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DICOM Glossary

A reference guide to DICOM terms and concepts to clarify medical and technical terminology.

Contents

General Terms

DICOM

Digital Imaging and Communications in Medicine

International standard for storing and transmitting medical images. Defines both the file format and communication protocols.

SOP (Service-Object Pair)

Combination of a DICOM service (store, print, etc.) with an object (such as a CT image). It defines which service operates on which object.

UID (Unique Identifier)

Unique number identifying each study, series, or image. Format: 1.2.840.10008.xxx.xxx

Example:

let studyUID = decoder.info(for: .studyInstanceUID)
// "1.2.840.113619.2.55.3.2831868264.123.1234567890.1"

Types of UID:

  • Study Instance UID - identifies an exam
  • Series Instance UID - identifies a related group of images
  • SOP Instance UID - identifies a specific image

Data Structure

DICOM Tag

Numeric identifier for each metadata field.

Format: (GGGG,EEEE) where:

  • GGGG = Group
  • EEEE = Element

Example:

(0010,0010) = Patient Name
(0008,0060) = Modality

In code:

// Recommended: Type-safe DicomTag enum
let name = decoder.info(for: .patientName)

// Legacy: raw hex (for custom/private tags)
let name = decoder.info(for: 0x00100010)

VR (Value Representation)

Defines the data type of a DICOM element.

VR Name Description Example
PN Person Name Person name "Doe^John"
DA Date Date (YYYYMMDD) "20240101"
TM Time Time (HHMMSS) "143025"
UI UID Unique identifier "1.2.840..."
US Unsigned Short 16-bit integer 512
DS Decimal String Decimal number "1.5"
CS Code String Short code "CT"
LO Long String Longer text "Head CT"

Transfer Syntax

Defines how DICOM data is encoded (endianness, compression).

Supported by the decoder:

  • Little Endian Implicit VR (default)
  • Little Endian Explicit VR
  • Big Endian Explicit VR
  • JPEG Lossless (Process 14, Selection Value 1) - native decoder
  • JPEG Baseline (ImageIO)
  • JPEG 2000 (ImageIO)

Not supported:

  • JPEG Lossless processes other than Process 14, Selection Value 1
  • RLE

To detect in code:

if decoder.compressedImage {
    print("Compressed image detected")
}

Images and Pixels

Pixel Data

Tag (7FE0,0010) - contains the raw pixel values of the image.

Formats:

  • 8-bit grayscale (0-255)
  • 16-bit grayscale (0-65535)
  • 24-bit RGB (color)
if decoder.bitDepth == 16 {
    let pixels = decoder.getPixels16()
} else if decoder.bitDepth == 8 {
    let pixels = decoder.getPixels8()
}

if decoder.samplesPerPixel == 3 {
    let pixels = decoder.getPixels24()
}

Bits Allocated vs Bits Stored

  • Bits Allocated: Space reserved per pixel (often 8 or 16).
  • Bits Stored: Bits actually used (may be fewer).

Example:

Bits Allocated: 16
Bits Stored: 12
High Bit: 11

Meaning: pixels occupy 16 bits, but only 12 bits carry data.

Photometric Interpretation

Describes how to interpret pixel values.

Value Meaning
MONOCHROME1 0 = white, max = black (X-ray)
MONOCHROME2 0 = black, max = white (default)
RGB RGB color
PALETTE COLOR Color lookup table 
let photoInterp = decoder.photometricInterpretation

if photoInterp == "MONOCHROME1" {
    // Invert values if you read raw pixels
}

Pixel Spacing

Physical distance between pixels in millimeters.

let spacing = decoder.pixelSpacingV2
print("Pixel: \(spacing.x) x \(spacing.y) mm")
print("Slice thickness: \(spacing.z) mm")

let widthMM = Double(decoder.width) * spacing.x
print("Physical width: \(widthMM) mm")

Window/Level

Window Center (Level)

Tag (0028,1050) - central value of the display window. Controls brightness.

Window Width

Tag (0028,1051) - width of the display window. Controls contrast.

Narrow window (high contrast) vs. wide window (low contrast)

Preset Windows

Common window/level settings for different tissues.

Examples for CT:

let lung = DCMWindowingProcessor.getPresetValues(preset: .lung)
// Center: -600 HU, Width: 1500 HU

let bone = DCMWindowingProcessor.getPresetValues(preset: .bone)
// Center: 400 HU, Width: 1800 HU

let brain = DCMWindowingProcessor.getPresetValues(preset: .brain)
// Center: 40 HU, Width: 80 HU

Hounsfield Unit (HU)

Density scale used in CT. Water = 0 HU.

