Monday, August 27, 2018

PACS troubleshooting tips and tricks series (part2): Addressing.

In my last blog, I discussed network and transmission issues, in this write-up I’ll discuss “addressing” issues. A DICOM device needs to be configured with three addresses:
  1.   The network (IP address)
  2.  The port number
  3.  The Application level address aka AE-Title

As an analogy, think about the IP-address as a “street address,” the port number as the apartment number (let’s say 38A), and the AE-title the tenant, which can be multiple if the apartment is shared by several occupants.

Imagine that a system cannot communicate; the causes could be due to:

Use "ping" to check network availability
  • IP issuesthis can be caused by incorrect VLAN configuration, router DHCP settings, wireless issues, VPN problems, firewalls or as simple as having the incorrect IP address of your destination. Remember that DICOM was defined in the early 1990’s when no-one thought we would run out of IP addresses, so it relies on fixed IP addresses and cannot handle the dynamic addresses typically issued by DHCP. (Actually there is an option in DICOM that allows it but it is rarely implemented). Testing if you can reach your destination is simple, you use the command to get to the DOS screen and use the ping command with the appropriate IP destination (see illustration 1). To find the IP address of your destination you can use the “ipconfig” command for windows or “ifconfig” for Unix at your destination computer. Wireless connections are challenging, as many devices now are becoming wireless, such as portable ultrasounds, portable x-ray units, and the digital acquisition plates for digital radiography.
  • The challenges with wireless are:
  1. The communication has to be encrypted (remember the metadata in the DICOM files that are transferred contain patient information)
  2.  The IP address has to be fixed, which could be a challenge when moving between different floors and connecting to a different wireless router
  3. There are still immature wireless applications out there, i.e. unstable connections at the devices themselves. Note that the fact that a device communicates today does not mean that it works tomorrow, as there could be a new router installed with incorrect IP settings by your IT department.
·        Port issues – the “well-known” DICOM port is 104, however some devices don’t allow you to allocate this port in this low number range and therefore, a relatively newly “approved” DICOM port is 11112. It is strongly encouraged to always use this officially assigned port (as defined by IANA) when you configure your system. There could be multiple DICOM applications listening to the same port, I had this happen to me after I installed a DICOM viewer on my laptop, which was listening to 104 in the background and grabbing my images instead of the archive I wanted to send to it, which took me an hour or so to figure out. The “netstat” command will show you information about your network, with some of the options including the port numbers and processes that are attached.

Use Verification to check DICOM
AE availability
·        AE-Title (AET) issues – Some systems use the computer host name as an AE-Title, which is poor practice as a single host could have multiple AE-Titles. Some systems have a fixed AE-Title which is even worse, as they should be configurable. It is recommended to have a local “standard” about AE-title assignments, for example, include the hospital abbreviation, location and modality such as BAYL_DT_OUTP_CT1 being CT number 1 in the outpatient department at Baylor Medical Center in Dallas downtown. A good practice is to use capital letters only for the AET to eliminate any case sensitivity issues. Remember, there can be multiple AE-Title’s listening to the same port at the same IP, for example, a PACS system might have different AE-T’s for the archive, database, and workflow provider. Duplicate AE-Titles on the same network will create problems, especially when retrieving information, as the AE-Title is part of the retrieval command.
Most PACS systems will need to add the initiating device to be added to its configuration file for image retrievals, which would explain the fact that you can sometimes list the studies on a workstation but that the subsequent retrieval will fail. Some of the PACS systems allow you to configure it in a “promiscuous” mode meaning that it will listen to any device accessing it at the correct IP and port, also something that is bad practice and should be discouraged. Testing if the DICOM application is running and is able to reply to an initiating device is done by executing a DICOM Verification, aka “DICOM-ping” or “echo” command. Some of the applications allow you to execute this from the application, sometimes you have to find it at a service or other menu, see illustration for a sample.

PACS addressing issues are relatively easy to troubleshoot, if you use ping and DICOM Verification, you should be able to solve 95% of the issues. Sometimes you might find that ping does not make it through because of very strict IT policies and subsequent router settings, and some rogue DICOM implementations might have trouble supporting Verification (which is a requirement for any device that is a DICOM listener). 

If you are new to DICOM O(or like to learn more about its fundamentals) you might want to check out the DICOM textbook, or sign up for our on-line or face-to-face DICOM training classes which have quite a bit of hands-on exercises.

Last but not least, look for more upcoming “troubleshooting tips” which will be on DICOM file support issues.

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