25 DICOM — Application Entities
25.1 Application Entities in Network Communication
In general network communication, Application Entities (AEs) are the end-points of communication at the application layer. They represent distinct software components or services that can communicate with each other over a network. Think of them as the “identities” that applications use when they need to exchange information.
┌─────────────────────────────────────────────────────────────┐
│ Application Layer │
│ │
│ ┌──────────────┐ ┌──────────────┐ │
│ │ Application │◄────────────────────►│ Application │ │
│ │ Entity A │ Network Protocol │ Entity B │ │
│ └──────────────┘ └──────────────┘ │
│ │ │ │
└────────┼───────────────────────────────────────┼────────────┘
│ │
┌────▼────┐ ┌────▼────┐
│ Lower │ │ Lower │
│ Layers │◄──────────────────────────►│ Layers │
└─────────┘ └─────────┘Key characteristics of Application Entities:
- Unique Identity: Each AE has a unique identifier in the network
- Protocol Understanding: AEs understand and implement specific application protocols
- Service Provider/User: Can act as either a service provider (server) or service user (client)
- Autonomous: Can initiate, respond to, or refuse communication requests
25.2 Application Entities in DICOM Communication
In DICOM (Digital Imaging and Communications in Medicine), Application Entities take on a very specific and crucial role. A DICOM Application Entity represents any software component that can send, receive, or process DICOM data.
25.2.1 DICOM AE Components
Each DICOM AE is defined by three key identifiers:
┌─────────────────────────────────────────────┐
│ DICOM Application Entity │
│ │
│ ┌────────────────────────────────────────┐ │
│ │ AE Title: "PACS_SERVER_01" │ │
│ │ (Max 16 characters, unique identifier) │ │
│ └────────────────────────────────────────┘ │
│ │
│ ┌────────────────────────────────────────┐ │
│ │ IP Address: 192.168.1.100 │ │
│ └────────────────────────────────────────┘ │
│ │
│ ┌────────────────────────────────────────┐ │
│ │ Port Number: 11112 │ │
│ │ (Common DICOM port: 104 or 11112) │ │
│ └────────────────────────────────────────┘ │
└─────────────────────────────────────────────┘25.2.2 DICOM AE Roles and Services
DICOM AEs can operate in different roles:
- Service Class User (SCU) - The client that requests services
- Service Class Provider (SCP) - The server that provides services
┌──────────────────┐ DICOM Protocol ┌──────────────────┐
│ Modality │ Association │ PACS Server │
│ │◄─────────────────────────────►│ │
│ AE: "CT_SCAN_1" │ │ AE: "PACS_MAIN" │
│ Role: SCU │──────► C-STORE Request ──────►│ Role: SCP │
│ (Sends Images) │◄────── C-STORE Response ◄─────│ (Stores Images) │
└──────────────────┘ └──────────────────┘25.2.3 Common DICOM Services by AE Type
┌─────────────────────────────────────────────────────────────────┐
│ DICOM Service Classes │
├───────────────────┬─────────────────────────────────────────────┤
│ Service │ Description │
├───────────────────┼─────────────────────────────────────────────┤
│ C-STORE │ Transfer and store DICOM images │
│ C-FIND │ Query for patient/study/series info │
│ C-MOVE │ Retrieve images from remote AE │
│ C-GET │ Direct retrieval of images │
│ C-ECHO │ Verify connectivity (like ping) │
│ Worklist │ Query scheduled procedures │
│ Storage Commit │ Confirm successful storage │
└───────────────────┴─────────────────────────────────────────────┘25.2.4 DICOM Association Process
When two DICOM AEs communicate, they establish an “Association”:
SCU (Client) SCP (Server)
│ │
│───────── A-ASSOCIATE Request ───────►│
│ (Propose capabilities) │
│ │
│◄──────── A-ASSOCIATE Accept ─────────│
│ (Negotiate services) │
│ │
│═══════ DICOM Services Active ════════│
│ │
│────────── C-STORE Request ──────────►│
│◄───────── C-STORE Response ──────────│
│ │
│────────── C-FIND Request ───────────►│
│◄───────── C-FIND Response ───────────│
│ │
│───────── A-RELEASE Request ─────────►│
│◄──────── A-RELEASE Response ─────────│
│ │25.2.5 Real-World DICOM AE Examples
In your radiology environment, you likely encounter these AE types:
- Modality AEs: CT scanners, MRI machines, X-ray systems
- PACS AEs: Picture Archiving and Communication Systems
- Workstation AEs: Diagnostic viewing stations
- RIS AEs: Radiology Information Systems
- AI/Processing AEs: Image processing or AI analysis nodes
┌────────────┐ ┌────────────┐ ┌─────────────┐
│ CT Scanner │────►│ PACS │────►│ Workstation │
│ AE: CT_01 │ │ AE: PACS_1 │ │ AE: WS_RAD1 │
└────────────┘ └─────┬──────┘ └─────────────┘
│
▼
┌─────────────┐
│ AI Engine │
│ AE: AI_PROC │
└─────────────┘25.2.6 Configuration Considerations
For DICOM AEs in your hospital network:
- AE Titles must be unique and often follow naming conventions (e.g., “RAMA_CT_01”)
- Port configuration requires firewall rules to allow DICOM traffic
- Security: Modern DICOM supports TLS encryption between AEs
- Timeout settings: Important for handling network latency
- Transfer syntax: AEs negotiate compression and encoding methods
Each AE maintains a configuration table of other AEs it can communicate with, containing their titles, IP addresses, and ports - this is often called the “DICOM node configuration” or “AE configuration table.”