Sources for dicom public dictionaries:
 * the official source is the part 6 of the dicom standard (see
   http://www.dclunie.com/dicom-status/status.html#BaseStandard2001).
 * http://www.fpimage.com/Manuals/Imp/dicomdic.html
   an html group based presentation.
 * ftp://rsbweb.nih.gov/pub/nih-image/documents/dicom-dict.txt
   a C syntax oriented version provided by NIH (Jim Nash)
 * http://afni.nimh.nih.gov/afni/AFNI_Dist/afni_src.tgz
   extract mri_dicom_hdr.c (tar zxvf afni_src.tgz afni_src/mri_dicom_hdr.c)
   and look for the definitions of CMD_dictionary, META_dictionary,
   ID_dictionary, PAT_dictionary...