DICOM Query/Retrieve

Troubleshooting Q/R Issues in PACS

The best way to learn is through a real life experience. The next best, is simulating one. Imagine a PACS administrator, Sarah, who works in a busy hospital’s radiology department. She receives a support ticket from one of the radiologists, Dr. Roberts, who is unable to retrieve a patient’s MRI study from the PACS into the advanced post-processing software they use for 3D reconstructions. Dr. Roberts reports that whenever he tries to query the PACS for the study, the software either returns no results or fails to retrieve the images entirely, causing delays in patient care.

Understanding the Issue: Sarah starts her investigation by considering the basic principles of how Query/Retrieve works in DICOM. She knows that this process involves two key components in the DICOM network: the Service Class Provider (SCP) and the Service Class User (SCU).

• Service Class Provider (SCP): In this context, the PACS acts as the SCP. It’s responsible for responding to query requests and providing the requested data. When Dr. Roberts queries the PACS for a specific study, the PACS, as the SCP, should search its database and return the relevant information.
• Service Class User (SCU): The post-processing software Dr. Roberts is using acts as the SCU. It initiates the query by sending a C-FIND request to the PACS and then, once the data is found, requests retrieval using C-MOVE or C-GET.

Given that the query isn’t returning results or the retrieval is failing, Sarah suspects there may be an issue in how the SCU (the post-processing software) is communicating with the SCP (the PACS).

Explaining Query/Retrieve in this Context: In DICOM, the Query/Retrieve service allows the SCU to search for and retrieve images from the SCP. Here’s how it generally works:

1. C-FIND (Query): The SCU sends a C-FIND request to the SCP (PACS), specifying criteria like patient ID, study date, or modality to locate the desired images. If the SCP finds a match, it responds with a list of studies or series that meet the criteria.
2. C-MOVE or C-GET (Retrieve): After the SCU receives the query results, it can use a C-MOVE request to ask the SCP to transfer the images to a specific destination, or a C-GET request to have the SCP send the images directly back to the SCU.

If something goes wrong during these steps, it could be due to various factors such as network issues, configuration mismatches, or even DICOM compatibility problems between the SCU and SCP.

Resolution: To resolve the issue, Sarah takes the following steps:

1. Replicate the Issue and Capture Logs: Sarah first replicates the issue to capture the relevant logs. By doing this, she ensures that the logs reflect the exact problem Dr. Roberts encountered, providing valuable data for troubleshooting.
2. Identify the Failing Operation (C-FIND or C-MOVE/C-GET): Sarah determines whether the issue occurs during the C-FIND operation (the initial query) or during the C-MOVE or C-GET operation (the retrieval). This helps her focus her troubleshooting efforts on the specific part of the Query/Retrieve process that is failing.
3. Test with a Different SCU: Sarah attempts to query and retrieve the same study using a different workstation configured as an SCU. If the retrieval works from this SCU, the issue is likely with the configuration of the original post-processing software.
4. Verify Network Connectivity: Sarah checks the network connection between the post-processing software (SCU) and the PACS (SCP). She ensures there are no firewalls or network policies blocking DICOM communications.
5. Check for Recent Changes: Sarah investigates if there have been any recent changes in the environment that could be affecting the Query/Retrieve operation. This includes checking for recent network security changes, system patches, software updates, or any modifications made by another analyst to the PACS or the post-processing software. She also verifies if any changes have been made to the AET, IP addresses, or ports that could cause the issue.
6. Review Logs and Errors to Find Culprit: Sarah reviews the DICOM logs on both the PACS and the post-processing software. She looks for specific error messages that could indicate whether the issue is with the SCP (PACS) or the SCU (post-processing software). The logs may reveal problems like unsupported DICOM commands, failed connections, or issues with SOP Classes.
7. Restart DICOM Services: Based on the log review, if Sarah identifies which system (either the SCP or SCU) is having issues, she restarts the relevant DICOM service. This is often necessary when services have been running for an extended period and might have encountered an error that can be cleared by a restart.
8. Reboot the Q/R Server: If restarting the individual service doesn’t resolve the issue, Sarah proceeds to reboot the Query/Retrieve server of the identified culprit (either PACS or the post-processing software server). Rebooting can resolve deeper issues that a simple service restart cannot fix, especially if there are underlying system resource constraints.
9. Check DICOM Configuration: After addressing potential service and server issues, Sarah verifies that the AE Titles, IP addresses, and ports are correctly configured in both the PACS and the post-processing software. A common issue is a mismatch in AE Titles, which would prevent the SCP from recognizing the SCU’s request.
10. Escalate to Vendor for Level 2 Support: If the issue remains unresolved after all these steps, Sarah escalates the problem to the vendor’s level 2 support team. The vendor might provide more advanced troubleshooting, including potential software patches or configuration changes.
11. Update or Patch Software: Finally, if the issue is determined to be related to software bugs or incompatibilities, Sarah checks for updates or patches for the post-processing software. Installing these updates may resolve the issue.

Outcome: After following these steps, Sarah successfully identifies and resolves the issue. The post-processing software is now able to query the PACS and retrieve the MRI study without any problems. Dr. Roberts can continue with the 3D reconstruction, ensuring that patient care is not delayed.

---

Can you identify what went wrong?

