Introduction

As more and more research is done on white matter morphology, the obvious question to ask is whether differences can be found between individuals or between groups, ie patients vs controls. Before we begin, I’d like to summarize some important acquisition issues.

Data Acquisition

• make sure you acquire data on each subject exactly the same. Be careful to use exactly the same TR/TE, b-value, voxel size and number of slices.
• always measure iso-voxel data. It’s easy to understand why: when acquiring non-isovoxel data, and if a fiber structure would be aligned parallel to the long axis of a voxel, more contrast is added there due to the summation of diffusivity. Ergo, more artificial signal decay in that direction!
• try to avoid interpolating your data. This has a dramatic effect on derived measures such as FA. This is illustrated below, where the FA distribution for a native resolution VMP versus interpolated VMP is shown for a ROI in the corpus callosum.

  

• be very careful with interpretation of differences in FA or ADC. They might not be of anatomical nature but due to the acquisition, data processing etc.
• include a T1 weighted high-resolution anatomy in the session.

About native and Talairach space

We recommend computing the tensor and derived measures in native space (that is the space as the data was scanned in), and to do the group analysis in common space by converting the scalar FA/ADC maps to TAL space. This prevents introducing artefacts in the tensor data by interpolation. Interpolation of a tensor is not trivial and may introduce tensor deformation in the processing. For more information, see reference [?].