Investigation of vibration-induced artifact in clinical diffusion-weighted imaging of pediatric subjects

What's New and What's Next in Diffusion MRI Preprocessing

Diffusion MRI (dMRI) provides invaluable information for the study of tissue microstructure and brain connectivity, but suffers from a range of imaging artifacts that greatly challenge the analysis of results and their interpretability if not appropriately accounted for. This review will cover dMRI artifacts and preprocessing steps, some of which have not typically been considered in existing pipelines or reviews, or have only gained attention in recent years: brain/skull extraction, B-matrix incompatibilities w.r.t the imaging data, signal drift, Gibbs ringing, noise distribution bias, denoising, between- and within-volumes motion, eddy currents, outliers, susceptibility distortions, EPI Nyquist ghosts, gradient deviations, B₁ bias fields, and spatial normalization. The focus will be on “what’s new” since the notable advances prior to and brought by the Human Connectome Project (HCP), as presented in the predecessing issue on “Mapping the Connectome” in 2013. In addition to the development of novel strategies for dMRI preprocessing, exciting progress has been made in the availability of open source tools and reproducible pipelines, databases and simulation tools for the evaluation of preprocessing steps, and automated quality control frameworks, amongst others. Finally, this review will consider practical considerations and our view on “what’s next” in dMRI preprocessing.

An MRI protocol for anatomical and functional evaluation of the California sea lion brain

BACKGROUND

Domoic acid (DOM) is a neurotoxin produced by some harmful algae blooms in coastal waters. California sea lions (Zalophus californianus) exposed to DOM often strand on beaches where they exhibit a variety of symptoms, including seizures. These animals typically show hippocampal atrophy on MRI scans.

NEW METHOD

We describe an MRI protocol for comprehensive evaluation of DOM toxicosis in the sea lion brain. We intend to study brain development in pups exposed in utero. The protocol depicts the hippocampal formation as the primary region of interest. We include scans for quantitative morphometry, functional and structural connectivity, and a cerebral blood flow map.

RESULTS

High-resolution 3D anatomical scans facilitate post hoc slicing in arbitrary planes and accurate morphometry. We demonstrate the first cerebral blood flow map using MRI, and the first structural tractography from a live sea lion brain.

COMPARISON WITH EXISTING METHODS

Scans were compared to prior anatomical and functional studies in live sea lions, and structural connectivity in post mortem specimens. Hippocampal volumes were broadly in line with prior studies, with differences likely attributable to the 3D approach used here. Functional connectivity of the dorsal left hippocampus matched that found in a prior study conducted at a lower magnetic field, while structural connectivity in the live brain agreed with findings observed in post mortem studies.

CONCLUSIONS

Our protocol provides a comprehensive, longitudinal view of the functional and anatomical changes expected to result from DOM toxicosis. It can also screen for other common neurological pathologies and is suitable for any pinniped that can fit inside an MRI scanner.

White matter compromise in autism? Differentiating motion confounds from true differences in diffusion tensor imaging

Common findings from diffusion tensor imaging (DTI) in autism spectrum disorder (ASD) include reduced fractional anisotropy (FA), and increased mean and radial diffusivity (MD, RD) of white matter tracts. However, findings may be confounded by head motion. We examined how group-level motion matching affects DTI comparisons between ASD and typically developing (TD) groups. We included 57 ASD and 50 TD participants, comparing three subsets at increasing levels of motion-matching stringency: full sample (FS); quality-controlled (QC); and quantitatively-matched (QM). Groups were compared on diffusivity measures using Tract-Based Spatial Statistics (TBSS) and probabilistic tractography. Two methods for estimating diffusivity were compared: dti-fit and restore. TBSS: In set FS, FA was reduced in the ASD compared to the TD group throughout the right hemisphere. This effect was less extensive in set QC and absent in set QM. However, effect sizes remained stable or increased with better quality-control in some regions. Tractography: In set QM, MD was significantly higher in ASD overall and RD was higher in bilateral ILF. Effects were more robust in QM than in FS or QC sets. Effect sizes in several tracts increased with stringent quality matching. Restore improved tensor estimates, with some increases in effect sizes, but did not fully compensate for reduced quality. Findings suggest that some previously reported DTI findings for ASD may have been confounded by motion. However, effects in the tightly matched subset indicate that tract-specific anomalies probably do exist in ASD. Our results highlight the need for careful quality-control and motion-matching. Autism Res 2017, 10: 1606-1620. © 2017 International Society for Autism Research, Wiley Periodicals, Inc.