BOLD contrast sensitivity enhancement and artifact reduction with multiecho EPI: Parallel‐acquired inhomogeneity‐desensitized fMRI

TLDR

Functional MRI typically uses gradient‑echo EPI to detect BOLD signal changes, but a single echo time cannot match the spatially varying T, and the long echo train makes the images highly susceptible to field‑inhomogeneity‑induced distortion. The study proposes a parallel‑imaging method that acquires multiple echo times after one excitation to reduce artifacts without lengthening TR. The method optimizes sensitivity by pixelwise weighted echo summation guided by local T or CNR, and its performance was assessed via differential BOLD CNR calculations in both unstimulated and Stroop task conditions. At 3 T, the technique increased BOLD sensitivity by at least 11 % across the brain, especially in susceptibility‑prone regions, and markedly reduced image distortion. Published in Magn Reson Med 2006; © 2006 Wiley‑Liss, Inc.

Abstract

Abstract Functional MRI (fMRI) generally employs gradient‐echo echo‐planar imaging (GE‐EPI) to measure blood oxygen level‐dependent (BOLD) signal changes that result from changes in tissue relaxation time T between activation and rest. Since T strongly varies across the brain and BOLD contrast is maximal only where the echo time (TE) equals the local T , imaging at a single TE is a compromise in terms of overall sensitivity. Furthermore, the long echo train makes EPI very sensitive to main field inhomogeneities, causing strong image distortion. A method is presented that uses accelerated parallel imaging to reduce image artifacts and acquire images at multiple TEs following a single excitation, with no need to increase TR. Sensitivity gains from the broadened T coverage are optimized by pixelwise weighted echo summation based on local T or contrast‐to‐noise ratio (CNR) measurements. The method was evaluated using an approach that allows differential BOLD CNR to be calculated without stimulation, as well as with a Stroop experiment. Results obtained at 3 T showed that BOLD sensitivity improved by 11% or more in all brain regions, with larger gains in areas typically affected by strong susceptibility artifacts. The use of parallel imaging markedly reduces image distortion, and hence the method should find widespread application in functional brain imaging. Magn Reson Med, 2006. © 2006 Wiley‐Liss, Inc.