The Digital Imaging and Communications in Medicine (DICOM) standard, now at version 3.0, is the backbone of modern medical imaging. Unlike standard image formats like JPEG or PNG, DICOM is not just about images—it’s about medical information. It provides a comprehensive framework for storing, transmitting, and managing imaging data in healthcare environments, from acquisition devices to PACS servers and AI systems.
- DICOM vs. JPEG/PNG: What’s the Difference?
JPEG/PNG are general-purpose image formats used for display or web usage. They store image pixels but lack clinical context.
DICOM images, however:
- Contain rich metadata: Patient info, acquisition parameters, device details.
- Follow a structured hierarchy: Patient → Study → Series → Image.
- Support medical-specific compressions (lossless JPEG, JPEG 2000, RLE).
- Carry diagnostic information, including windowing, scaling, and orientation.
- Enable integration with hospital systems like PACS, RIS, and EMR.
- DICOM File Structure: Key Concepts
🔹 Transfer Syntax
Defines how data is encoded (endian-ness, compression). Examples:
- Implicit VR Little Endian (default)
- JPEG Lossless (for image compression)
- Explicit VR Big Endian
🔹 Groups and Tags
DICOM files are composed of Data Elements identified by unique (Group, Element) tags. Example:
- (0010,0010) – Patient Name
- (0028,0010) – Rows
- (7FE0,0010) – Pixel Data
🔹 VR (Value Representation)
Specifies the data type for each tag (e.g., PN = Person Name, DA = Date, UI = Unique Identifier).
🔹 Compression Techniques
DICOM supports:
- JPEG Lossless / Lossy
- JPEG 2000
- RLE (Run-Length Encoding)
- MPEG2/MPEG4 for video loops
🔹 Still Image, Sequence, and Loops
- Still: Single-frame (e.g., X-ray)
- Sequence: Multi-frame (e.g., CT slices, MR series)
- Loop: Cine images or ultrasound clips
🔹 Window Leveling Techniques
Enhance contrast for diagnostic viewing.
- Window Width/Level (WW/WL): Linear contrast control
- Window LUT: Lookup tables for pixel value mapping
- Non-linear Windows: Sigmoid or logarithmic mappings for specific use cases
🔹 DICOM Overlay
Separate graphic layer (bitmap) for annotations, measurements, or AI markings. Stored in tags like (6000,3000) and rendered over the image.
🔹 Real-World Coordinates (3D Mapping)
Image position and orientation tags map pixels to real-world 3D coordinates (X, Y, Z):
- (0020,0032) – Image Position (Patient)
- (0020,0037) – Image Orientation (Patient) Essential for 3D reconstructions and navigation.
🔹 Information Object Definitions (IODs)
Each modality has a defined IOD. Examples:
- CR/DR – X-ray IOD
- CT – CT Image Storage IOD
- MR – MR Image Storage IOD
- MG – Mammography IOD
- US – Ultrasound Multi-frame IOD
🔹 Example: DICOM File Structure (CT Image)
| Group | Description | Tag (Example) |
| 0010 | Patient | (0010,0010) Name |
| 0020 | Study/Series | (0020,000D) Study UID |
| 0020 | Image | (0020,0013) Instance # |
| 0028 | Image Attributes | (0028,0010) Rows |
| 7FE0 | Pixel Data | (7FE0,0010) |
| 0028 | Window LUT | (0028,1050/1051) WL/WW |
| 6000 | Image Overlay | (6000,3000) Overlay |
- DICOM Communication Protocol: SCU/SCP and Services
🔹 SCU vs. SCP
- SCU (Service Class User): Initiates communication (e.g., modality sending image).
- SCP (Service Class Provider): Responds to requests (e.g., PACS receiving image).
🔹 Association & Feedback
A DICOM Association is a network session where SCU and SCP negotiate supported services and transfer syntaxes. If an operation fails, a feedback/status code is returned.
🔹 C-STORE Service
Used to send/receive images.
- SCU: Sends image data to PACS.
- SCP: Receives and stores image data.
Example: CT modality (SCU) sends image to PACS (SCP) via C-STORE.
🔹 C-FIND / C-MOVE / C-GET: Query/Retrieve (Q/R)
- C-FIND (SCU): Queries PACS (SCP) for patient/study info.
- C-MOVE: Asks PACS to send images to another node.
- C-GET: Requests images directly within the session.
Workflow: A viewer queries PACS for a patient study (C-FIND), then retrieves images via C-MOVE.
🔹 Modality Worklist (MWL) and MPPS
- MWL: Allows a modality (e.g., US machine) to pull scheduled exams from RIS.
- Reduces manual entry, ensures consistency.
- MPPS (Modality Performed Procedure Step): Sends status updates (started, completed) back to RIS.
Use Case: RIS schedules a CT scan → CT pulls data (MWL) → Sends back exam status (MPPS).
🔹 DICOM Print Service
- Sends image data to DICOM printers using Film Session, Film Box, and Image Box objects.
- Supports Grayscale or Color Image Boxes depending on image type.
Example: Mammogram images are printed in grayscale on a DICOM printer.
- DICOM in Real-World Integrations
🔹 DICOMweb (WADO-RS, STOW-RS, QIDO-RS)
RESTful web services for:
- WADO-RS: Retrieve DICOM objects/images
- STOW-RS: Store DICOM objects via HTTP
- QIDO-RS: Query for studies/series/images
Enables browser-based PACS and AI systems to integrate without traditional DICOM DIMSE protocols.
🔹 DICOM Segmentation (SEG) and AI
- Encodes AI outputs as structured segmentations.
- Enables standardized labeling, overlay, and interoperability with PACS and viewers.
AI lung nodule detection creates DICOM SEG objects viewable in radiology workstations.
🔹 DICOM SR, Encapsulated PDF & MP4
- Structured Report (SR): Codified, searchable clinical content.
- Encapsulated PDF: Embeds documents (e.g., consent forms, lab results).
- Encapsulated MP4: Stores video clips from modalities or scopes.
Use Case: AI-generated reports stored as SR and integrated with the hospital EMR.
Thank You!
DICOM 3.0 is a vast, powerful standard critical to modern imaging and healthcare interoperability. If you’d like a deeper dive into any of the above topics, feel free to reach out—I’m happy to help!