Terminology of advanced features


An “Orthanc peer” is another instance of Orthanc, possibly running on a remote computer.

Contrarily to a “modality”, it is possible to communicate with a peer through the HTTP/HTTPS protocol (i.e. through the REST API of Orthanc), and not through the DICOM protocol. This implies a much easier configuration: It is only required to know the URL, the username and the password to communicate with an Orthanc peer. This contrasts with DICOM query/retrieve, that is quite complex and that involves a lot of pitfalls (cf. the FAQ entry about troubleshooting DICOM communications) that can be bypassed if using HTTP.

Furthermore, as HTTP(S) communications are generally not blocked by firewalls (contrarily to the DICOM protocol that is inherently an Intranet protocol and that often requires the setup of VPN tunnels), it is much easier to setup communications of medical images through the Internet with Orthanc peers.


Because of its focus on low-end computers, Orthanc implements disk space recycling: The patient that has been stored for the longest time inside Orthanc can be automatically deleted when the disk space used by Orthanc grows above a threshold, or when the number of stored patients grows above a threshold. This feature enables the automated control of the disk space dedicated to Orthanc.

Recycling is controlled by the MaximumStorageSize and the MaximumPatientCount options in the Orthanc configuration file.

It is possible to prevent important data from being automatically recycled. This mechanism is called protection. Each patient can be individually protected against recycling by using the Unprotected/Protected switch that is available from Orthanc Explorer.

Note that protection is only available at the patient level. It protects all the studies/series/instances of the patient against recycling. The rationale is that we think it is important to keep available all the data related to one patient. Unwillingly losing a study/series that is part of the same patient might lead to a loss in consistency with respect to the medical history of this patient.


If your disk space is limited, besides Recycling/Protection, you should also consider using disk space compression. When compression is enabled, Orthanc compresses the incoming DICOM instances on-the-fly before writing them to the filesystem, using zlib. This is useful, because the images are often stored as raw, uncompressed arrays in DICOM instances: The size of a typical DICOM instance can hopefully be divided by a factor 2 through lossless compression. This compression process is transparent to the user, as Orthanc automatically decompresses the file before sending it back to the external world.

Compression is controlled by the StorageCompression option in the Orthanc configuration file.

Metadata & attachments

Metadata consists in an associative key-value array (mapping a integer key in the range [0,65535] to a string value) that is associated with each DICOM resource stored inside Orthanc (may it be a patient, a study, a series or a DICOM instance). Metadata can be either referred to using the integer key, or using a symbolic name (if configured). Metadata records information that is not readily available in the DICOM tags.

In spirit, the metadata mechanism is similar to the attachment mechanism. However, metadata is stored directly inside the database, whereas attachments are stored as separate files on the filesystem (the database only stores a reference to the attachments). Choosing between metadata and attachments is a matter of trade-off: Metadata must be kept small (as a rule of thumb, under 1KB) and used if fast access is needed, whereas attachments can be used to store arbitrarily large piece of data.

Also note that metadata and attachments are only available for resources stored inside Orthanc. Once one DICOM instance leaves the Orthanc ecosystem, its associated metadata and attachments are lost.

Core metadata

Here are the main metadata handled by the Orthanc core:

  • ReceptionDate records when a DICOM instance was received by Orthanc. Similarly, LastUpdate records, for each patient/study/series, the last time a DICOM instance was added to this resource.
  • RemoteAet records the AET of the modality that has sent some DICOM instance to Orthanc.
  • ModifiedFrom and AnonymizedFrom hold from which original resource, a resource was modified or anonymized. The presence of this metadata indicates that the resource is the result of a modification or anonymization that was carried on by Orthanc.
  • Origin records through which mechanism the instance was received by Orthanc (may be Unknown, DicomProtocol, RestApi, Plugins, or Lua).
  • IndexInSeries records the expected index of a DICOM instance inside its parent series. Conversely, ExpectedNumberOfInstances associates to each series, the number of DICOM instances this series is expected to contain.
  • Starting with Orthanc 1.2.0, TransferSyntax and SopClassUid respectively stores the transfer syntax UID and the SOP class UID of DICOM instances, in order to speed up the access to this information.

Metadata listed above are set privately by the Orthanc core. They are read-only from the perspective of the end user, as Orthanc internally relies on them.

User-defined metadata

The metadata described above where handled by the core of Orthanc. Orthanc users are however allowed to define their own user-defined metadata. Such metadata are associated with an integer key that is greater or equal to 1024 (whereas keys below 1023 are reserved for core metadata).

You can associate a symbolic name to user-defined metadata using the UserMetadata option inside the configuration of Orthanc:

"UserMetadata" : {
  "SampleMetaData1" : 1024,
  "SampleMetaData2" : 1025

Accessing metadata

Metadata associated with one DICOM resource can be accessed through the REST API, for instance:

$ curl http://localhost:8042/instances/cb855110-5f4da420-ec9dc9cb-2af6a9bb-dcbd180e/metadata
$ curl http://localhost:8042/instances/cb855110-5f4da420-ec9dc9cb-2af6a9bb-dcbd180e/metadata/RemoteAet
$ curl http://localhost:8042/instances/cb855110-5f4da420-ec9dc9cb-2af6a9bb-dcbd180e/metadata/SampleMetaData1

User-defined metadata can be modified by issuing a HTTP PUT against the REST API:

$ curl http://localhost:8042/instances/cb855110-5f4da420-ec9dc9cb-2af6a9bb-dcbd180e/metadata/1024 -X PUT -d 'hello'
$ curl http://localhost:8042/instances/cb855110-5f4da420-ec9dc9cb-2af6a9bb-dcbd180e/metadata/1024

User-defined attachments

Orthanc users are allowed to define their own user-defined attachments. Such attachments are associated with an integer key that is greater or equal to 1024 (whereas keys below 1023 are reserved for core attachments).

You can associate a symbolic name to user-defined attachments using the UserContentType option inside the configuration of Orthanc. Optionally, the user may specify a MIME content type for the attachment:

"UserContentType" : {
  "samplePdf" : [1024, "application/pdf"],
  "sampleJson" : [1025, "application/json"],
  "sampleRaw" : 1026

Accessing attachments

Attachments associated with one DICOM resource can be accessed through the REST API, for instance:

$ curl http://localhost:8042/instances/cb855110-5f4da420-ec9dc9cb-2af6a9bb-dcbd180e/attachments/samplePdf/data
$ curl http://localhost:8042/instances/cb855110-5f4da420-ec9dc9cb-2af6a9bb-dcbd180e/attachments/sampleJson/data

User-defined attachments can be modified by issuing a HTTP PUT against the REST API:

$ curl http://localhost:8042/instances/cb855110-5f4da420-ec9dc9cb-2af6a9bb-dcbd180e/attachments/samplePdf -X PUT --data-binary @sample.pdf
$ curl http://localhost:8042/instances/cb855110-5f4da420-ec9dc9cb-2af6a9bb-dcbd180e/attachments/sampleRaw -X PUT -d 'raw data'

Central registry of metadata and attachments

Obviously, one must pay attention to the fact that different applications might use the same key to store different user-defined metadata, which might result in incompatibilities between such applications. Similarly, incompatibilities might show up for user-defined attachments.

Developers of applications/plugins that use user-defined metadata or attachments are therefore kindly invited to complete the central registry below: