Opening a NII File

The NII file type is primarily associated with FSL.

NII	File extension: NII
	File type:	3D medical image file

What is a NII file

NII files mostly belong to FSL. NII files store volumetric neuroimaging data from MRI scans. They use the Neuroimaging Informatics Initiative standard to combine both image data and a header that details voxel dimensions and spatial orientation.

Format structure: Contains a header plus imaging data in one file.

Primary use: Designed for precise brain imaging analysis.

Common applications: Tools like FSL (e.g., FSL5.0 and FSL3.2beta) and MRIcron are used to open and process these files.

FilExt.com note: You can find further information on the FSL documentation page.

How to open a NII file

You need a suitable software like FSL to open a NII file. Without proper software you will receive a Windows message "How do you want to open this file?" or "Windows cannot open this file" or a similar Mac/iPhone/Android alert. If you cannot open your NII file correctly, try to right-click or long-press the file. Then click "Open with" and choose an application. You can also display a NII file directly in the browser:. Just drag the file onto this browser window and drop it.

Online NII Text Viewer

Choose your .nii file to analyze

or drag and drop it here

Online File Viewer

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Programs that open and convert NII files:

FSL
See the previous paragraphs to learn more about the main application. NII files are often referred to as FSL 3d images because this type of file is primarily created or used by this software.
NIfTI-1 Data Format by Neuroimaging Informatics Technology Initiative
NIfTI-1 is adapted from the widely used ANALYZE 7.5 file format. The hope is that older non-NIfTI-aware software that uses the ANALYZE 7.5 format will still be compatible with NIfTI-1. NIfTI-1 uses the empty space in the ANALYZE 7.5 header to add several new features. The identifying characters used for this association are - Hex: 00 00 01 5C 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00, ASCII: \ .

NII file format:

To find out what program is needed to open NII files, you need to determine the file format. The initial characters of a file are called signature, sometimes also referred to as "magic bytes". The signature can be used to infer the file format. Frequently, however, one file extension is used by different programs for different file formats, and one file format for different file extensions. When there is a problem with a file, the first step to solving it is to determine the file format. Our evaluation of the NII files looks like this:

NII is a very widely used file extension. The file type mainly uses a special format. The two most common formats are as follows:

96% of all NII files are associated with NIfTI-1 data format. These are binary files, so they do not contain words or text. The files are often between 1 MB and 150 MB in size. The file type was created 10 years ago, but is still used today. Files like these have the following tags: brats, flair, coronacases, segmentation, mask, volume, brain and training.
1% of all NII files are NIfTI-2 data format files. These files are not plain text files, so they also contain binary data. NII files are between 390 KB and 190 MB in size. The following filename is common: 20180601.dtseries.nii. Files like these have the following tags: dtseries, dscalar, msmall, atlas, dlabel and thickness.

The remaining 3% of all NII files are different files with different signatures, the following 3 formats exist: JetDB. Tagged as covid, study, bold, mprage, task or atlas. Just click the "Choose your .nii file to analyze" button on this page to find out what your NII file is.

Technical Data for NII File Extension

File classification:

3D Image

Identifier:

Hex: 00 00 01 5C 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00, ASCII: \ (provided by TrID database)

Related files:

nii.gz, hdr, nrrd, mnc, gz, zip, nil, ni, tar, nii 2, lml, mha, nni, json, img, txt, mri, jpg, pdf, npy, tgz, dcm

Mime type:

image/nia, image/nie, image/nii

a nii 3D medical image file is a special file format and should only be edited and saved with the appropriate software.

How to solve problems with NII files

Associate the NII file extension with the correct application.
Update your software that should actually open 3d medical image files. Because only the current version supports the latest NII file format. Search, therefore, e.g. on the manufacturer website after an available FSL update.
To make sure that your NII file is not corrupted or virus-infected, get the file again and scan it with Google's virustotal.com.
Click here to open your .NII file online - secure, fast, and no downloads needed.