Quantitative Evaluations of Geometrical Distortion Corrections in Cortical Surface-Based Analysis of High-Resolution Functional MRI Data at 7T

Abstract

Background: Although 7T functional MRI (fMRI) provides better signal-to-noise ratio and higher spatial resolution than 3T fMRI, geometric distortions become more challenging because fMRI is more susceptible to distortions than structural MRI. Accurate alignment of 7T fMRI to structural MRI data is critical for precise cortical surface-based analysis.

Purpose: To quantify the effectiveness of distortion corrections of 7T fMRI data.

Study type: Prospective.

Subjects: Fifteen healthy individuals aged 19-26 years (mean: 21.9 years).

Field strength/sequence: Multiband gradient-echo echo-planar imaging sequence at 7T; 3D T₁ /T₂ -weighted sequences (magnetization prepared rapid acquisition with gradient echo [MPRAGE] and sampling perfection with application optimized contrast using different flip angle evolution [SPACE]) at 3T.

Assessment: fMRI data at 7T were registered to cortical surfaces reconstructed from 3T structural data acquired in the same subjects. Distortions induced by B₀ inhomogeneity and gradient nonlinearity (B₀ and gradient distortions) were evaluated as cortical fallout (misregistration of noncortical areas) and displacement (misregistration along gray matter).

Statistical tests: Repeated measures analyses of variance with post-hoc t-tests with Bonferroni correction.

Results: The accuracy of fully corrected fMRI images based on the intensity distribution was 89.2%. Without any corrections, 9.7% of vertices in the whole surfaces were fallout and the average displacement was 0.96 mm for the rest of the vertices. B₀ and gradient distortion corrections significantly reduced the fallout (to 2.1% and 8.7%) and displacement (to 0.29 mm and 0.86 mm). These corrections were effective even around regions with moderate distortions (the somatosensory and visual cortices for B₀ distortion, and the anterior frontal, inferior temporal, and posterior occipital cortices for gradient distortion).

Data conclusion: B₀ distortion correction is crucial for surface-based analysis of fine-resolution fMRI at 7T. Gradient distortion correction should be considered when regions of interest include regions distant from the isocenter of scanners.

Evidence level: 1 TECHNICAL EFFICACY STAGE: 1.

Keywords: B0 inhomogeneity; cortical surface-based analysis; geometrical distortion; gradient nonlinearity; ultra-high field MRI.