.DCM file signature
A DICOM file begins with the bytes 44 49 43 4D (DICM in ASCII) at offset 128 (0x80). The signature is the same whatever the file is named, which is why a renamed or extensionless DICOM file is still recognisable.
Signature
| Offset | Hex | ASCII |
|---|---|---|
| 128 | 44 49 43 4DAfter a 128-byte preamble. Some old files omit the preamble entirely. | DICM |
MIME type: application/dicom · usual extensions: .dcm, .dicom
How to check a file's signature
Without installing anything: drop the file on our detector. It reads the bytes in your browser and shows the hex dump alongside the verdict; nothing is uploaded.
macOS and Linux (Terminal):
xxd -l 32 file.dcm
file file.dcmWindows (PowerShell):
Format-Hex -Path file.dcm -Count 32This signature sits at offset 128, so look there rather than at the first bytes: xxd -s 128 -l 16 file.dcm.
If the bytes do not match
A file named .dcm whose first bytes are something else is not a DICOM file. The common explanations are a wrong or deliberately misleading extension, a download that saved an HTML error page instead of the file, or a container such as ZIP wrapping the real content. The detector reports what the bytes say; the DICOM guide explains what a genuine file should contain.