Antipodally reflected bvecs for sagittal DWIs

[Lestropie]

Describe the bug

Where DWIs are acquired with sagittal slice orientation, the saved bvecs are antipodally reflected with respect to the diffusion sensitisation gradients that the sequence was instructed to apply. This happens irrespective of phase encoding direction or slice order.

To reproduce

Build and run the Docker image at: https://github.com/Lestropie/DWI_metadata

Expected behavior

The gradient table utilised during acquisition contains three fiducials at the head, applying diffusion sensitisation gradients along the three axes of the Device Coordinate System. The software tool reads these three columns from bvecs, transforms them to RAS real space according to my understanding of the bvecs format (which is seemingly confirmed correct via other tests), and compares them to the fiducials. If these correspond, no warning will be issued.

Version

The Docker image builds the most recent tag, v1.0.20240202, from source.

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PS. In case it's a clue: I found that with Eigen used by MRtrix3, when trying to access the 3x3 portion of an affine transform, .rotation() gives a different result to .linear() only for sagittal acquisition, as the former actually invokes a decomposition.