Free water elimination and mapping from diffusion MRI

TLDR

DTI is limited when voxels contain free water, such as CSF or edema, because the indices no longer reflect underlying tissue properties. The study proposes a method to separate tissue diffusion properties from surrounding free water and map the free water volume. The method fits a bi‑tensor model with a stabilizing mathematical framework and can be applied to conventional DTI acquisitions, integrating easily with existing pipelines. The method produced corrected DTI indices, improved tractography adjacent to ventricles and edema, enabled edema segmentation by underlying tissue condition, introduced free‑water volume as a new quantitative contrast, and revealed that free water extends beyond parenchymal borders, influencing neuronal bundle architecture. Published in Magn Reson Med 2009 and © 2009 Wiley‑Liss, Inc.

Abstract

Abstract Relating brain tissue properties to diffusion tensor imaging (DTI) is limited when an image voxel contains partial volume of brain tissue with free water, such as cerebrospinal fluid or edema, rendering the DTI indices no longer useful for describing the underlying tissue properties. We propose here a method for separating diffusion properties of brain tissue from surrounding free water while mapping the free water volume. This is achieved by fitting a bi‐tensor model for which a mathematical framework is introduced to stabilize the fitting. Applying the method on datasets from a healthy subject and a patient with edema yielded corrected DTI indices and a more complete tract reconstruction that passed next to the ventricles and through the edema. We were able to segment the edema into areas according to the condition of the underlying tissue. In addition, the volume of free water is suggested as a new quantitative contrast of diffusion MRI. The findings suggest that free water is not limited to the borders of the brain parenchyma; it therefore contributes to the architecture surrounding neuronal bundles and may indicate specific anatomical processes. The analysis requires a conventional DTI acquisition and can be easily merged with existing DTI pipelines. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.