3D in utero quantification of T2* relaxation times in human fetal brain tissues for age optimized structural and functional MRI.

PURPOSE: Maximization of the blood oxygen level-dependent (BOLD) functional MRI (fMRI) contrast requires the echo time of the MR sequence to match the T2* value of the tissue of interest, which is expected to be higher in the fetal brain compared with the brain of a child or an adult. METHODS: T2* values of the cortical plate/cortical gray matter tissue in utero in healthy fetuses from mid-gestation onward (20-36 gestational weeks) were measured using 3D T2* maps calculated from 2D dual-echo T2*-weighted data corrected for between-slice motion and reconstructed in 1.0 mm3 isotropic resolution from a sequence of multiple time points, together with 1.0 mm3 isotropic resolution T2-weighted structural data. RESULTS: Mean T2* relaxation times of the cortical tissue were about twice as high as those reported previously in adults. In a supporting experiment applying single seed analysis, default mode and auditory networks appeared better localized and less noisy while using an echo time of 100 ms versus 43 ms. The results of the previous study reporting a trend for T2* values to decrease with fetal age were reproduced and extended to include cortical tissues and subjects in earlier gestation (20-26 gestational weeks). CONCLUSION: The first measurement of T2* values in fetal cortical tissues suggested the appropriate echo time range for fetal BOLD fMRI protocol optimization to be 130-190 ms. Magn Reson Med 78:909-916, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Detecting default mode networks in utero by integrated 4D fMRI reconstruction and analysis.

Recently, there has been considerable interest, especially for in utero imaging, in the detection of functional connectivity in subjects whose motion cannot be controlled while in the MRI scanner. These cases require two advances over current studies: (1) multiecho acquisitions and (2) post processing and reconstruction that can deal with significant between slice motion during multislice protocols to allow for the ability to detect temporal correlations introduced by spatial scattering of slices into account. This article focuses on the estimation of a spatially and temporally regular time series from motion scattered slices of multiecho fMRI datasets using a full four-dimensional (4D) iterative image reconstruction framework. The framework which includes quantitative MRI methods for artifact correction is evaluated using adult studies with and without motion to both refine parameter settings and evaluate the analysis pipeline. ICA analysis is then applied to the 4D image reconstruction of both adult and in utero fetal studies where resting state activity is perturbed by motion. Results indicate quantitative improvements in reconstruction quality when compared to the conventional 3D reconstruction approach (using simulated adult data) and demonstrate the ability to detect the default mode network in moving adults and fetuses with single-subject and group analysis. Hum Brain Mapp 37:4158-4178, 2016. © 2016 Wiley Periodicals, Inc.