bvec signs for GE DTI with Coronol + ROW (#970)

[mr-jaemin]

Summary

dcm2niix (v1.0.20250626) produces incorrect bvec signs for GE DTI acquired with Coronal + ROW phase-encoding (with warnings). Other plane/phase combinations appear correct (Axial/Sagittal with COL/ROW and Coronal with COL).

Root cause: For Coronal + ROW, performing the ROW swap/negate before the plane-flip yields an incorrect combination of sign flips.

Proposed fix: Apply the plane‑dependent flip first, then apply the ROW remap, and negate the first element after the swap. This preserves existing, validated behavior for all other cases while correcting Coronal+ROW.

Impact: Affects GE DTI with Coronal+ROW only

Order Change:

**BEFORE**: ROW → plane‑dependent flip → always-negate-Y → negate-all
**AFTER**:  plane‑dependent flip → always-negate-Y → ROW → negate-all

Implementation (GE → FSL bvec)
While dcm2niix GE → FSL bvec code is somewhat complicated (derived from xiangruili's dicm2nii) with some differences:

• Y-direction handled opposite to dicm2nii
• Negate all components for compatibility

Current sequence:

1. ROW swap first (if !col)
   - swap X ↔ Y, then negate Y
2. Apply plane-dependent flip
3. Always negate Y
4. Negate all components (compatibility)

Proposed sequence:

1. Apply plane-dependent flip first
2. Always negate Y
3. ROW swap last (if !col)
   - swap X ↔ Y, then negate X
4. Negate all components (compatibility)

See #970 for Validation Results and Investigation Details

[neurolabusc]

Thanks!