Friday, July 13, 2012

DICOM configuration and Unique Addressing

Unique addressing is equally important
for electronic as well as physical  mail

A medical device that intends to exchange information over a network requires unique addressing at various levels for network cards, routers and applications in order to communicate. Unique addressing requires that there is only one address within a certain domain and therefore guarantees that any information routed to that destination will be delivered only to that unique address. It is no different than the street address used by postal mail. Interestingly enough, I discovered only a few years ago that several countries have no unique address, and homes are often identified with descriptions such as “3rd house with the green door across from the Catholic church.” Obviously if this type of addressing does not keep up with new construction, the mail is often returned undelivered. This would not be acceptable for the delivery of patient images and related information.

Application level addressing for DICOM devices is done by assigning an Application Entity (AE) Title. The protocol to negotiate a DICOM association requires the declaration of a Calling and Called AE Title. The Calling AE Title identifies the association initiator, which is in almost all cases the Service Class User (SCU), and the Called AE Title identifies the destination, or in almost all cases the Service Class Provider (SCP). A device such as a modality will typically have multiple destinations, for example, a CT might be routed to a PACS, an image processing workstation, and one or more Teleradiology destinations to send the images to after hours.

The AE Titles are typically managed by a PACS administrator similar to IP address management, which is usually done by IT. Uniqueness is important, and some kind of “system” does make sense as well. As an example, when I visited Alaska, I found that AE Titles for each device are given a prefix using the town’s airport code (virtually every little town in Alaska has its own airstrip and therefore code). Others use site ID’s or abbreviated hospital names. A typical AE Title could be STJOE_CT01_ER. Remember that the length of AE Titles is 16 bits and case sensitive, i.e. STJOE is a different AE Title than StJoe. Some might argue that the Calling AE Title does not have to be unique as there is no SCP capability, however, I disagree on best practices grounds since many destinations check the calling AE Title and in cases where the Calling AE Title is not on the configuration list, the association request is refused. This is only the case if the system to be connected is what we call non-promiscuous, as promiscuous systems do not typically perform any checking of the calling AE Title.

Unique IP addressing is also critical. Unfortunately, the DICOM communication protocol does not facilitate dynamic IP addressing, which is sometimes an issue, especially when using mobile and/or portable devices. There is actually a DICOM protocol defined to allow for dynamic addressing as part of the so-called DICOM configuration management protocol, but that is rarely implemented.

The last time that I heard issues about using duplicate IP addresses was several years back and this no longer seems to be a big issue. One needs to be careful with re-imaging a computer configuration on different machines using the same image, especially when using virtual machine technologies, as the IP address might get re-imaged as well.

A network connection also requires a unique port number at the receiving device. There is some confusion about the port number to use for DICOM Associations as the most common or “well known” port number, 104, sometimes cannot be used due to restrictions by some operating systems. Vendors have been resourceful and used other, non-standard numbers such as 6001 and others. However, the best port number to use is 11112 as this is the Internet Assigned Numbers Authority (IANA) officially assigned port for DICOM applications. I have experienced issues in the past when there are multiple applications running on your computer, some of them automatically start with assigning port 104 to DICOM applications. In such cases, images being received might get “lost” as another application might be taking over control of that port. A port scanner may be useful in that case, which is available as a free tool on-line.
Configuring a modality with the AE Title(s), port number and IP address is relatively straight forward, a demo of an example using a modality simulator can be seen here.

In summary, one needs to make sure that AE Titles are unique; these are typically managed by a PACS administrator. The same requirement for uniqueness applies for IP addresses, and the recommended port number for DICOM connections is 11112.