Enhancing measured diffusion anisotropy in gray matter by eliminating CSF contamination with FLAIR

Abstract

In this work, the effect of fluid-attenuated inversion recovery (FLAIR) on measured diffusion anisotropy was investigated in gray matter. DTI data were obtained with and without FLAIR in six normal volunteers. The application of FLAIR was experimentally demonstrated to lead to a consistent increase in fractional anisotropy (FA) in gray-matter regions, which was attributed to suppressed partial volume effects from CSF. In addition to these experimental results, Monte Carlo simulations were performed to ascertain the effect of noise on the measured FA under the experimental conditions of this study. The experimentally observed effect of noise was corroborated by the simulation, indicating that the increase in the measured FA was not due to a noise-related bias but to an actual increase in diffusion anisotropy. This enhanced measurement of diffusion anisotropy can be potentially used to differentiate directionally dependent structure and tracking fibers in gray matter. By measuring the diffusion characteristics of water in tissue along at least six noncolinear directions in space, diffusion tensor imaging (DTI) maps the directional dependence of water diffusion and is useful for visualizing microstructural tissue organization in the human brain (1). The advent of DTI in the last decade allows investigators to noninvasively visualize neural fiber tracts in vivo (2,3), which facilitates the study of brain development and the pathology of diseases associated with white-matter damage and disruption. Recent advances in tract-tracking techniques have further advanced the exploration of neural connectivity and neurological pathways in the human brain (4 -7).The application of DTI assumes a parallel relationship between the direction of maximum diffusion (i.e., the principal eigenvector of the diffusion tensor) and the direction of fiber fascicles traversing the imaged voxel. The measurement of diffusion-weighted MR signals, however, suffers from several limitations and is prone to artifacts that compromise accuracy in the derived diffusion tensor. For instance, cerebrospinal fluid (CSF) can degrade diffusion mapping, resulting in an overestimate of the trace of the diffusion tensor (8,9). Therefore, a technique that reduced the inaccuracy caused by CSF contamination in diffusion tensor measurements would allow better tract-tracking in both white and gray matter, especially in gray-matter areas such as the hippocampus, thalamus, and cortex. This would increase our understanding of the brain anatomy of neural fibers, neurological pathways, and disruptions in disease states.Recent studies have suggested that applying fluid-attenuated inversion recovery (FLAIR) to suppress CSF (10) before the diffusion weighting could reduce CSF contamination in the measured diffusion tensor (11,12). Although FLAIR has been used to address CSF contamination in ADC measurements in acute stroke (12) and epilepsy (13), and diffusion anisotropy measurements in white matter (11), the effect of CSF contamination on the measurement of diffusion anis...

Three distinct fiber pathways of the bed nucleus of the stria terminalis to the amygdala and prefrontal cortex

“…These indicate a predominant direction of diffusion in the head of the CN that is oriented mostly parallel to the IC and the transversal plane. In addition, in those parts of the head of the CN that were traversed by the anterior projection, fractional anisotropy (FA), as a measure for the directionality of diffusion, almost exclusively ranged above .2 (reaching values of up to .5) and therefore was considerably more pronounced than FA in other grey matter structures such as the frontal, parietal, and occipital lobe (Ma, Kadah, LaConte, & Hu, 2004).…”

Pseudolesions Arising from Unfolding Artifacts in Diffusion Imaging with Use of Parallel Acquisition: Origin and Remedies

“…Use of the saturation band to remove the eyeball signals illustrates the first method. In cases in which the unfolding artifacts originate from the CSF, the combination of fluid-attenuated inversion recovery with DTI acquisition [13][14][15] is preferred to the saturation band method, at the expense of prolonged scan time and slightly reduced signal-to-noise ratio. Matching the RF coil profile by acquisition by using EPI with the same phase-encoding bandwidth (ie, equal distortions) is an alternative option but is often beyond the radiographer's control.…”

Diffusion properties of cortical and pericortical tissue: regional variations, reliability and methodological issues

“…One recommended solution to this problem is to use FLAIR DTI sequences [51,52]. However, FLAIR DTI sequences are unavailable on many clinical scanners, require more time to acquire and reduce the signal to noise ratio of DTI measures of deep WM tracts that may also be useful to analyze alongside cortical structures.…”