Multi-echo fMRI at 7T

12:03 pm 20 Dec 2017

Available online 5 December 2017

We thank Aiman Al-Najjar, Nicole Atcheson, and Steffen Bollmann for help with data collection. We thank Steve Cauley for providing a multi-echo enabled variant of the MGH slice-GRAPPA reconstruction and Himanshu Bath for details on the sequence-recon interface. This research was supported by the National Health and Medical Research Council (APP 1088419). MB acknowledges funding from Australian Research Council Future Fellowship grant FT140100865. SB acknowledges support through the Australian Government Research Training Program Scholarship. The authors acknowledge the facilities of the National Imaging Facility (NIF) at the Centre for Advanced Imaging, University of Queensland.

The nuclei of the basal ganglia pose a special problem for functional MRI, especially at ultra-high field, because T2* variations between different regions result in suboptimal BOLD sensitivity when using gradient-echo echo-planar imaging (EPI). Specifically, the iron-rich lentiform nucleus of the basal ganglia, including the putamen and globus pallidus, suffers from substantial signal loss when imaging is performed using conventional single-echo EPI with echo times (TE) optimized for the cortex. Multi-echo EPI acquires several echoes at different echo times for every imaging slice, allowing images to be reconstructed with a weighting of echo times that is optimized individually for each voxel according to the underlying tissue or T2* properties. Here we show that multi-echo simultaneous multi-slice (SMS) EPI can improve functional activation of iron-rich subcortical regions while maintaining sensitivity within cortical areas. Functional imaging during a motor task known to elicit strong activations in the cortex and the subcortex (basal ganglia) was performed to compare the performance of multi-echo SMS EPI to single-echo SMS EPI. Notably within both the caudate nucleus and putamen of the basal ganglia, multi-echo SMS EPI yielded higher tSNR (an average 84% increase) and CNR (an average 58% increase), an approximate 3-fold increase in supra-threshold voxels, and higher t-values (an average 39% increase). The degree of improvement in the group level t-statistics was negatively correlated to the underlying T2* of the voxels, such that the shorter the T2*, as in the iron-rich nuclei of the basal ganglia, the higher the improvement of t-values in the activated region.


Alexander M. Puckett, Saskia Bollmann, Benedikt A. Poser, Jake Palmer, Markus Barth, Ross Cunnington

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