Introduction

High spatial resolution functional MRI(fMRI) allows the investigation of mesoscopic cortical functional units, such as columns and laminae. For this, both signal-to-noise ratio(SNR) and the spatial specificity of the signal are critical. Generally, the gradient-echo(GE)-blood oxygenation level-dependent(BOLD) signal is highest at the surface of the cortex due to the high density of draining veins^1-2, which is distal to neuronal activation. On the other hand, the spin-echo(SE)-BOLD signal is expected to be localized within gray matter with higher spatial accuracy³. However, the SE-BOLD signal is less sensitive than the GE-BOLD signal4. In this work, we applied a filter designed based on the ΔR₂^*/ΔR₂-ratio to enhance both the sensitivity and specificity of the BOLD signal using spin- and gradient-echo(SAGE)-Echo planar imaging (EPI) sequence^4-5. fMRI experiments during fist-clenching with touching with 0.8mm isotropic resolution were performed and we directly compared the layer profile of SAGE-BOLD with that of GE- and SE-BOLD in the primary sensory and motor cortices. Vessel-size tuned SAGE-BOLD-fMRI provided enhanced laminar specificity and sensitivity in the gray matter region, making it an efficient tool for high spatial resolution fMRI studies to resolve mesoscopic functional units.

Methods

Six subjects participated in this study. All procedures followed the guidelines of the IRB of Sungkyunkwan University, South Korea. All measurements were performed on a 7T-scanner (MAGNETOM-Terra, Siemens-Healthineers), equipped with a 32-channel head-coil (NOVA-Medical). The SAGE-BOLD signal was calculated as S_GASE=S_GE^α×S_SE, where α is a filter defined as a sigmoid function α=−0.5×tanh(0.3×(ΔR₂^*/ΔR₂−10))+0.5 to enhance micro-vasculature while suppressing macro-vessels. α is close to one for ΔR₂^*/ΔR₂<<10, 0.5 for ΔR₂^*/ΔR₂=10, and close to zero for ΔR₂^*/ΔR₂>>10. The subjects performed fMRI experiments using a 3.8-min unilateral fist-clenching with touching stimulation paradigm (initial 20-s resting and 8-blocks of alternating 6-s clenching and 20-s resting) to demonstrate enhanced sensitivity and specificity in the micro-vascular region of SAGE-BOLD signal at 0.8mm isotropic resolution. The imaging parameters for multi-shot⁶ SAGE-EPI were as follows: 0.8mm³ isotropic resolution, Rin-plane=9 (for each shot; effective Rin-plane=3 by 3-shots), FOV=120×120 mm², 24slices, flip-angle-pair=50°-180°, partial-Fourier=6/8, TR=2000 ms, and TEGE/TESE=18/58ms. fMRI experiments were repeated 10-times to ensure sufficient sensitivity. Anatomical images were acquired using a FLASH sequence with the same imaging parameters as the SAGE-EPI except for TEs=3.3,6.3, and 20ms, and the flip-angle was 50°. The boundary between gray matter and CSF was delineated and a B0-field map for distortion correction of the EPI images was obtained.

Results

In Fig 1, z-score maps of GE-, SE-, and SAGE-BOLD signals were compared in one representative slice in each subject. Anatomical images obtained from FLASH sequences (the 1st row) with a TE of 3.0 ms clearly delineated CSF, gray matter, and white matter. Layers for M1 and S1 were manually drawn and overlaid onto SE-EPI images (the 2nd row). The anterior and posterior gray matter bank of the central sulcus corresponds to M1 and S1, respectively. For the case of GE-BOLD signal (the 3rd row), the z-score values peaked at or outside the cortical surface near high-intensity CSF areas. However, for the case of SAGE-BOLD signal (the 5th row), without significant activation near the CSF areas, M1 and S1 activation areas were also clearly distinguished and showed higher z-score values than SE-BOLD (the 4th row). Similar differences between GE-BOLD and SAGE-BOLD signal were consistently observed across all participants. In Fig 2A, scatter plot between SE-BOLD and GE-BOLD percent signal changes was plotted for two groups, macro-vessel area (ΔR₂^*/ΔR₂ > 10) and micro-vasculature area (ΔR₂^*/ΔR₂ ≤ 10). Micro-vasculature area (filled blue circles) showed linear relationship between SE-BOLD and GE-BOLD. However, macro-vessel area (red open circles) showed supra-linear relationship, which means GE-BOLD is more sensitive to macro-vessel than SE-BOLD. When α filter was applied (Fig 2B), percent signal changes of SAGE-BOLD were similar to SE BOLD in macro-vasculature (reduced significantly compared to GE BOLD) but were greatly enhanced in micro-vasculature area. Next, Fig 3 shows z-score values which reflect the sensitivity of various BOLD signals. SAGE-BOLD improves CNR only in micro-vasculature areas (ΔR₂^*/ΔR₂ ≤ 10), resulting in the enhanced sensitivity and specificity. Finally, z-score levels of GE-, SE-, and SAGE-BOLD were obtained along the cortical depth profiles by averaging signals from five slices in each subject and plotted in Fig 4. The functional response of GE-BOLD was highest at the cortical surface, where the concentration of draining vessels is high, and decreased monotonically with increasing cortical depth. On the other hand, for the case of SAGE-BOLD, z-score level was reduced at the cortical surface and peaked in the gray matter for both M1 and S1, which is consistent with SE-BOLD behavior with improved sensitivity.

Discussion and Conclusion

We proposed and demonstrated the feasibility of SAGE-EPI sequence at 7T to achieve better sensitivity and specificity than SE-BOLD at 0.8-mm in-plane resolution for laminar fMRI. Experimental results confirmed that SAGE-BOLD with micro-vasculature-tuned filter has higher CNR than conventional SE-BOLD. Different filter functions can be applied for obtaining SAGE-BOLD to tune different vessel sizes. SAGE-BOLD may play an important role for investigating mesoscopic cortical-circuits in the human brain with high specificity and sensitivity.

Acknowledgements

This work was supported by the Institute of Basic Science under grant IBS-R015-D1.

References

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