Improved Selectivity in 7 T Digit Mapping Using VASO-CBV

Abstract

Functional magnetic resonance imaging (fMRI) at Ultra-high field (UHF, ≥ 7 T) benefits from significant gains in the BOLD contrast-to-noise ratio (CNR) and temporal signal-to-noise ratio (tSNR) compared to conventional field strengths (3 T). Although these improvements enabled researchers to study the human brain to unprecedented spatial resolution, the blood pooling effect reduces the spatial specificity of the widely-used gradient-echo BOLD acquisitions. In this context, vascular space occupancy (VASO-CBV) imaging may be advantageous since it is proposed to have a higher spatial specificity than BOLD. We hypothesized that the assumed higher specificity of VASO-CBV imaging would translate to reduced overlap in fine-scale digit representation maps compared to BOLD-based digit maps. We used sub-millimeter resolution VASO fMRI at 7 T to map VASO-CBV and BOLD responses simultaneously in the motor and somatosensory cortices during individual finger movement tasks. We assessed the cortical overlap in different ways, first by calculating similarity coefficient metrics (DICE and Jaccard) and second by calculating selectivity measures. In addition, we demonstrate a consistent topographical organization of the targeted digit representations (thumb-index-little finger) in the motor areas. We show that the VASO-CBV responses yielded less overlap between the digit clusters than BOLD, and other selectivity measures were higher for VASO-CBV too. In summary, these results were consistent across metrics and participants, confirming the higher spatial specificity of VASO-CBV compared to BOLD.

Keywords: 7 T; BOLD; Digit; High-resolution; Selectivity; Somatotopic; Specificity; VASO; Vascular space occupancy.

Fig. 1

Overview of the acquisition method and the topographic digit mapping of a subset of fingers (D1, D2, and D5) using VASO-CBV and BOLD in the 6 participants. A We carefully positioned the imaging slab (0.79 × 0.79 × 1.5 mm³) to cover the left primary sensorimotor area. B The participants performed individual finger movement on a block-designed task, using thumb (blue, D1), index (green, D2), and little (red, D5). C Each voxel was assigned to a single digit using a winner-take-all algorithm, creating a subset of digit representation maps. The digit representations are orderly organized (thumb-index-little) along the central sulcus, predominantly in S1. No smoothing has been applied on VASO-CBV or BOLD data

Fig. 2

DICE similarity coefficients (DSC) for each pair-digit cluster in S1 and M1. BOLD yielded higher scores than VASO-CBV (Paired t-test, p < 0.05) in both regions. A higher similarity score represents a higher overlap between each digit pair. The error bar is the standard deviation. Please see Supplementary Fig. 3 for Jaccard measurements

Fig. 3

Average z-scores of the dominant finger (columns) in relation to the average z-score of the other voxels (digit voxels). The lines between each digit cluster denote each participant's average z-score. We found similar results for M1 and S1, with higher z-scores for the non-dominant digit voxels in the BOLD data than VASO-CBV data

Fig. 4

Response selectivity indices for the VASO-CBV and BOLD responses in M1 and S1. A Overall selectivity, and B Digit selectivity with D1 (thumb), D2 (index), and D5 (little) according to Eqs. 1 and 2 (see Methods). Both selectivity indices describe the magnitude of the fMRI response to the movement of a specific digit relative to the motion of other digits. For the overall selectivity, VASO-CBV showed higher cortical selectivity for all participants (lines denote each participant result) and both M1 and S1. The digit selectivity indices are also higher for VASO-CBV for all participants in both M1 and S1 when compared with BOLD