DICOM data elements, tags, and privacy review

This is a working guide, not a complete DICOM dictionary or an attribute confidentiality profile. pydicom 3.0.2 bundles the 2024c public dictionary; use the live DICOM PS3.3/PS3.6 and the selected IOD when correctness depends on a newer edition.

Privacy boundary

DICOM metadata and pixels may contain PHI. Do not print a complete Dataset, serialize the full dataset to JSON, or copy arbitrary values into logs. Tag names that appear technical can still identify a person through site-specific values, free text, private data, UIDs, dates, devices, or linkage with external records.

DICOM PS3.15 Annex E says that applying attribute actions does not guarantee that the Information Object is de-identified. A valid workflow must select a profile/options for its context and include expert verification and re-identification risk review.

pydicom access model

from pydicom import dcmread
from pydicom.tag import Tag

ds = dcmread(
    "authorized/input.dcm",
    stop_before_pixels=True,
    specific_tags=["SOPClassUID", "Modality", "Rows", "Columns"],
)

modality = ds.get("Modality", "UNSPECIFIED")
element = ds.get_item(Tag(0x0008, 0x0016))

• Keyword access (ds.Modality) returns the value and raises AttributeError when absent.
• ds.get("Modality", default) is safer for optional elements.
• Tag indexing (ds[0x0008, 0x0016]) returns a DataElement; read .value only when authorized.
• A tag consists of a 16-bit group and 16-bit element.
• Standard public tags generally use even groups. Private data uses odd groups and private creator blocks.
• Dataset contains DataElement objects. A value with VR SQ is a Sequence of nested Dataset items.

Narrow technical allowlist

The following values are commonly useful for bounded technical inventory. They do not make an entire record safe to disclose.

BurnedInAnnotation=NO is a declaration, not proof that pixels are clean. Absence, YES, or another value requires review. Even NO does not address recognizable facial/anatomic features or matching against source images.

Instance, relationship, and spatial elements

These values are technically important but can enable linkage or reveal individual context. Do not emit them in default reports.

Do not sort a series only by SliceLocation or assume SliceThickness equals inter-slice spacing. Reconstruct geometry from the applicable IOD, orientation, position, frame functional groups, and validated series membership.

For the Image Plane Module with BIPED or absent Anatomical Orientation Type, patient coordinates increase left, posterior, and toward the head (LPS). PixelSpacing = [row_spacing, column_spacing] in mm. For zero-based NumPy indices [row, column], the pixel center is:

ImagePositionPatient
  + column * PixelSpacing[1] * ImageOrientationPatient[:3]
  + row    * PixelSpacing[0] * ImageOrientationPatient[3:]

For a consistent parallel stack, sort positions projected onto the cross product of the two orientation vectors, then check duplicates, spacing, orientation, Frame of Reference, and acquisition dimensions. Series membership alone does not separate time points/echoes/diffusion directions. Enhanced multi-frame ordering uses Dimension Index Values and frame functional groups. A PNG does not retain patient geometry; resampling/rotation requires corresponding affine updates and interpolation appropriate to the data. QUADRUPED conventions differ.

Direct and quasi-identifiers

The following examples are not exhaustive. PS3.15 Table E.1-1 and the chosen options control action selection, including nested occurrences.

Required IOD type matters. A PS3.15 action can remove (X), zero (Z), replace with a valid dummy value (D), replace a UID consistently (U), keep (K), or clean (C), with conditional combinations. Blind deletion can make an instance non-conformant.

Dates and times

Common date/time elements include:

VR syntax:

• DA: YYYYMMDD
• TM: HHMMSS.FFFFFF with permitted truncation
• DT: YYYYMMDDHHMMSS.FFFFFF&ZZXX with permitted truncation

Date/time handling is not solved by replacing every value with a constant. Review:

• whether full dates or modified dates are allowed by the selected PS3.15 option;
• one consistent shift across the intended longitudinal scope;
• leap days, range limits, partial precision, time zones, and midnight crossings;
• standalone TM values that cannot be shifted safely without a paired date;
• interval preservation and external event linkage;
• IOD Type 1/2 requirements and scientific utility.

Record the policy and caveats without logging original values.

UIDs: replace instance relationships, not semantics

UID VR is UI, but not every UID is an identifier to pseudonymize.

Usually structural/semantic and preserved:

• Transfer Syntax UID
• SOP Class UID and Referenced SOP Class UID
• coding/context/template UIDs defined by standards
• implementation UID handling according to rebuilt File Meta Information

Often instance/reference linkage requiring profile-directed, consistent replacement:

• Study, Series, SOP Instance, and Frame of Reference UIDs
• Device UID (PS3.15 default action U; explicit retention options may apply)
• Referenced SOP Instance UIDs in sequences
• synchronization, concatenation, tracking, specimen, and transaction UIDs

Use one-to-one replacement over the declared scope. A keyed deterministic mapping can maintain consistency, but the key/map is sensitive. Replacing UIDs does not itself prevent pixel or metadata matching and must not create false confidence.

Sequences and recursive traversal

Identifiers may occur at any nesting depth:

def visit(dataset):
    for element in dataset:
        if element.VR == "SQ":
            for item in element.value:
                visit(item)
        else:
            review(element.tag, element.keyword, element.VR)

Bound recursion depth and total elements for untrusted files. Do not print values from the callback. pydicom's Dataset.walk() is also recursive by default, and remove_private_tags() uses recursive traversal.

Private data

Private elements use odd group numbers and a private creator block. Their semantics are vendor-defined and names may be unknown or non-unique. Access by tag or PrivateBlock, not by the descriptive display name.

private_count = sum(1 for element in ds.iterall() if element.tag.is_private)

Dataset.remove_private_tags() recursively removes private elements, but:

• private removal alone is not de-identification;
• standard elements, sequences, pixels, graphics, and overlays still matter;
• some private elements may be scientifically necessary;
• the PS3.15 Retain Safe Private Option requires evidence that retained elements are safe and removal/processing of all others.

Default to remove or reject private data. Explicit retention needs a reviewed allowlist and provenance.

Pixel, graphics, and structured content

Potential identifying content is not limited to (7FE0,0010) PixelData:

• Float/Double Float Pixel Data
• overlays in repeating 60xx groups
• retired curves in 50xx groups
• presentation-state graphics and annotations
• Structured Report text/content items
• waveforms, encapsulated documents, spectra, and other bulk content
• full-face images and recognizable head/neck reconstructions

The PS3.15 Clean Pixel Data, Clean Recognizable Visual Features, Clean Graphics, and Clean Structured Content options address different risks. Human review may be required, and cleaning can impair utility.

DICOM JSON

Dataset.to_json() and to_json_dict() preserve DICOM element content. Binary data is either base64 InlineBinary or represented by BulkDataURI. Therefore:

• JSON is not a safe metadata summary;
• full JSON can contain the same PHI as the source dataset;
• a bulk-data handler must enforce storage and retrieval authorization;
• pydicom 3.0.2 documents JSON support as beta.

Use scripts/extract_metadata.py for allowlisted aggregate inventory.

Sources (verified 2026-10-01)

• pydicom 3.0.2 dataset basics
• pydicom core elements
• pydicom private elements
• pydicom DICOM JSON tutorial
• DICOM PS3.3 2026d, Information Object Definitions
• DICOM PS3.3 Image Pixel Module
• DICOM PS3.3 Image Plane Module
• DICOM PS3.15 Table E.1-1 actions
• DICOM PS3.5, Data Structures and Encoding
• DICOM PS3.5 private elements
• DICOM PS3.6, Data Dictionary
• DICOM PS3.15 2026d, Annex E confidentiality profiles