Boyd Taylor, H. | bioRxiv Science

bioRxiv2019

Anatomy-guided Inverse-phase-encoding Registration Method for Correcting Susceptibility Artifacts in Sub-millimeter fMRI

Echo planar imaging (EPI) is a fast and non-invasive magnetic resonance imaging (MRI) technique that supports data acquisition at spatial and temporal resolutions suitable for brain function studies. However, susceptibility artifacts are unavoidable distortions in EPI. These distortions are especially strong in high spatial resolution images and can lead to misrepresentation of brain function in fMRI experiments. A common method for correcting susceptibility artifacts is based on a registration scheme which uses two EPI images acquired using identical sequences but with inverse phase-encoding (PE) directions. In this paper, we present a new method for correcting susceptibility artifacts by integrating a T1-weighted (T1w) image into the inverse-PE based registration, since the T1w structural image is considered as a ground-truth measurement of the brain. Furthermore, the T1w image is used as a criterion to select automatically the regularization parameters of the proposed image registration. Evaluations on two high-resolution EPI-fMRI datasets, acquired at 3T and 7T scanners, confirm that the proposed method provides more robust and sharper corrections and runs faster compared with two other state-of-the-art inverse-PE based susceptibility artifact correction methods, i.e. HySCO and TOPUP.

neuroscience↗

bioRxiv2018

Vascular effects on the BOLD response and the retinotopic mapping of hV4

Despite general acceptance that the retinotopic organisation of human V4 (hV4) takes the form of a single, uninterrupted ventral hemifield, measured retinotopic maps of this visual area are often incomplete. Here, we test hypotheses that artefact from draining veins close to hV4 cause inverted BOLD responses that may serve to obscure a portion of the lower visual quarterfield -- including the lower vertical meridian -- in some hemispheres. We further test whether correcting such responses can restore the missing retinotopic coverage in hV4. Subjects (N=11) viewed bowtie, ring, drifting bar and full field flash stimuli. Functional EPIs were acquired over approximately 1.5h and analysed to reveal retinotopic maps of early visual cortex, including hV4. Normalised mean maps (which show the average EPI signal amplitude) were constructed by voxel-wise averaging of the EPI time course and used to locate venous eclipses, which can be identified by a decrease in the EPI signal caused by deoxygenated blood. Inverted responses are shown to cluster in these regions, and correcting these responses improves maps of hV4 in some hemispheres, including restoring a complete hemifield map in one. A leftwards bias was found in which 11/11 hV4 maps in the left hemisphere were classified as incomplete, while this was the case in only 3/11 right hemisphere maps. Incomplete hV4 maps did not correspond with venous artefact in many instances, with incomplete maps being present in the absence of a venous eclipse and complete maps coexisting with a proximate venous eclipse. We also show that mean maps of upper surfaces (near the boundary between cortical grey matter and CSF) provide highly detailed maps of veins on the cortical surface. Results suggest that venous eclipses and inverted voxels can explain some incomplete hV4 maps, but cannot explain the remainder nor the leftwards bias in hV4 coverage reported here.

neuroscience↗