at ultrahigh fields.

Research highlights
► bSSFP fMRI was performed with two TRs and four phase cycling angles at 9.4 T.
► Spatial specificity of bSSFP fMRI is changed with phase cycling angle.
► The homogeneity of bSSFP fMRI in the middle of cortex was higher at shorter TR.
► Sensitivity of bSSFP fMRI is higher than GRE fMRI with the same TE/TR.