By Garrett Seeley
In continuation of the previous article, one of the essential concepts that we did not fully explain was that of a Picture Archive and Communications System (PACS). Regardless of the complexity, people using this term refer to the place the local modalities (CT, MRI, or digital X-ray, etc.) store their images. Sometimes PACS is a single computer, sometimes it is a whole network of machines working together to accomplish the task. Regardless of the back-end structure, PACS is where the images are stored.
However, the PACS network has a little more complexity than just a place to store images. What a radiology department really needs is a way to get patient information to the X-ray machine, a way for the machine to understand what the patient needs to have done and therefore how to configure itself for that study, a way to store the images with a station for a doctor to review, and a text result transcribed back to the hospital’s patient record.
This is what a modern system looks like:
To understand this, we must first cover HL7, the communication protocol for the Electronic Medical Record (EMR) system. It is used by the hospital as the main patient record system. HL7 is text based. Although that allows for quick exchanges of text information, it does not store images or video; that is PACS’ job. Therefore, a hospital network is twofold: the machines that use text to communicate with each other (represented in red on the graphic), and those that exchange pictures and videos with each other (represented in green). The EMR is where the patient records are kept, it is also where a doctor enters in their request for a medical study. This request is forwarded, in HL7 format, to the Radiology Information System (RIS). The RIS is a scheduler, plain and simple. Radiology uses its own system to build a list of what studies to do. A RIS keeps the schedule for the radiology department, but it is often still in text format. To send patient and study information to an imaging modality, a worklist is used. A worklist is literally a list of what modalities must do that day, but in DICOM format. Recall that a DICOM image has a header and headers have text in them. Therefore, DICOM can be used to exchange text information in the form of a header without an image. That is what the worklist stores, and it communicates this information to the modalities, so they know who the next patient is and what study is being done. That study is called an accession. It is a DICOM study number and is one of the most critical bits of information for the rad techs to verify before acquiring the images. Just keep in mind that this patient schedule was built using information from the EMR using the RIS, stored in the worklist server.
Once the worklist is constructed, a queue is built for the modalities. The modalities then perform a query to find their next patient. The user clicks the correct accession number for the patient study and the modality retrieves the partial DICOM file from the worklist. What comes to the modality is a DICOM file with the patient and study data, but without an image. The modality then performs the study, creating a series of images for each task required in the study. All of this is then pushed to the PACS where it is stored until a radiologist requires the image. The radiologists have their own “to do” list, which is stored on the PACS and can be as simple as an unread or read flag on the accession number. The radiologists select a patient, lock the study, read the image, transcribe their findings to HL7 to be stored back in the EMR, and then release the study in PACS with the status as read. The final goal of the transcription is to give the requesting doctor the information from the radiologists in text format, so that the information can guide a diagnosis, therapy or intervention as required.
Understanding the workflow of the overall network will help in advanced troubleshooting of DICOM networks. It helps to see where the image is coming from, where we are in the chain of the transfer, and what to look for in the breakdown of communication. Keep in mind that DICOM requires both sides of a communication be set up to talk to each other. That means that each modality must be set in each server, and at a minimum, each modality must at least have the settings of the worklist server and the PACS. Since there are often additional servers with specialized functions, and over 100 modalities (or more) in a hospital system, there are hundreds of points of failure for DICOM settings. This is one of the main reasons why network troubleshooting has become a full third of a biomed’s job.
