Infrastructure

What Is PACS?

A PACS (Picture Archiving and Communication System) is the system that receives medical images from imaging equipment, stores them for the long term, and serves them to the radiologists and referring physicians who read them. It also bridges out to the EHR, reporting, and billing. DICOM is the language a PACS speaks; the PACS itself is the database, the file store, and the access layer around them.

The four components

  1. Acquisition gateway: DICOM endpoint that receives C-STOREs from modalities. Often does on-the-fly routing, de-duplication, and transfer-syntax conversion.
  2. Archive: the durable store. Relational DB for metadata + blob store (local disk tier → SSD hot tier → tape or cloud cold tier) for pixel data.
  3. Reading workstations: where radiologists interpret. Pull via C-MOVE, C-GET, or (modern) WADO-RS. Includes hanging protocols, measurement tools, and voice dictation integration.
  4. Web distribution: zero-footprint viewers for referring physicians, patient portals, and remote reading. OHIF, Weasis, MicroDICOM, and PACS-vendor web viewers all fill this slot.
One study, four stops. Images arrive from the modality at an acquisition gateway, land in a tiered archive, and are served to two very different audiences: radiologists reading diagnostically, and everyone else on a zero-footprint viewer. Every hop speaks DICOM, which is what lets any one box be replaced.

VNA: Vendor-Neutral Archive

A VNA is a storage layer that separates image archival from the PACS application tier. The idea: when you replace your PACS vendor every 5-8 years, you should not have to migrate 10 PB of imaging data with them. A VNA speaks DICOM (and usually XDS-I / DICOMweb) and owns the long-term storage; the PACS caches what's hot and reads from the VNA on demand.

VNAs shine in multi-facility health systems where different sites run different PACS vendors but want one logical archive. They also decouple the archive-sizing calculation from the PACS-licensing calculation, which tends to save money at scale.

VNA vs PACS archive: a PACS archive is typically vendor-locked: the database schema, storage format, and retrieval APIs are proprietary. A VNA is neutral by design: any DICOM-compliant PACS can point at it, and data migration to the next VNA is a DICOM-level export rather than a database-dump-and-rewrite.

The difference is where the archive lives. In a single-vendor PACS the application tier and the archive ship together, so replacing the vendor means moving the images. A VNA pulls archival out from under the application, and several PACS can read the same logical archive.

Cloud PACS

"Cloud PACS" encompasses three distinct patterns. Don't confuse them:

  • Hosted PACS: a vendor-managed PACS running in their cloud tenant. You still have a PACS; it's just not in your data center.
  • Cloud-native archive: AWS HealthImaging, Google Cloud Healthcare API, Azure Health Data Services. Object-storage-backed DICOM stores with native APIs plus DICOMweb.
  • Cloud-bridged archive: on-prem PACS extended with cloud tiers for cold storage. Transparent to clinical users; controlled via lifecycle policies.

See the vendor support matrix: cloud support by attribute →

How a study moves through a PACS

  1. Modality (CT scanner) C-STOREs images to the acquisition gateway after each patient.
  2. Gateway validates, routes, optionally compresses, then C-STOREs to the archive.
  3. Archive persists to hot storage; kicks off an MPPS/Storage Commitment confirmation back to the modality.
  4. Radiologist opens the patient in their worklist; workstation C-MOVEs the current study + priors from the archive to the workstation's local cache.
  5. Radiologist dictates findings; the Structured Report (SR) is C-STOREd back to the archive.
  6. Hot-to-cold tiering runs nightly: studies older than 90 days move to cold storage. Access from cold adds latency but the lifecycle policy is transparent to clients.

Integration points around the PACS

  • RIS / EHR: HL7 v2 ADT + ORM + ORU messages drive the worklist and carry report text back. Modern stacks use FHIR ImagingStudy + DiagnosticReport.
  • Modality Worklist (MWL): modalities C-FIND against RIS/PACS for the day's scheduled procedures.
  • Reporting / speech: PowerScribe, Fluency Direct integrations via proprietary SDKs plus DICOM SR for structured findings.
  • Enterprise viewer: zero-footprint web viewer accessible from the EHR via context launch.
  • AI / CAD: inference pipelines that consume images via DICOMweb, produce annotations as DICOM SR, Presentation States, or Segmentation objects.

Next step

Planning a PACS migration or weighing a VNA?

Saga IT builds the gateways, routers, and DICOM plumbing between modalities, archives, and the EHR, including the migrations that come with swapping either one out.

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