HL7 and DICOM. What’s the difference?

PACS Training - DICOM Basics 101

Here’s the oversimplified layman’s explanation. Think of DICOM as the file type for medical images. These file types communicate in a very specific way. This requires the applications that handle these files to be compatible to certain standards. DICOM is sort of limited to imaging. Whereas Health Layer Seven (HL7) is the protocol for messages. Think of HL7 as a common language that various healthcare applications use to communicate with each other. This can be used to send textual information to and from different systems. So how is DICOM related to HL7?

How HL7 orders work with DICOM MWL

A patient can be checked in to their MRI (Magnetic Resonance Imaging) appointment when they arrive at an outpatient imaging center. In order for the procedure to take place, a physician must place an order for the MRI exam. This order is sent to the Radiology Information System (RIS). This is called a HL7 ORM (order entry) message. The HL7 message will contain important patient identifiers such as name, date of birth (DOB) and Medical Record Number (MRN). It will also include details specific to the MRI exam including, but not limited, to modality, study description and accession number. The RIS will populate this information on a DICOM Modality Worklist (MWL). Basically, the patient study details will appear on a list of studies that need to be performed that day. MRI modalities in the clinic can query this list. This query is performed through a DICOM operation called a C-FIND. The modality sends a C-FIND Request query to the RIS and the RIS responds with a C-FIND response with a list of studies that match the query parameters. The MRI technologist can search through the modality worklist and expect to see the checked-in patient on the list. By selecting the patient from the list, the study details will be applied to the images as DICOM tags (metadata). The image plus the tags make up a DICOM file which can then be stored in the PACS using the DICOM C-Store operation.

HL7 in Final Report Viewing

Another way HL7 is used with DICOM is for report association. Upon completion of this MRI exam, the results need to be read by a radiologist. This is typically done in a separate application from the PACS. The voice dictation system. Upon completion, the textual report generated by the radiologist is sent from the voice dictation system to the patient’s record (EMR) and to the PACS. It’s sent via.. you guessed it.. an HL7 message. The HL7 ORU (Observation Result) message. This message becomes available in the PACS so that end users can read the final text report while viewing the DICOM images.


Hope this was a helpful explanation of how DICOM is related to HL7. DICOM and HL7 are each distinct standards that help transmit patient data. DICOM is restricted to imaging while HL7 is a broader language that all health information software are required to speak. This was a very simplified explanation. For more information go directly to the Health Layer Seven and DICOM Standard websites for more details.

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