If a segmentation mask appears shifted, rotated, mirrored, or clipped against a medical image, first check whether both files describe the same physical coordinate space. Matching array dimensions do not prove alignment, and different dimensions do not prove a mismatch. Correct a bad reference or transform before resampling; resampling is appropriate when the physical alignment is already valid but the voxel grids differ.
What “misaligned” means: check physical space, not just array size
A volume’s voxel indices become physical locations through its geometry: voxel spacing, origin, axis directions, and extent. Two arrays can have the same dimensions yet place their voxels at different physical locations. A segmentation and image can also have different dimensions while occupying the same physical space, for example when they use different sampling or extents. 3D Slicer’s Physical Space vs. Image Space FAQ explains why image-space indices and physical-space positions are distinct.
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Before editing anything, preserve unmodified copies and confirm that the mask belongs to the correct subject, image series, acquisition phase, and time point. A mask tied to another series can look like a registration failure even if its own geometry is valid.
Inspect the files’ geometry and references
For NIfTI volumes
Compare dimensions, voxel sizes, origin, axis directions, and the voxel-to-world affine for the image and mask. Inspect both qform and sform, along with qform_code and sform_code. Check which coordinate spaces the forms represent and whether their handedness is consistent with the intended anatomy. NIfTI guidance describes qform as suitable for scanner coordinates and rigid-body orientation, while sform can encode a more general affine mapping, including standard-space mappings. See the NIfTI-1 FAQ and recommended qform and sform usage.
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Do not infer the stored coordinate orientation from whether a viewer displays radiological or neurological convention. Those are display choices, not proof of how the file encodes coordinates; the NIfTI qform/sform brief guidance distinguishes the forms’ roles.
For DICOM image series and segmentations
Confirm that the segmentation references the intended source instances or series and, when applicable, the same Frame of Reference UID. For image geometry, use Pixel Spacing, Image Orientation (Patient), and Image Position (Patient) to determine pixel orientation and slice locations. Do not reconstruct slice positions from slice thickness alone: position data is needed to establish where slices lie. The 3D Slicer DICOM documentation covers import and troubleshooting considerations.
A DICOM SEG may have a different sampling grid or extent from its referenced image while still relating to it in physical space. Compare geometry and references rather than requiring identical array dimensions. DICOM’s Segmentation IOD describes the segmentation object and its references.
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Check whether a transform or coordinate frame is wrong
Registration transforms have a direction: they map coordinates from a source frame to a target or registered frame. Verify the direction, coordinate frames, and referenced images before applying a matrix; using the inverse when the forward transform is required (or vice versa) can move a mask in the wrong direction. DICOM defines rigid, rigid-scale, and affine spatial registration matrices as mappings between homogeneous coordinate systems in its Spatial Registration Module and discusses registration in PS3.17.
For NIfTI, changing voxel coordinates requires corresponding changes to the spatial transforms. The NIfTI guidance specifies that an affine voxel-coordinate change updates the original form using the inverse voxel transform. Avoid changing header numbers by eye just to make an overlay look plausible; establish the correct physical-space relationship first.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Choose between correcting geometry and resampling
| Situation | What to do | Why |
|---|---|---|
| The header, reference, or transform is wrong | Correct or establish the valid physical-space relationship first. | Resampling cannot discover an unknown registration or make unrelated anatomy overlap. |
| Physical coordinates are valid, but the image and mask use different grids | Resample the mask onto the intended image reference grid. | The output can then use the target’s origin, spacing, directions, and extent. |
| The image and mask do not overlap in physical space | Resolve the reference or registration problem before resampling. | Resampling may return zeros or omit some or all of the input. |
For labelmaps, use nearest-neighbor interpolation so resampling selects existing categorical label values rather than creating intermediate values. Linear and higher-order interpolation are generally for intensity images, not discrete masks. Check that the input and reference overlap physically before running the operation; this limitation is noted in Slicer’s physical-space FAQ.
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Fix segmentation geometry in 3D Slicer
Changing the source volume selected in Segment Editor does not automatically redefine an existing segmentation’s labelmap geometry. The source volume used when the segmentation is first created determines that geometry. If the segmentation needs a different extent, resolution, or set of axes, use the segmentation’s geometry specification controls and select an appropriate source geometry. See the project’s Segment Editor documentation and Image Segmentation documentation.
When the coordinates are already correct and only the grid needs to change, use a resampling operation with the intended image as the reference and nearest-neighbor interpolation for the labelmap. Slicer describes geometry through origin, spacing, axis directions, and extents; binary labelmap resampling can make small representational changes. Avoid repeated conversions between grids where possible.
Validate the saved result
- Overlay the mask on the intended image in axial, coronal, and sagittal views, and inspect multiple slices rather than a single location.
- Check anatomical landmarks and left/right orientation; a visually plausible overlay on one slice does not rule out a flipped or rotated volume.
- Inspect the mask’s bounds and output extent to catch clipping or unexpected empty regions.
- Save, reload the output, and repeat the geometry and overlay checks so validation is not limited to the in-memory display.
For clinical use, follow the institution’s validation and data-handling procedures. A software overlay check alone is not clinical validation.
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