DICOM transfer syntaxes explained
“Unsupported transfer syntax” is the most common reason a DICOM file will not open. This page explains what the term means, how to find it in your file, and what each common value is.
Last reviewed 18 October 2026. Written by the dicompeek editors; sources are listed at the end.
What a transfer syntax is
A transfer syntax is the set of encoding rules used for a DICOM data set. It answers three questions: are the data types written out in every element (explicit VR) or implied by the dictionary (implicit VR); which byte order do multi-byte numbers use (little or big endian); and is the pixel data stored raw or compressed, and if compressed, with which method. It is identified by a UID and stored in the file's meta header asTransfer Syntax UID, tag (0002,0010). A program must know the transfer syntax before it can read anything after the header, which is why the header itself is always written the same fixed way.
How to find it in your file
Open the file in the tag viewer and look at (0002,0010), or open it in thedicompeek viewer: when it cannot show the picture it names the transfer syntax and still lists the tags. Compare the UID with the table below.
The common ones
| UID | Name | What it is | dicompeek |
|---|---|---|---|
1.2.840.10008.1.2 | Implicit VR Little Endian | The default every DICOM system must support. Uncompressed. | Opens |
1.2.840.10008.1.2.1 | Explicit VR Little Endian | Uncompressed, the usual choice for files. | Opens |
1.2.840.10008.1.2.1.99 | Deflated Explicit VR Little Endian | Explicit VR, with the whole data set deflate-compressed (as in zip). | Not yet |
1.2.840.10008.1.2.2 | Explicit VR Big Endian | Retired from the standard, still met in old files. | Opens |
1.2.840.10008.1.2.5 | RLE Lossless | Simple run-length compression. Pixel values are exact. | Opens |
1.2.840.10008.1.2.4.50 | JPEG Baseline (Process 1) | Lossy JPEG, 8 bits per sample. | Opens |
1.2.840.10008.1.2.4.51 | JPEG Extended (Process 2 & 4) | Lossy JPEG, up to 12 bits. | Not yet |
1.2.840.10008.1.2.4.57, .70 | JPEG Lossless (Process 14; .70 is selection value 1) | Lossless JPEG. The standard names .70 as its default transfer syntax for lossless JPEG. | Not yet |
1.2.840.10008.1.2.4.80 | JPEG-LS Lossless | Lossless compression designed to be simple and fast. Pixel values are exact. | Opens |
1.2.840.10008.1.2.4.81 | JPEG-LS Lossy (Near-Lossless) | Each pixel may differ from the original by a fixed small amount. | Opens |
1.2.840.10008.1.2.4.90 | JPEG 2000 (Lossless Only) | Wavelet-based, exact pixels. | Opens |
1.2.840.10008.1.2.4.91 | JPEG 2000 | Wavelet-based, lossy or lossless. | Opens |
1.2.840.10008.1.2.4.201 to .203 | High-Throughput JPEG 2000 | A faster variant of JPEG 2000. | Not yet |
1.2.840.10008.1.2.4.100 and up | MPEG2, MPEG-4 AVC/H.264, HEVC/H.265 | Video, for endoscopy and similar recordings. | No |
Many more exist, most of them retired. The complete list is in PS3.6 Annex A. “Opens” means that dicompeek can show the pixels of files in that syntax; for any other file it still lists all the tags.
Lossless or lossy?
Lossless methods (RLE, JPEG Lossless, JPEG-LS Lossless, JPEG 2000 Lossless) give back exactly the pixel values that were stored.Lossy ones (JPEG Baseline and Extended, JPEG 2000 Lossy, JPEG-LS Near-Lossless) trade some accuracy for a smaller file. The file'sLossy Image Compression attribute, (0028,2110), is set to 01 when any lossy step has been applied. Whether lossy compression is acceptable for a task is a question for the people responsible for it; it is not a technical default.
Why one scanner writes many syntaxes
The file you hold may not be in the syntax the scanner used. Archives and export software can recompress files, and the media storage profiles for discs restrict which syntaxes may be used on them. So the same series can arrive in different syntaxes depending on the route it took.
If your viewer says “unsupported transfer syntax”
- Read the UID in the message or the tag viewer and find it in the table or in PS3.6 Annex A.
- Try another viewer. Support for the older lossless JPEG and for High-Throughput JPEG 2000 varies between programs.
- Convert the file to an uncompressed syntax with a command-line toolkit such as DCMTK or GDCM, both open source. They have tools that decompress DICOM files; check each toolkit's documentation for the exact command and run it on a copy.
- If you only need pictures, ask the person who exported the files to export them again as uncompressed or as JPEG 2000.
Do not upload the file to a website you do not trust to convert it: it contains patient data. dicompeek decodes in your browser, with two small WebAssembly decoders that are part of this site (OpenJPEG for JPEG 2000 and CharLS for JPEG-LS) and are fetched from dicompeek.com only when a file needs them.
Sources
- DICOM PS3.6 Annex A, Registry of DICOM Unique Identifiers (every transfer syntax UID and name)
- DICOM PS3.5 chapter 10, Transfer Syntaxes and Annex A, Transfer Syntax Specifications
- DICOM PS3.10 chapter 7, DICOM File Format (the meta header is always Explicit VR Little Endian)
- DICOM PS3.11, Media Storage Application Profiles (which syntaxes a given kind of disc may use)
- OpenJPEG and CharLS, the open-source codecs used here (BSD licences)
Not for clinical diagnosis. See the medical disclaimer.