NIfTI is a type of file format for neuroimaging. Technically there are two NIfTI file format versions, the NIfTI-1 and NIfTI-2 file formats 1. The NIfTI-2 format is an update on NIfTI-1 that allows more data to be stored 1.
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Clinical applications
NIfTI files are used very commonly in imaging informatics for neuroscience and even neuroradiology research 1,2.
In clinical care DICOM files are standard, however, some imaging informatics tools can convert DICOM files to NIfTI format automatically 2.
History and etymology
NIfTI is an eponymous acronym that comes from the Neuroimaging Informatics Technology Initiative which was a working group at the NIH (National Institutes of Health in the USA) in the early 2000s.
The Neuroimaging Informatics Technology Initiative wanted to create a file type that was similar to but improved from the Analyze file format, another type of medical imaging file format. At that time Minc file format (from the Montreal Neurological Institute) and Analyze were the common file types used in neuroimaging research.
Practical Points
NIfTI, DICOM, Minc, and Analyze file formats are all still used (as of 2019) in various contexts. Programs meant for one file type will not work well on another, and therefore file conversion is often necessary. File types are easily recognizable by their extension e.g. .nii for NIfTI or .dcm for DICOM.