Short answer: HL7 and DICOM are both healthcare standards, but they carry different things. HL7 moves information about the patient and the order: registrations, orders, schedules and finished reports between the EHR, RIS and other systems. DICOM moves the images themselves, plus the data that describes them, between modalities, PACS, viewers and archives. A working radiology department needs both, and most “the study is missing” or “the report never arrived” problems happen where the two meet.
This guide explains each standard in plain terms, shows how they work together through one imaging exam, and points out the handoffs that most often fail.
What is HL7?
HL7 (Health Level Seven) is a family of standards for exchanging clinical and administrative data. In most hospitals and imaging centers, the workhorse is still HL7 version 2, a text-based format made of segments separated by pipe (|) characters. Each message starts with an MSH header, and segments like PID (patient identification), PV1 (visit), ORC/OBR (order details) and OBX (results) carry the content.
Common HL7 v2 message types in imaging include:
- ADT: admit, discharge, transfer and demographic updates (for example, a patient’s name or medical record number changes).
- ORM / OMI: new, changed or canceled imaging orders.
- SIU: scheduling information.
- ORU: results, such as the finalized radiology report going back to the EHR.
- MDM: document notifications.
HL7 v2 messages usually travel over TCP using a simple wrapper called MLLP, and the receiving system answers with an acknowledgment (ACK). The newer HL7 FHIR standard uses web-style APIs and is growing, but many radiology interfaces still run on v2.
What is DICOM?
DICOM (Digital Imaging and Communications in Medicine) is the standard for medical images. A DICOM file holds the pixel data and a header of tags describing it: patient name and ID, accession number, study date, modality, body part and unique identifiers (UIDs) for the study, series and image.
DICOM is also a network protocol. Key services include:
- C-STORE: send images from a modality to PACS, or from PACS to an archive.
- C-FIND / C-MOVE / C-GET: search for and retrieve prior studies.
- Modality Worklist (MWL): lets the scanner pull scheduled exams so demographics aren’t typed by hand.
- MPPS: the modality reports what was actually performed.
- Structured Reports (SR): measurements and dose information in DICOM form.
Systems identify each other by AE title, IP address and port, and each pair must be configured to trust the other.
HL7 vs. DICOM at a glance
| HL7 (v2) | DICOM | |
|---|---|---|
| Main job | Patient, order, schedule and report data | Images and image metadata |
| Typical systems | EHR, RIS, interface engine, billing | Modalities, PACS, VNA, viewers |
| Format | Text messages with pipe-delimited segments | Binary files with tagged headers |
| Key identifiers | MRN, order number, accession number | Patient ID, accession number, Study Instance UID |
How HL7 and DICOM work together: one exam, start to finish
- Registration (HL7 ADT): the patient is registered in the EHR, and demographics flow to the RIS and PACS.
- Order (HL7 ORM): a provider orders a CT. The order, including the accession number, goes to the RIS or worklist provider.
- Worklist (DICOM MWL): the CT scanner queries the worklist and loads the patient and accession number.
- Images (DICOM C-STORE): the scanner sends images to PACS. The accession number in the DICOM header matches the order.
- Reading: the radiologist opens the study in PACS and dictates, often in a reporting system like PowerScribe.
- Report (HL7 ORU): the signed report goes back to the EHR so the ordering provider can see it, and may also be faxed or routed elsewhere.
The accession number and patient identifiers are the thread that ties the HL7 world to the DICOM world. When they don’t match, studies end up unverified, orphaned or attached to the wrong order.
Where HL7 and DICOM integrations usually break
- Identifier mismatches: the modality was used without the worklist, so the accession number or MRN was typed by hand and doesn’t match the order.
- Merges and updates not reaching PACS: an ADT merge happens in the EHR, but PACS never receives or processes it, leaving split patient records.
- Rejected or unacknowledged HL7 messages: the interface engine queues messages because the receiving system returned an error (
AE/AR) or no ACK at all. - Field mapping differences: a procedure code, location or ordering provider is sent in a field the receiving system doesn’t expect.
- DICOM association failures: wrong AE title, port or IP, or a firewall change blocks the connection.
- Report routing failures: the ORU leaves the reporting system but never lands in the EHR, or a fax or print route silently fails.
A good first step is always the same: pick one exam and follow its identifiers through every hop, checking the interface engine logs on the HL7 side and the DICOM logs on the PACS side.
What clients say
“In our PACS Hospital IT environment, we encountered a critical issue with reports failing to send via fax, which disrupted our operations. Cesar’s deep knowledge and troubleshooting skills were instrumental in resolving the problem. He swiftly pinpointed the issue and implemented an effective solution, ensuring our reporting process remained efficient and reliable. His expertise made a significant difference in maintaining the smooth flow of our radiology services.” — Jeff C. in Georgia (Sacred Heart Imaging Solutions, Our Story)
If the root cause turns out to be the network itself, such as switches, firewalls or office workstations at a medical practice, our partner A. Computer Doctor provides IT and network support for medical offices.
Frequently asked questions
Is DICOM part of HL7? No. They are separate standards from separate organizations. HL7 is developed by HL7 International; DICOM is maintained by the DICOM Standards Committee, with NEMA as its secretariat. They are designed to work side by side.
Does PACS use HL7 or DICOM? Both. PACS uses DICOM to receive, store and send images, and usually receives HL7 messages (orders and demographic updates) so studies can be matched to the right patient and order.
What is the accession number? It’s the identifier for a specific imaging order or exam. It appears in HL7 order messages and in the DICOM image header, which is how the two are linked.
Who should troubleshoot an HL7 or DICOM problem? It often spans the interface team, PACS administrators, the RIS or EHR team and vendors. One person coordinating across them usually shortens the fix.
Get help with your PACS integrations
Sacred Heart Imaging Solutions brings 20+ years of healthcare IT experience to PACS and CPACS support, integration with EHR and RIS systems, vendor coordination and workflow optimization. If orders, images or reports aren’t flowing the way they should, contact us, call 951-595-7758 or email [email protected] (Mon–Fri, 8:00 AM–5:00 PM Pacific, remote from Murrieta, CA).
