THE FILE FIELD GUIDE

.nii

NIfTI Image Data

Data & Science

NII is NIfTI’s single-file form, often used for neuroimaging research; .nii.gz is commonly its compressed form. Inspect dimensions, voxel sizes and spatial transforms, including affine/qform/sform, rather than appearance alone.

Main use: Store image arrays with spatial-coordinate information
Data type: Binary data

All sections, questions and sources

Content reviewed：2026-10-19 · Editor：Kai

Opening steps

Choose your environment or purpose to see its steps.

NIfTI viewer / NiBabel

Preserve the original and inspect a working copy in a compatible application.

Open a copy in a NIfTI-capable viewer.
Inspect dimensions, voxel sizes and orientation.
If a reference is available, inspect alignment without overwriting the original.

Inspect the received file and its requirements

Establish dependencies and settings before proceeding.

Identify NIfTI version and compression.
Establish 3D/4D structure and axis meaning with the producer.
Inspect qform/sform and processed-data spatial conditions.

Menu labels and support vary by app, OS and version. Preserve the original before making changes.

Troubleshooting by symptom

Left/right appears different: inspect viewer conventions and spatial metadata.
Misalignment: inspect affine, qform/sform and reference space.
Time axis absent: inspect 4D shape and selected volume.
Compressed file rejected: check .nii.gz support or decompression requirements.

NII → research data / display image

Select required volumes/slices in compatible software and export separately.
Verify array shape, coordinates, units and de-identification/sharing conditions.

Slice images such as PNG do not preserve full 3D/4D arrays, values and spatial coordinates.

Header-only changes can change meaning

Changing a spatial matrix changes relationships even when voxel values remain unchanged. Do not edit headers merely to make images look aligned; establish the intended reference space and processing.

Source for these steps：NIMH · NIfTI-1 Data Format / NiBabel project · Working with NIfTI images

Worked example: checks before handoff

Example: compare source/result shape and affine and provide the reference image and processing conditions when sharing an analysis.

Source for these steps：NIMH · NIfTI-1 Data Format / NiBabel project · Working with NIfTI images

Related-format differences

Related-format differences
Format / method	Purpose and preserved information
.nii	Store image arrays with spatial-coordinate information
DCM / PNG	DCM is DICOM and PNG display imagery, with different spatial/metadata structures.
NII → research data / display image	Slice images such as PNG do not preserve full 3D/4D arrays, values and spatial coordinates.

Handling and security

Headers may retain participant/medical information; follow agreed sharing and de-identification checks.

Questions and answers

Is NII.GZ a different image format?

It is commonly gzip-compressed NIfTI; supporting tools may read it directly.

Can JPEG recreate the original?

Dimensions, precision and spatial metadata are not preserved equivalently.

Should I flip the array if orientation looks wrong?

Check display conventions versus coordinates first; visual-only flipping can break spatial relationships.

Format facts: name, steward and MIME type

Formal name: NIfTI Image Data
Steward / specification: NIMH
MIME type: Consult the producer or delivery system. The extension alone does not establish a MIME type.

Primary and official sources

StandardNIfTI-1 Data Format
NIfTI single-file structure and specification.
NIMH · Link checked：2026-10-16
Software documentationWorking with NIfTI images
Shape, affine transforms, qform/sform and scaling.
NiBabel project · Link checked：2026-10-16