12.3 PACS, Display & Image Management

PACS, DICOM, and the Information Systems

A PACS (Picture Archiving and Communication System) is the digital network that acquires, stores, distributes, and displays medical images, replacing film and file rooms. Its four functional parts are the imaging modalities (the x-ray units), the secured network, archive/storage servers, and display workstations. PACS speaks a single language: DICOM (Digital Imaging and Communications in Medicine), the standard that defines both the image file format and the network protocols used to move images between devices. Every DICOM image carries a header with patient demographics (name, ID, birth date), study/series data, and technical parameters — the header is what lets any DICOM-compliant workstation open the study.

PACS does not work alone. The RIS (Radiology Information System) manages radiology scheduling, exam tracking, and reports; the HIS (Hospital Information System) manages hospital-wide admissions, orders, and billing. A DICOM modality worklist pulls the scheduled patient and exam data from the RIS directly to the modality so the technologist selects the patient rather than typing demographics — this prevents mismatched studies and wrong-patient errors. Image movement uses DICOM services: C-STORE (send/store an image), C-FIND (query for studies), and C-MOVE (retrieve studies) — the query/retrieve workflow.

Archiving and Redundancy

Medical images are legal records with retention requirements, so archiving must be permanent and fault-tolerant. PACS archives use redundant storage (for example RAID arrays), on-site short-term storage for rapid retrieval, and off-site/cloud backup for disaster recovery. Prior studies are auto-retrieved (pre-fetched) from the archive when a new exam is scheduled so the radiologist can compare. A well-designed system guarantees no single hardware failure loses images.

Display (Monitor) Quality Control

Diagnostic displays must be QC-tested so subtle contrast is not lost. Key metrics:

• Luminance and luminance ratio: primary (diagnostic) monitors must be bright and high-contrast. AAPM Task Group 18 (TG-18) sets a minimum maximum luminance of 170 cd/m² and a luminance ratio (Lmax/Lmin) greater than 250 for primary displays; practical guidelines recommend higher luminance (≈350 cd/m²) for chest and mammography reading.
• DICOM GSDF calibration: monitors are calibrated to the Grayscale Standard Display Function (GSDF, DICOM Part 14) so equal steps in pixel value appear as perceptually equal brightness steps. Conformance is verified to within about 10% using a photometer.
• Test patterns: the AAPM TG-18 (and updated TG-270) patterns — such as TG18-QC — are viewed to check contrast, resolution, luminance uniformity, and low-contrast visibility. Luminance nonuniformity should stay under about 30%.
• Ambient light: reading-room lighting must be low and controlled because glare raises the effective black level and buries low-contrast detail.

PACS / Display Component → Purpose → QC or Tolerance

Image Identification and Annotation

Every permanent image must be correctly and permanently identified. Required data include patient name and ID, date, and facility, plus a legally required anatomic side (lead) marker. As tested on the exam, the right/left lead marker is placed within the collimated field, at the time of exposure, without obscuring anatomy of interest — it is primary evidence of side. Electronic (post-processing) annotation of side is a fallback that departments restrict, because a flipped or mislabeled digital marker is a serious medicolegal error; a physical marker exposed with the beam is preferred. A complete permanent record ties the DICOM header (name, unique ID, date/time, accession number) to the visible anatomic marker, and departments audit for unmarked or electronically marked images because an unlabeled side is a repeatable liability. Windowing (window level/width) and cropping are post-processing display adjustments and must never be used to hide clipped anatomy or to substitute for a real marker.

Workflow, Compression, and Technologist Duties

In a filmless department the technologist is the first quality gatekeeper: verifying identity against the worklist, confirming the correct exam, checking the exposure index, adding markers, and cropping/masking only unexposed borders before sending the study with C-STORE. Images may be compressed for transmission — lossless compression preserves every pixel and is used for diagnostic archives, whereas lossy compression discards data and is limited to non-diagnostic uses. Diagnostic monitors need a warm-up period and stable, dimmed ambient lighting before primary reading. Teleradiology extends the workflow off-site, so the same DICOM/GSDF calibration standards must hold on the remote display. A brief scenario: a chest study opens with a prior patient's name still in the header — the technologist must correct the demographic mismatch at the source (worklist/RIS), not merely relabel the image, because the DICOM header is the legal identifier that follows the study everywhere.

Common exam trap: DICOM is the standard/format and transfer protocol, PACS is the storage-and-display system, RIS/HIS are the information systems. If a question asks what carries the patient demographics that must match the order, the answer is the DICOM header; if it asks what prevents typing the wrong patient, the answer is the modality worklist.