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

Abstract

Functional MRI (fMRI) generally employs gradient-echo echoplanar imaging (GE-EPI) to measure blood oxygen level-dependent (BOLD) signal changes that result from changes in tissue relaxation time T* 2 between activation and rest. Since T * 2 strongly varies across the brain and BOLD contrast is maximal only where the echo time (TE) equals the local T* 2 , 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* 2 coverage are optimized by pixelwise weighted echo summation based on local T* 2 or contrast-tonoise 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.

Frontal Eye Fields Control Attentional Modulation of Alpha and Gamma Oscillations in Contralateral Occipitoparietal Cortex

“…Participants completed an fMRI version of the cued attention task, which was used to localize left and right FEF (see above). fMRI data were acquired using a 1.5T Avanto MRI scanner (Siemens) using a multi-echo (Poser et al, 2006) echoplanar sequence (TR ϭ 2.18 s, TEs ϭ 9.4, 21.2, 33 and 45 ms, flip angle ϭ 90°, 31 axial slices, slice thickness ϭ 3 mm, FoV ϭ 224 ϫ 224 mm, in plane voxel size ϭ 3.5 ϫ 3.5 mm). A high-resolution T1-weighted image (TR ϭ 2250 ms, TE ϭ 2.58 ms, flip angle ϭ 15°, 208 sagittal slices, in plane voxel size ϭ 1 ϫ 1ϫ1 mm, FOV ϭ 224 ϫ 224 mm) was acquired for coregistration of functional images, TMS neuro-navigation and MEG source analysis.…”

“…Image processing and statistical analysis was performed using SPM8 (www.fil.ion.ucl.ac.uk) and MATLAB 2012b (MathWorks). All multi-echo images were realigned to the first image of the series and a weighted image of the four echo images was calculated per TR (Poser et al, 2006). Functional images were then coregistered to the anatomical T1 image and smoothed using a 6 mm full-width at halfmaximum isotropic Gaussian kernel.…”

Blocking the Mineralocorticoid Receptor in Humans Prevents the Stress-Induced Enhancement of Centromedial Amygdala Connectivity with the Dorsal Striatum

“…Blood-oxygenation-level-dependent (BOLD) T2*-weighted multi-echo GRAPPA images (Poser et al, 2006) were obtained with the following parameters: repetition time (TR) ¼ 2.14 s, echo times (TEs) ¼ 9/21/33/44/56 ms, 34 transversal slices, ascending acquisition, distance factor 17%, effective voxel size ¼ 3.3 Â 3.3 Â 3.0 mm, field of view (FOV) ¼ 212 mm. We used this multiecho sequence for its improved BOLD sensitivity and lower susceptibility for artifacts, especially for ventral regions (Poser et al, 2006). In addition, we acquired high resolution T1-weighted anatomical image (TR ¼ 2.73 s, TE ¼ 2.95 ms, 176 sagittal slices, FOV ¼ 256 mm, voxel size ¼ 1.0 Â 1.0 Â 1.0 mm).…”

Structurally-informed Bayesian functional connectivity analysis

“…Multi-echo images obtained using the rs-fMRI acquisition protocol were combined using a custom Matlab script (MATLAB 7.7, The MathWorks Inc., Natick, MA, USA) which implements the procedure described in (Poser et al, 2006) and also incorporates motion correction using functions from the SPM5 software package (Wellcome Department of Imaging Neuroscience, University College London, UK). Of the 1030 combined volumes, the first six were discarded to allow the system to reach a steady state.…”