AFNI doesn't deal with the gzipped files. The IMA files are DICOM files, so that is usually not a problem, unless they are corrupted or not written by Siemens correctly. gunzip all the files first and try again. Unfortunately if it still can't be read by to3d or Dimon, you won't be able to read the mosaic files with the raw format specifier except by cutting up the mosaic into single slices using imcutup as in this very old message posting. (You can also try reading the files with another conversion program like mri_convert in the FreeSurfer package, mriconvert, medcon, MRICRO)
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afni.nimh.nih.gov]
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afni.nimh.nih.gov]