Ewout Kramer's presentation on DICOM from a FHIR perspective is a good presentation of first impression issues to consider when dealing with images and FHIR. Most important is his recognition of the differences between a normalized and composite (document) view of the world.
At SIIM the prespective will be that the PACS is at the center of the universe. The more general DICOM perspective is:
- The PACS is crucially important, but there will be more than one PACS involved. The PACS centric view dominates the acquisition and early hours of activity, but then things change.
- The PACS will share data with other PACS systems by network. This is sometimes a partnership of equals, and sometimes a federation. But it does mean that data exchange takes place between autonomous partners.
- The PACS may share data with other PACS systems by media (e.g., DVD). This is often overlooked in network centric discussions. Over 300 million patient studies are exchanged annually on CD and DVD. This may be after a significant time delay when the patient brings old studies to a new provider. Media transfers are a significant use of DICOM. The effect on tertiary providers has been about a 20% reduction in procedures performed and reimbursements.
- External workstations, outside reviewers, and other systems are a much lower volume, but exchanges with them are a crucial part of DICOM workflows.
The normalized versus composite (document) organization of data is a very important difference and it deserves all the emphasis it is given in the lecture. It will need considerable discussion and thought.
DICOM has both normalized and composiste forms. The decision to use composite for image objects is an old decision that was driven by many of the same considerations that led to the document model in CDA.
- It matches the medical workflows that were well established by 100 years of medical practice in radiology at the time DICOM was introduced. Matching the system design to the user workflow is a good idea.
- It meets the need for autonomy of operation between independent PACS, workstation, and other systems. Managing one normalized view over many independent systems is very hard.
- It meets the clinical need to present the state of knowledge at the time of capture. Medical decisions need access to both the state of knowledge at the time of examination and the current state of knowledge. You can't compare an old with a new study without having both.
Missing from the video, and likely missing from the SIIM hackathon, are the implications of sheer size. Medical images are more than just "large". You cannot just say "use a faster machine and network" when a single complex medical study can exceed 1 gigabyte in size and a collection of studies must be usable on within the short time limits allowed by current medical practice workflows. PACS systems expect to deal with data volumes measured in terabytes per day. This must be designed in.