Transcoding of DICOM files

General information

As of release 1.5.7, Orthanc does not feature support for transcoding DICOM instances yet. In other words, the Orthanc core never changes the transfer syntax of some DICOM instance when it has to send it to another modality using the DICOM protocol.

Adding support for transcoding is one of the features that is pending on our roadmap, and for which we are looking for industrial sponsors.

Motivation for transcoding

Let’s consider the following basic workflow, in which some imaging workstation must access a medical image that originates from a PACS and that is served through an Orthanc proxy:

This is quite a common situation, e.g. in university hospitals where researchers must access medical images without having authorization to log in the clinical PACS. It is also common if the main PACS restricts the number of workstations that can directly be connected to it, or if Orthanc acts as gateway through Internet.

The problem is that the software running on workstations might not be able to display some DICOM transfer syntaxes. This is especially true in research software, that is often limited to uncompressed transfer syntaxes. For instance, let’s consider the following scenario where a workstation wants to access an image from the PACS:

A typical PACS system will decide, when requested to export an image using DICOM C-Store, to compress the image in order to reduce the network bandwidth and the storage requirements. Orthanc is fine with it: As a vendor neutral archive, Orthanc can basically receive/store/transmit any DICOM transfer syntax. Unfortunately, this might not be the case of the target workstation, that is often limited to some selected transfer syntaxes. As a consequence, the workstation will complain about not being to read the DICOM file (in the situation depicted above, because the PACS has decided to send the DICOM image using the JPEG2k transfer syntax).


There are basically 4 solutions to this issue. The first one, as stated above, would be to implement transcoding in Orthanc. Feel free to get in touch with us if you want to sponsor this development.

The second solution consists in making Orthanc refuse to accept the transfer syntaxes that are not supported by the workstation. This is depicted in the following diagram:

If Orthanc tells the PACS that is doesn’t accept, say, DICOM JPEG2k, the source PACS will be aware of this, and will transcode the DICOM file before it is sent to Orthanc. This is the role of the following configuration options that specifies which transfer syntaxes are accepted by Orthanc:

  "DeflatedTransferSyntaxAccepted"     : true,
  "JpegTransferSyntaxAccepted"         : true,
  "Jpeg2000TransferSyntaxAccepted"     : true,
  "JpegLosslessTransferSyntaxAccepted" : true,
  "JpipTransferSyntaxAccepted"         : true,
  "Mpeg2TransferSyntaxAccepted"        : true,
  "RleTransferSyntaxAccepted"          : true,
  "UnknownSopClassAccepted"            : false

If all of those options are set to false, Orthanc will only receive uncompressed transfer syntaxes (obviously provided that the source PACS supports DICOM transcoding).

The third solution consists in applying an external conversion tool to every DICOM image that is received by Orthanc. The standard command-line tools gdcmconv from GDCM or dcmconv from DCMTK can be used to change the transfer syntax of a given DICOM file. These tools can be invoked from a Lua script (check out OnStoredInstance() callback) or from an Orthanc plugin (check out OrthancPluginRegisterOnStoredInstanceCallback() function). A sample Lua script that converts every incoming DICOM file to the JPEG2k transfer syntax is part of the Orthanc sources.

Finally, as a fourth solution, it is possible to combine two Orthanc servers, the first one being configured to accept any transfer syntax, and the second one being responsible to serve the DICOM files after conversion to uncompressed transfer syntax (which should be compatible with any workstation):

In this solution, a plugin or an external script continuously monitors the content of the first Orthanc server thanks to its REST API. Whenever a DICOM instance is received by the first Orthanc, the plugin/script uses external conversion tools to convert the instance to an uncompressed transfer syntax, then forward it to a second Orthanc server. In other words, the first Orthanc server acts as a transient buffer for decompression. Contrarily to the third solution, this solution has the advantage of better scalability (as decompression implemented in a Lua callback blocks Orthanc as long as the Lua script has not returned).