Digital Imaging and Communications in Medicine (DICOM)

Wikipedia has a very good high level description about DICOM and the protocol specifications can be found at the DICOM Homepage. This page will focus on wireshark specific topics.

Disclaimer: 

Wireshark is not a Medical Device and therefore exported files MUST NOT be used for any clinical processes. The exported file are solely for data and communication interpretation purposes, and the implementation does not claim to be a reference. For certain network captures, the different PDUs are not resembled in the correct order, i.e. leading to invalid DICOM files. So when looking at the files e.g. using a DICOM editor, apply the necessary care, as far as the interpretation of the results go.

History

DICOM is the third version of a standard developed by American College of Radiology (ACR) and National Electrical Manufacturers Association (NEMA) and was released in 1993. Previous standards did not include network support. For more information about the history, please refer to [1] & [2]

Protocol dependencies

Example traffic

Following screenshot shows a DICOM communication containing a C-ECHO followed by C-STORE request.

dicom_assoc_accept.png

The accepted or rejected presentation contexts are decoded, to quickly identify negotiation issues.

Example capture file

XXX - Add a simple example capture file to the SampleCaptures page and link from here (see below). Keep this file short, it's also a good idea to gzip it to make it even smaller, as Wireshark can open gzipped files automatically.

Wireshark

Starting with Wireshark 1.2.1, the DICOM dissector has many new features.

Known issues

Currently, the biggest issue is still the proper reassembly of more than one PDU [3]. This also affects the export.

Preference Settings

Following settings are available to influence DICOM dissection.

dicom_default_pref.png

Display Filter

A complete list of DICOM display filter fields can be found in the display filter reference

Capture Filter

You cannot directly filter DICOM protocols while capturing. However, if you know the TCP port used (see above), you can filter on that one.

DICOM Export

First make sure to have a valid DICOM capture, including Association Request. Then, select File -> Export -> Objects -> DICOM.

dicom_export.png

Depending on the minimum size defined in the preferences, you will see more or less items in the list.

The Save all dialog is a little tricky, if the 'Browse for other folders' is expanded. Make sure to be in the parent directory and only highlight the target directory, don't open it.

dicom_export_save_all.png

Conformance statement

For the DICOM Export, following UIDs are used. Since the SOP Class UID (0008,0016) and SOP Instance UID (0008,0018) are mandatory elements in the meta header, they are created if needed.

Troubleshooting DICOM

Wireshark is an ideal starting point to troubleshoot DICOM connectivity problems. Most often, the involved DICOM devices run on different operating systems, are from different vendors and sometimes are rather closed devices. In addition, the log files on those devices and cannot show both ends.

Basic connectivity problems can be identified just by using Wireshark captures. If it is more than that, it should at least be possible to tell, at which end to start.

To help quickly identify common scenarios, the DICOM dissector is creating 'Expert Info' marks as shown next.

dicom_expert_info.png

In the Paket Details tree, warnings are highlighted as follows:

dicom_expert_highlight.png

Association Request & Response

If one can't get beyond this, it's most likely a DICOM configuration or network setup issue.

Transfer Syntax

To better understand this topic, it is helpful to understand the term 'Presentation Context'. In modern terms, it can best be described as a channel. So, in a single TCP Session (association) one can have multiple channels (presentation context). Each channel has a particular purpose (abstract syntax) and an agreed encoding (transfer syntax). It's part of the association request and response, to agree on a single transfer syntax per presentation context. This can be seen pretty well in wireshark.

The device issuing the A-ASSOCIATE request, will put the most preferred syntax as first element in a presentation context. The provider will then make a decision based on the offered list and priority, which one to use and tell that in the A-ASSOCIATE reply. Since 'Implicit Little Endian' is the least common denominator, there should no be a problem here, unless there's an implementation error.

Query and Retrieve

This is the tricky part. Things can go wrong in the query part, or in the transition to the object transfer.

C-FIND Issues

C-MOVE Issues

Performance

Looking at the timestamps is by far the best method to figure out why a transfer is slow.

Tag Data

If things still don't work, despite of proper communication, it may be an issue of the actual data being transferred. Here Wireshark is only of limited use. For this purpose, the DICOM export was introduced, to see whether the data can be read from a file. If not, it time to contact the respective device vendor(s).

Discussion

Protocols/dicom (last edited 2010-04-06 21:25:33 by GuyHarris)