DICOM Standard

Section 5 of 12
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Overview of DICOM

DICOM (Digital Imaging and Communications in Medicine) is the international standard for medical imaging and related information. It defines both a file format for medical images and a network protocol for transmitting medical imaging data.

Maintained by: NEMA (National Electrical Manufacturers Association) First Released: 1993 Current Version: Continuously updated (annual supplements)

When and Where DICOM is Used

Primary Use Cases:

  • Medical image storage and transfer
  • Picture Archiving and Communication Systems (PACS)
  • Radiology Information Systems (RIS) integration
  • Cardiology image management
  • Pathology whole slide imaging
  • 3D printing from medical images
  • Radiation therapy planning
  • Ophthalmology imaging

Typical Settings:

  • Hospitals and imaging centers
  • Radiology departments
  • Cardiology departments
  • Pathology laboratories
  • Radiation oncology centers
  • Dentistry practices
  • Veterinary clinics
  • Teleradiology services

Imaging Modalities:

  • CT (Computed Tomography)
  • MRI (Magnetic Resonance Imaging)
  • X-Ray/Radiography
  • Ultrasound
  • PET (Positron Emission Tomography)
  • Nuclear Medicine
  • Mammography
  • Endoscopy
  • Pathology (Whole Slide Imaging)

DICOM Information Model

Patient-Study-Series-Image Hierarchy:

DICOM Information Modelsaravanansubramanian.comPatient - Study - Series - Image containment, from person down to pixel dataPatient (Root)Patient ID - Name - DOB - SexStudy (single imaging exam)Study Instance UID - Accession Number - Study DateSeriesSeries Instance UID - ModalityImageImageImageSeriesSeries Instance UID - ModalityImageImageStudya second imaging exam for the same patientSeriesImage

Example Identifiers:

  • Patient ID: Unique identifier for the patient
  • Study Instance UID: Unique identifier for a study (e.g., entire CT scan session)
  • Series Instance UID: Unique identifier for a series (e.g., CT abdomen sequence)
  • SOP Instance UID: Unique identifier for an individual image

DICOM File Structure

A DICOM file consists of:

  1. Preamble: 128 bytes (often unused)
  2. DICOM Prefix: “DICM” (4 bytes)
  3. Data Elements: Tag-value pairs containing image data and metadata

Data Element Structure:

  • Tag: Group number + Element number (e.g., 0010,0010)
  • VR (Value Representation): Data type (e.g., PN = Person Name)
  • Length: Size of the value
  • Value: The actual data

Common DICOM Tags:

(0008,0005) Specific Character Set
(0008,0018) SOP Instance UID
(0008,0020) Study Date
(0008,0030) Study Time
(0008,0050) Accession Number
(0008,0060) Modality (CT, MR, US, etc.)
(0008,0070) Manufacturer
(0008,1030) Study Description
(0010,0010) Patient's Name
(0010,0020) Patient ID
(0010,0030) Patient's Birth Date
(0010,0040) Patient's Sex
(0018,0015) Body Part Examined
(0018,1030) Protocol Name
(0020,000D) Study Instance UID
(0020,000E) Series Instance UID
(0020,0011) Series Number
(0020,0013) Instance Number
(0028,0010) Rows (Image height)
(0028,0011) Columns (Image width)
(0028,0100) Bits Allocated
(7FE0,0010) Pixel Data

DICOM Services (DIMSE)

DICOM defines network services using DIMSE (DICOM Message Service Element):

Storage Service (C-STORE):

  • Send images to PACS or workstation
  • Most commonly used DICOM service

Query/Retrieve Services:

  • C-FIND: Query for patients, studies, series, images
  • C-MOVE: Retrieve images from PACS
  • C-GET: Alternative retrieve method

Verification Service (C-ECHO):

  • Test network connectivity
  • Verify DICOM node is alive

Modality Worklist Service (C-FIND):

  • Query scheduled procedures
  • Retrieve patient demographics for imaging

Storage Commitment Service (N-ACTION):

  • Confirm images are safely stored
  • Verify data integrity

DICOM Network Communication

Application Entity (AE):