System Information

Here’s what we know about the system. In the latest documentation, here is what the configuration for PACS and the Post-Processing software should be:

• PACS System (SCP):
  • AE Title: PACS_MAIN
  • IP Address: 192.168.10.10
  • Port: 104
• Post-Processing Software (SCU):
  • AE Title: POSTPROC_WORKSTATION
  • IP Address: 192.168.10.20
  • Port: 105

Here is the result of the PACS DICOM logs that were captured in step 1.

Based on the logs above, have you identified the problem?

So what went wrong?

Explanation:

1. C-FIND Operation:
   • The log shows a successful C-FIND operation where the SCU (POSTPROC_WORKSTATION) queries the SCP (PACS_MAIN) and finds a matching study.
2. C-MOVE Operation:
   • The SCU then sends a C-MOVE request to retrieve the study. However, the initial request fails because the SCU requests to send the images to an incorrectly configured AE Title (POSTPROC_WORKSTATION_DEST) with the wrong IP and port (192.168.10.30:106 instead of the correct 192.168.10.20:105).
   • In query retrieve the SCU can request to send the images to another AE. In this case, the Post-Processing workstation is configured to request images to be sent to POSTPROC_WORKSTATION_DEST instead of the original AE, POSTPROC_WORKSTATION.
3. Error and Resolution:
   • The error is logged, and upon investigation, Sarah finds that the AE Title configuration was incorrect. After updating the configuration, the C-MOVE operation is retried and succeeds, with the images being correctly sent to the intended destination.

Root Cause Analysis During her investigation, Sarah discovers that the issue stems from a recent upgrade performed by the vendor. Initially, the post-processing software was set up across several servers to handle different tasks. Specifically, a web server was responsible for handling the C-FIND queries. This web server, AE Title: POSTPROC_WORKSTATION would request that images be sent to a separate, high-performance server called the image server. This image server had the AE Title of POSTPROC_WORKSTATION_DEST.

However, the recent upgrade by the vendor consolidated these functions into a single, more powerful server, eliminating the need for the separate rendering server. Unfortunately, the PACS administrator responsible for managing this change did not update the AE Title configuration to reflect the new setup, nor did they perform any tests post-upgrade. As a result, when the C-MOVE operation was attempted, the Post-Processing application still requested images to be moved to the outdated image server, which no longer existed. This part in the log:

[2024-09-01 10:15:27] [ERROR] C-MOVE operation failed. Unable to establish connection to destination AE Title: POSTPROC_WORKSTATION_DEST [192.168.10.30:106]. Error: “No response from remote AE.”

Understanding DICOM Security and Node Configuration: In DICOM, security is a critical aspect of the system’s design, particularly when it comes to Query/Retrieve operations. You don’t want just any system to query retrieve images from your PACS. In order to safeguard the images in PACS, the promiscuous setting for Q/R is usually turned off. Only allowed nodes can request images. For a Query/Retrieve operation to happen, the nodes (i.e., the devices or software systems involved) must be explicitly configured and recognized by each other. This is done through the configuration of AE Titles, IP addresses, and ports. An unknown node cannot request images from the PACS.

• Why Nodes Need to Be Added: DICOM requires that each device or system (node) that wishes to communicate with another must be registered in both systems. This means that the PACS system (SCP) needs to have the post-processing software’s AE Title, IP, and port correctly configured, and vice versa. This registration acts as a security measure to ensure that only authorized systems can access and retrieve sensitive medical images. Without proper configuration, a Query/Retrieve request could be rejected, or worse, data could be sent to the wrong destination, as happened in this scenario.

Resolution: Once Sarah identified the root cause of the issue, she took the following steps to resolve it:

1. Correct the Configuration: Sarah updated the AE Title configuration to reflect the new setup where the post-processing software now operates on a single, consolidated server. She replaced the outdated AE Title POSTPROC_WORKSTATION_DEST with the correct AE Title, IP address, and port for the newly upgraded server.
2. Verify the Configuration: After updating the settings, Sarah carefully verified all related configurations to ensure no other remnants of the old setup were still in place. This included checking the AE Titles, IP addresses, and ports for both the PACS system and the post-processing software. She Q/Red test images.
3. Replicate the Operation: Sarah then replicated the Query/Retrieve operation with the actual study to confirm that the issue had been resolved. The log now showed a successful C-MOVE operation, with the images being correctly sent to the intended post-processing workstation.
4. Document the Change: To avoid confusion in the future, Sarah documented the change in the system configuration and communicated the update to all relevant stakeholders, including her fellow PACS administrators and IT staff.

Outcome: After correcting the configuration error, the post-processing software was able to successfully query the PACS and retrieve the MRI study without any further issues. Dr. Roberts could proceed with the 3D reconstruction, ensuring that patient care was not delayed.

---

Conclusion

This scenario highlights the critical importance of keeping system configurations up-to-date, especially following major upgrades or changes in infrastructure. DICOM’s security protocols necessitate that all nodes involved in Query/Retrieve operations are correctly configured to ensure that sensitive medical data is properly transmitted. Missteps in this process, such as failing to update configurations after a system upgrade, can lead to significant workflow disruptions. By following a structured approach to troubleshooting and resolution, PACS administrators can maintain the integrity of the imaging workflow and ensure that their systems function smoothly and securely.

Understanding the roles of SCP and SCU in the DICOM network is essential for diagnosing and resolving these issues efficiently. By following a structured troubleshooting approach, PACS administrators can quickly identify and resolve problems, minimizing disruptions to patient care and maintaining the integrity of the imaging workflow.

---