Standards Watch | DICOM & HL7: Friend or Foe?

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The cooperation between the DICOM and HL7 standardization organization has been significantly improved over the past several years. There is a joint DICOM/HL7 working group, a.k.a. WG 20 for DICOM or for HL7, the Imaging and Integration Special Interest Group, that meets at every HL7 event (which is typically three times a year) to work on interfacing issues between both communication standards. The good news is that each of the standards has its own, distinct domain and therefore, there is relatively little, if any, overlap between both of them so that the coordination activity in this working group is focused on the boundary between the imaging (DICOM) and IT (HL7) domain.

When teaching, I get the question often, "why couldn't this be done with a single standard?" The reason? Each one is specialized to meet specific needs for its domain. For example, it would be an unnecessary burden to add the DICOM standard with the extensive number of different data types that are defined in HL7 to facilitate the variety of messages. In addition, burdening HL7 with artifacts that are defined, for example, to manage image quality, or to facilitate the representation of multi-dimensional datasets such as those used when acquiring 4D ultrasound, would be a gross overkill as well for the HL7 standard. There also is the issue that the encoding used in the current HL7 standard would cause a very inefficient encoding of the image pixel data. However, for the new version 3.0, this should be less of an issue.

From an input to imaging perspective, the HL7 to DICOM interface is defined by the so-called DICOM Modality Work List (MWL), which communicates the patent demographics, radiology scheduling and order details to the modalities in the radiology department. This interface is well defined, and is implemented by almost every new imaging modality today. Mapping between the corresponding HL7 transactions that contain the admission information and orders and this work list, is defined as part of the Integrating the Healthcare Enterprise (IHE) Scheduled Workflow Profile. (See www.rsna.org/ihe for more details on these profiles.) The joint DICOM/HL7 working group is defining the corresponding interface and mapping between the new version 3.0 of HL7 and the DICOM MWL to allow early 3.0 implementations (version 3.0 is about to be implemented in a couple of clinical sites).

From the imaging output perspective, i.e. considering the information that typically flows from the PACS to the RIS, there is status information exchanged about the examinations, as well as reports, measurements, observations, identification of key images and/or presentation states of those images, as well as images themselves, that need to be provided to a wider distribution within the healthcare enterprise. The support of status information by a RIS such as completion/canceling of the procedure, number of images generated and procedure details, in the form of the DICOM Modality Performed Procedure Step (MPPS), is unfortunately not keeping up to par with the availability of this relatively new service by the new digital modalities.

I sometimes compare the HL7 world with an elephant, which takes its time to move in a certain direction, versus the DICOM jaguar, that is agile and even maybe somewhat more aggressive. It seems that the same comparison can be made for the imaging industry and IT; changes in standardization are readily adopted, or at least within a few years for acquisition modalities and PACS, while the IT world definitely takes its time. Some of that is due to the sheer size of the installed base; however, I believe there is no real good excuse to not implementing the MPPS more rapidly by RIS vendors. The impact on the user when the MPPS is implemented is rather significant: if implemented, a radiologic technologist, does not have to go to a RIS terminal to change an order, for example if the decision is made to append the procedure based on what is observed during the first part of the procedure, or just merely cancel it in case a patient feels unwell. The information will be automatically communicated from the modality via the so-called MPPS manager back to the RIS. Support of MPPS at the RIS definitely has a positive impact on the workflow of the technologist allowing him or her to be more efficient.

Another important interface issue has to do with the distribution of images outside the radiology department for the physicians. Vendors are mostly using web-based