  • Logical endpoint for DICOM communication
  • Identified by AE Title (up to 16 characters)
  • Example: “PACS_SERVER”, “CT_SCANNER_1”

Association Establishment:

1. C-ASSOCIATE-RQ (Request)
   - Calling AE Title (client)
   - Called AE Title (server)
   - Supported abstract syntaxes (services)
   - Supported transfer syntaxes (encodings)

2. C-ASSOCIATE-AC (Accept) or C-ASSOCIATE-RJ (Reject)
   - Negotiated parameters

3. DIMSE Operations (C-STORE, C-FIND, etc.)

4. C-RELEASE-RQ and C-RELEASE-RP (Orderly release)
   or A-ABORT (Emergency abort)

Transfer Syntaxes:

  • Implicit VR Little Endian: Default, most compatible
  • Explicit VR Little Endian: More precise
  • Explicit VR Big Endian: Less common
  • JPEG Lossy/Lossless: Compressed images
  • JPEG 2000: Advanced compression
  • RLE: Run-length encoding

DICOM Conformance Statement

Every DICOM device must have a conformance statement documenting:

  • Supported SOP Classes (service-object pairs)
  • Supported transfer syntaxes
  • Association parameters (max PDU size, etc.)
  • Character sets supported
  • Implementation details

Example SOP Classes:

  • CT Image Storage
  • MR Image Storage
  • US Image Storage
  • X-Ray Image Storage
  • Radiation Therapy Storage
  • Structured Report Storage
  • Presentation State Storage

DICOM Web Services (DICOMweb)

Modern RESTful services for DICOM:

WADO-RS (Web Access to DICOM Objects - RESTful Services):

  • Retrieve studies, series, or instances
  • Get rendered images (JPEG, PNG)
  • HTTP GET requests

Example:

GET /studies/1.2.3.4.5/series/1.2.3.4.5.6/instances/1.2.3.4.5.6.7
Accept: application/dicom

STOW-RS (Store Over Web - RESTful Services):

  • Store DICOM objects via HTTP POST
  • Upload studies to PACS

QIDO-RS (Query based on ID for DICOM Objects - RESTful Services):

  • Query for patients, studies, series, instances
  • HTTP GET with query parameters

Example:

GET /studies?PatientName=Doe&StudyDate=20231119

DICOM SR (Structured Reports)

DICOM SR allows encoding clinical information in standardized format:

Use Cases:

  • Radiology reports
  • Measurement reports
  • CAD (Computer-Aided Detection) results
  • Radiation dose reports
  • Procedure logs

SR Document Types:

  • Basic Text SR: Simple text reports
  • Enhanced SR: Rich formatting and coding
  • Comprehensive SR: Full clinical context
  • Mammography CAD SR: Breast imaging analysis
  • Chest CAD SR: Chest imaging analysis

Implementation Considerations

PACS Architecture:

PACS Architecturesaravanansubramanian.comfour-tier image flow from modality to viewing workstationsModalityCT · MR · Ultrasound · …C-STOREDICOM Router / Gatewayrouting · anonymization · retryPACS Servercentral storage archiveC-FIND / C-MOVEViewing Workstationsradiologists · referring physicians

Key Design Decisions:

  • Choose appropriate transfer syntaxes (compression vs. quality)
  • Implement proper error handling and retry logic
  • Plan for large dataset handling (CT scans can be 1000+ images)
  • Consider network bandwidth and storage requirements
  • Implement anonymization for privacy compliance
  • Plan for long-term archive strategy

Performance Optimization:

  • Use compressed transfer syntaxes when appropriate
  • Implement parallel connections for bulk transfers
  • Cache frequently accessed studies
  • Use thumbnail/preview generation
  • Implement tiered storage (SSD for recent, HDD for archive)

Security Considerations:

  • Use TLS for network encryption
  • Implement authentication (LDAP, OAuth)
  • Audit all DICOM operations (as required by HIPAA)
  • Implement role-based access control
  • De-identify PHI when needed

Quiz: DICOM Standard

Question 1 of 5

What is the hierarchical structure of DICOM's information model from top to bottom?