It is possible that there might be multiple images in the DICOM file. One of which may be a thumb nail. Perhaps, you can use the QUERY_DICOM routine to determine if there are multiple images in the file. If there are 2 images in the file, you can use the IMAGE_INDEX keyword to specify which image you want to read into IDL.
http://www.harrisgeospati...#Q_815714925_1045913