What Is a DICOMDIR File, and How Do You Open It?

What a DICOMDIR file is

The DICOM standard, which almost every scanner and hospital uses, defines how scans are written to removable media. Each disc gets one index file named DICOMDIR, always in capitals and with no extension, at the top of the disc. Inside it is a tree:

• Patient: name, ID and date of birth.
• Study: one visit, for example “CT chest, 1 October”.
• Series: one set of images, for example the axial slices, or a scout image.
• Image: the path of each image file.

Viewers read the index so they can show you the studies and series instead of hundreds of loose files.

What a .dcm file is

A DICOM file (often .dcm) holds one image (one slice of a CT or MRI, or one X-ray) together with details about it: who it belongs to, the scanner, the date, the size of each pixel in millimetres, and the brightness window the scanner chose. That is why an image viewer such as Photos cannot open it: it is not a picture file but a medical record with a picture inside.

Why the files on the disc have no extension

The disc format allows only short file names, up to eight capital letters or digits, so the images are often called IM000001, I0000123 or similar, with no .dcm at the end. They are still DICOM files. Do not rename them: viewers find them through the DICOMDIR.

How to open a DICOMDIR

1. Install a DICOM viewer (see the comparison of viewers).
2. Open the DICOMDIR file from the viewer, or open the whole disc. In SlideScope, opening the DICOMDIR opens the disc and lists each series once; Home also has Got a scan on a CD or USB? Open it, which finds the disc for you.
3. If a disc has no DICOMDIR, open the DICOM folder instead: viewers read the files’ own headers and group them into series.

Converting DICOM to JPG or PDF

Many viewers can save the slice on screen as an ordinary image. That is fine for showing someone a picture, but a JPG loses the scan’s measurements and details, and one picture is only one slice of a CT or MRI. To share the whole scan with another doctor, give them a copy of the disc or the folder.

SlideScope for CT, MRI and X-ray discs: the facts

• SlideScope opens the DICOM files on a hospital CD, DVD or USB stick on Windows 10/11 and macOS 14 or later.
• Home has a button, “Got a scan on a CD or USB? Open it”, that finds the disc by its DICOMDIR index, whatever drive letter it got.
• Opening the disc’s DICOMDIR file opens the whole disc; each CT or MRI series is listed once, by name and number of images.
• All slices of a CT or MRI series open as one scrollable stack, in the order the scanner took them, with the real distance between slices.
• CT scans can be shown in the soft tissue, lung, bone and brain windows, or the windows the scanner saved.
• Distances are measured in millimetres.
• A disc can be copied to the computer with “Copy this scan to your computer”, into a SlideScope Scans folder in Documents.
• Medical scans stay on the computer: SlideScope does not send them to AI, save them to the cloud or share them by link.
• SlideScope is for viewing, not diagnosis. It does not interpret scans; your doctor or the radiologist’s report explains what a scan shows.

Questions people ask