Siemens often stores DICOM images with the IMA extension. However, in the days before DICOM, Siemens also used the IMA extension for their proprietary Magnetom format. In short, if dcm2niix processes them, they are in DICOM format. However, if dcm2niix complains (and the images were acquired more than a decade ago), you may want to see if my deprecated dcm2nii software handles these obsolete images.