DICOMDIR explained: how a patient scan CD is organised

DICOMDIR is the index file on a patient scan CD or USB stick. It is a table of contents that lists which patient, studies, series and images are on the disc, and where each image file is stored. The images themselves live in other files, usually in subfolders.

What a scan disc looks like

If you open a typical patient CD in a file explorer, you will usually see:

• a file named DICOMDIR, with no extension
• a folder, often called DICOM or similar, holding the images
• subfolders inside it with short or numeric names
• image files with names like IM0001, or just a string of digits, often with no .dcm ending
• frequently a Windows viewer program and some support files

The exact names differ between hospitals and scanner makers. The pattern of one index plus many image files is the common part.

What DICOMDIR contains

DICOMDIR is itself a DICOM file, but it holds no pictures. It holds a directory, organised in the same levels that DICOM uses for the images:

• Patient
• Study, one examination
• Series, one run or sequence within the study
• Image, pointing to one image file

For each image, the index stores a path to the file on the disc. That is how a viewer program can show the full list of scans without opening every image first. For the meaning of study and series, see CT vs MRI, study and series.

Why the file names look odd

The names are short and plain because the older disc standards limit how file names can look. The names carry no meaning to a person. The information about what each image is lives inside the file, in its tags, not in its name. See what is a DICOM file.

This is also why renaming files or moving them out of their folders can cause problems. The index points to files by their path, and a moved file can no longer be found through the index.

One disc, hundreds of files

A single CT can consist of several hundred image files, one per slice. A disc with a few studies can hold well over a thousand files. This is normal. You do not open them one by one. A viewer reads the index or walks through the folders and groups the files into series for you.

The bundled Windows viewer

Many discs include a small viewer that starts on Windows. It is useful on a Windows computer without installing anything. It will not run on a phone or tablet. The images do not depend on it, so you can ignore it and open the same data in another viewer.

What a viewer does with DICOMDIR

Viewers handle the index in one of two ways. Some read DICOMDIR and show the contents as listed. Others scan the whole folder tree and read every file directly. The second method also works when the index is missing, damaged or incomplete, which is why a viewer that can search nested subfolders is useful.

Radix DICOM opens whole patient-CD folders, including nested subfolders and files without an extension.

If something is missing

• No DICOMDIR on the disc. Not every disc has one. The images can still be opened by pointing a viewer at the folder.
• Only some series appear. Check that the whole disc was copied, including every subfolder. See how to open a DICOM CD on your phone.
• A file is skipped. It may be damaged or use an unsupported format. See why a DICOM file will not open.

A word of caution

The index, like the images, contains the patient’s name and other personal details. Treat the whole folder as private, and share it only with people who should see it. As always, what any scan means for a person’s health is a question for their doctor.