Tissue/Material HU Range
Air -1000
Lung -500 to -700
Fat -100 to -50
Water 0
Blood +30 to +45
Muscle +10 to +40
Liver +40 to +60
Trabecular bone +300 to +400
Dense bone +700 to +3000
Metals > +3000

Pixel to HU conversion:

let hu = decoder.applyRescale(to: pixelValue)
// HU = slope x pixel + intercept

Modalities

CT (Computed Tomography)

Rotational X-ray creating cross-sectional images.

Characteristics:

  • 16-bit grayscale
  • Values in HU
  • Multiple series (contrast phases)

Useful presets: lung, bone, brain, liver, abdomen.

MR (Magnetic Resonance)

Magnetic fields for soft-tissue imaging.

Characteristics:

  • Strong soft-tissue contrast
  • Multiple sequences (T1, T2, FLAIR, etc.)
  • No ionizing radiation
  • Does not use Hounsfield Units

CR/DX (Computed/Digital Radiography)

Digital X-ray.

Characteristics:

  • Often MONOCHROME1
  • 2D
  • High resolution

US (Ultrasound)

Sound-based imaging.

Characteristics:

  • Often color (RGB)
  • Frequently multi-frame
  • Doppler for blood flow

MG (Mammography)

Specialized X-ray for breast tissue. High resolution; often MONOCHROME1.

Clinical Terms

Patient Demographics

Identifying patient information.

let info = decoder.getPatientInfo()
// ["Name": "Doe^John", "ID": "12345", "Sex": "M", "Age": "045Y"]

Key tags:

  • (0010,0010) - Patient Name
  • (0010,0020) - Patient ID
  • (0010,0030) - Birth Date
  • (0010,0040) - Sex (M/F/O)

Study

A full medical exam that can contain multiple series.

Example: "CT Chest with Contrast"

  • Series 1: Non-contrast
  • Series 2: Arterial phase
  • Series 3: Venous phase
  • Series 4: Reconstructions
let study = decoder.getStudyInfo()

Series

Related images within a study.

Example: "Axial T1 Series"

let series = decoder.getSeriesInfo()
print(series["SeriesNumber"] ?? "")

Instance

A single DICOM image.

Hierarchy:

Study
  `-- Series 1
      |-- Instance 1
      |-- Instance 2
      `-- Instance 3
  `-- Series 2
      |-- Instance 1
      `-- Instance 2

Technical Terms

Endianness

Byte order in multi-byte values.

  • Little Endian: least significant byte first
  • Big Endian: most significant byte first

The decoder handles both.

Implicit vs Explicit VR

  • Explicit VR: VR is specified for each tag.
  • Implicit VR: VR is inferred from the tag dictionary.

The decoder detects both automatically.

Multi-frame Image

A DICOM file with multiple frames (similar to a short video).

if decoder.isMultiFrame {
    print("Frames: \(decoder.nImages)")
}

Common in ultrasound, angiography, and some dynamic CT studies.

Rescale Slope/Intercept

Formula for converting pixel values to clinical units.

Real Value = (Pixel x Slope) + Intercept

For CT (HU):

let params = decoder.rescaleParametersV2
let hu = params.apply(to: pixelValue)

Overlay

Graphics drawn over the image (annotations, measurements). Not processed by this decoder.

PACS (Picture Archiving and Communication System)

Hospital system for storing and distributing DICOM images.

WADO (Web Access to DICOM Objects)

Protocol for accessing DICOM images over HTTP.

Quick Tag Reference

Common tags (DicomTag enum and hex):

// Patient
.patientName       // 0x00100010 - Name
.patientID         // 0x00100020 - ID
.patientBirthDate  // 0x00100030 - Birth Date
.patientSex        // 0x00100040 - Sex

// Study
.studyInstanceUID  // 0x0020000D - Study Instance UID
.studyDate         // 0x00080020 - Study Date
.studyTime         // 0x00080030 - Study Time
.studyDescription  // 0x00081030 - Study Description

// Series
.seriesInstanceUID // 0x0020000E - Series Instance UID
.seriesNumber      // 0x00200011 - Series Number
.seriesDescription // 0x0008103E - Series Description
.modality          // 0x00080060 - Modality

// Image
.rows              // 0x00280010 - Rows
.columns           // 0x00280011 - Columns
.bitsAllocated     // 0x00280100 - Bits Allocated
.pixelRepresentation // 0x00280103 - Pixel Representation
.windowCenter      // 0x00281050 - Window Center
.windowWidth       // 0x00281051 - Window Width
.pixelData         // 0x7FE00010 - Pixel Data

Additional Resources

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