Gradient-echo and spin-echo blood oxygenation level-dependent functional MRI at ultrahigh fields of 9.4 and 15.2...

Sensitivity and specificity of blood oxygenation level-dependent (BOLD) functional MRI (fMRI) is sensitive to magnetic field strength and acquisition methods. We have investigated gradient-echo (GE)- and spin-echo (SE)-BOLD fMRI at ultrahigh fields of 9.4 and 15.2  Tesla.

BOLD fMRI experiments responding to forepaw stimulation were performed with 3 echo times (TE) at each echo type and B₀ in α-chloralose-anesthetized rats. The contralateral forelimb somatosensory region was selected for quantitative analyses.

At 9.4 T and 15.2 T, average baseline T₂^* (n = 9) was 26.6 and 17.1 msec, whereas baseline T₂ value (n = 9) was 35.7 and 24.5 msec, respectively. Averaged stimulation-induced ΔR₂^* was -1.72 s^-1 at 9.4 T and -3.09 s^-1 at 15.2 T, whereas ΔR₂ was -1.19 s^-1 at 9.4 T and -1.97 s^-1 at 15.2 T. At the optimal TE of tissue T₂^* or T₂ , BOLD percent changes were slightly higher at 15.2 T than at 9.4 T (GE: 7.4% versus 6.4% and SE: 5.7% versus 5.4%). The ΔR₂^* and ΔR₂ ratio of 15.2 T to 9.4 T was 1.8 and 1.66, respectively. The ratio of the macrovessel-containing superficial to microvessel-dominant parenchymal BOLD signal was 1.73 to 1.76 for GE-BOLD versus 1.13 to 1.19 for SE-BOLD, indicating that the SE-BOLD contrast is less sensitive to macrovessels than GE-BOLD.

SE-BOLD fMRI improves spatial specificity to microvessels compared to GE-BOLD at both fields. BOLD sensitivity is similar at the both fields and can be improved at ultrahigh fields only for thermal-noise-dominant ultrahigh-resolution fMRI.