Why a DICOM file will not open, and what to try
A DICOM file that will not open is usually one of a few things. It may not be an image at all, the copy may be incomplete, or the pictures may be in a different folder from the file you tried. Most of these are quick to check.
These checks are about files, not about what is in them. Questions about what a scan shows belong with the patient’s doctor.
It may not be a picture file
Not every file in a scan export holds pixels. A CD or download often contains several kinds of file, and only some of them are images.
- DICOMDIR. This is an index of the disc. It lists the patients, studies and series and where their files are. It has no pictures of its own, so opening it alone shows nothing useful. See DICOMDIR explained.
- Reports and structured data. Some DICOM objects hold a report, measurements or display settings instead of an image. They are valid files and they have no pixel data to show.
- Other files in the export. Viewer programs, readme files,
.xmlor.htmlpages and.ziparchives often sit beside the scan. They are not DICOM.
The name may be misleading
DICOM files are often called .dcm, but the extension is only a habit. Many exports have no extension at all, with names like IM000001 or a long string of numbers. A real DICOM file can have any name, and a file named .dcm can be something else.
A quick test is to look at the file’s bytes. A standard DICOM file has 128 bytes of preamble followed by the letters DICM. A hex viewer on a computer will show it. If those letters are missing, the file may still be DICOM in an older style, or it may not be DICOM at all.
Also check that the file is not a renamed .jpg, .png or .zip. If a .dcm opens in an image viewer as a normal photo, it was never DICOM. If it is a zip, extract it first and open the contents.
The copy may be incomplete
A file that was cut short during a download, an email transfer or a copy from a disc will often fail. Signs of a bad copy:
- The size is much smaller than the original or than similar files in the same folder.
- The CD had scratches and some files copied with errors.
Copy the files again, preferably straight from the disc or the original download to a folder on the phone or computer. If you received it through a chat app, ask the sender to send it as a file or a zip, because some apps change or shrink what they send. If the same file fails after a fresh copy, ask the sender or the imaging centre for another export.
The images may be in another folder
On a patient CD the images are usually split across many subfolders, one per series, often with short cryptic names. If you copied only one folder, or only the top level, you may have the index and none of the pictures. Open the whole disc folder, not a single file.
Keep the folder structure as it was. Moving files around can break the links the index relies on.
Compression and unusual encodings
DICOM images can be stored in several ways: uncompressed, or compressed with methods such as JPEG, JPEG 2000 or RLE. Each file declares how it was stored, in a field called the transfer syntax. Most software handles the common ones. A viewer may still fail on an unusual or rarely used encoding, or on a file that claims one encoding and contains another.
If a file opens on one device and not on another, the encoding is a likely cause. Try a different viewer, or ask the imaging centre to re-export the study in a common format. They can usually do this without redoing the scan.
What the app should tell you
A good viewer says plainly when a file cannot be shown, instead of showing a blank screen. Radix DICOM opens single .dcm files and whole patient-CD folders, including nested subfolders and files with no extension, and it reports when a file fails to open. That message is a prompt to run the checks above.
If it still fails
If the file is a real image, the copy is complete and a second viewer cannot open it either, contact the clinic or imaging centre that produced it. They can supply a new copy. For anything about what the images mean, ask the patient’s doctor.