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

Abstract

Purpose: 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.

Methods: 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.

Results: 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.

Conclusion: 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.

Keywords: BOLD; fMRI; gradient-echo; spin-echo; ultrahigh field.

Figure 1

(A) Experimental protocol of 9.4 T and 15.2 T. Three TE GE‐BOLD runs and 3 TE SE‐BOLD runs were performed sequentially. The protocol was repeated 3 to 8 times. Each BOLD run was separated by a rest of 1 minute. (B) Statistical maps with GE‐ (TE = 20 msec) and SE‐BOLD (TE = 40 msec) overlaid on corresponding BOLD images at 9.4 T. Color bar represents the cross‐correlation values. (C) Averaged CCC map from 2 selected slices of GE‐ and SE‐BOLD overlaid on averaged BOLD images. (D) Selected 9 voxels by 2 slices. The superficial ROI was selected as a region with 3 superficial voxels by 2 slices (red), and the parenchyma ROI was selected as a region with 6 inner voxels by 2 slices (blue)

Figure 2

Statistical activation maps from 3 TE GE‐ and SE‐BOLD overlaid on the corresponding EPI images at 9.4 T (A) and 15.2 T (B). Color bar represents percent changes

Figure 3

(A), (B) GE‐BOLD fMRI time courses obtained from the S1FL ROI (n = 9) for TE of 15 msec (blue), 20 msec (green), and 30 msec (red) at 9.4 T and 15.2 T, respectively. (C) SE‐BOLD fMRI time courses obtained from the S1FL ROI (n = 9) for TE of 30 msec (blue), 40 msec (green), and 50 msec (red) at 9.4 T. (D) SE‐BOLD fMRI time courses obtained from the S1FL ROI (n = 9) for TE of 20 msec (blue), 30 msec (green), and 40 msec (red) at 15.2 T. Activation duration was defined as the period of 6 to 20 seconds after the onset of stimulation at time = 40 seconds (black horizontal bar), and poststimulus duration was defined as the period of 10 to 30 seconds after the offset of stimulation at time = 60 seconds (red horizontal bar). (E) TE‐dependent BOLD percent signal changes for GE (red) and SE (blue) at 9.4 T (filled circle) and 15.2 T (open circle). Fitted lines by a linear function were overlapped with solid lines (9.4 T) and dashed lines (15.2 T)

Figure 4

TE‐dependent GE‐ (solid line) and SE‐BOLD (dashed line) percent signal changes for the superficial ROI (red) and parenchyma ROI (blue) at 9.4 T (A) and 15.2 T (B), respectively