I don't think that this is a file format that is supported by IDL out of the box. I think you might want to try entering "Nifti idl" into google (or some other search engine). It looks like there might be some users who have written programs to do this.

I have not been able to find any IDL nifti image file reader/writer with google (as suggested) or anywhere else. Given IDL's customer base, this functionality should be available out of the box. If anyone knows of currently maintained IDL tools for working with nifti, please share!