DICOMDIR: the index file on a DICOM disc
If your imaging CD, DVD or USB stick has a file called DICOMDIR with no extension, and image files with names likeIM000001 in folders next to it, this page explains what you are looking at.
Last reviewed 18 October 2026. Written by the dicompeek editors; sources are listed at the end.
What a DICOMDIR is
A DICOMDIR is the directory of a DICOM file-set: the collection of files written together on one disc or other medium. The standard requires exactly one file with the reserved name DICOMDIR in every file-set. It is itself a DICOM file, encoded in Explicit VR Little Endian, and it holds the “Media Storage Directory”. It is not an image. Opening it in a viewer shows no picture.
The directory content can be empty. The standard allows that for environments that do not need one, in which case only the file-set identification is present. In practice, discs from hospitals list their contents.
What is inside
The directory is a tree of directory records. The usual shape mirrors how imaging data is organised (seewhat is DICOM): a patient record, under it study records, under eachseries records, and under each the image records. Every image record holds a Referenced File ID, which is the path of the actual file on the disc, plus a few attributes copied from it so a program can list the contents without opening every file: patient name and ID, study date, modality, series and instance numbers.
The file-set as a whole is identified by a File-set UID, and may also have a short human-readable File-set ID of up to 16 characters, which is often the same as the disc's volume label. The DICOMDIR lists only files inside its own file-set.
Why the image files have strange names
The standard limits file names on media. A file is identified by a File ID of one to eight components, and each component is one to eight characters long, made only of the capital letters A to Z, the digits 0 to 9 and the underscore. That is why discs show structures such asDICOM\S0001\I0000001 or IMAGES\IM000001, and why many files have no.dcm extension. The standard notes that files may also be given an alternate name or link with a longer name or an extension such as.dcm, but nothing requires it. The name carries no meaning; the folders and numbers are just a way to keep names unique. Seehow to open a .dcm file if you are unsure whether a file without an extension is DICOM.
Opening the images from a disc
- Copy the whole disc to a folder first if you can. Some discs also contain a small viewer program for Windows; you do not need it.
- Find the image files. They are in the folders next to the DICOMDIR (often named
DICOM,IMAGESor similar), not in the DICOMDIR itself. - Open them in the dicompeek viewer by dropping them on the page. Select all files of one series together; they are sorted by instance number into a stack. dicompeek reads the images directly and does not read the DICOMDIR, so drop the image files and leave the DICOMDIR out.
A file-set that has lost or damaged its DICOMDIR does not conform to the standard, but the image files are still ordinary DICOM files and open as usual. Programs that import whole discs usually look for the DICOMDIR first, so for those, copy the folder structure with the DICOMDIR intact.
Privacy: the DICOMDIR names the patient
Because the directory copies patient name, ID and dates from the images, the DICOMDIR is as sensitive as the images. If you share files, share theanonymized images, not the DICOMDIR. Anonymizing the images alone leaves the old DICOMDIR pointing at files with different names and UIDs, so it would be wrong as well as revealing. The dicompeek anonymizer refuses a DICOMDIR for this reason. A new directory can be generated by tools that write DICOM media.
Quick reference
| Question | Answer |
|---|---|
| Is it an image? | No. It is an index of the images. |
| Can I delete it? | You can, to view the images, but keep it on the disc you were given. |
| Why no extension? | The reserved File ID is exactly DICOMDIR. |
| Does it contain patient data? | Yes, names, IDs and dates of everything on the disc. |
| Does dicompeek open it? | No, it shows the images. Drop the image files. |
Sources
- DICOM PS3.10 section 8.6, Reserved DICOMDIR File ID (one DICOMDIR per file-set, Explicit VR Little Endian, may be empty)
- DICOM PS3.10 section 8.2, File IDs and 8.5, Character Set (one to eight components of one to eight characters; A to Z, 0 to 9, underscore)
- DICOM PS3.10 chapter 8, DICOM File Service (file-set, File-set UID and ID)
- DICOM PS3.3 section F.3, Basic Directory IOD
Not for clinical diagnosis. See the medical disclaimer.