Every DICOM object, whether a CT slice, an ultrasound clip, or a structured report, is a list of data elements. Each element holds one piece of information, such as the patient's name, the modality, or the pixel data itself. A tag is the number that says which element it is.
How to read a tag
A tag is two four-digit hexadecimal numbers, written as (group,element). (0010,0020) is group 0010, element 0020: Patient ID. The group loosely gathers related elements. Group 0010 holds patient details, 0008 general study and instance details, 0020 the identifiers that tie images into series and studies, and 0028 how the pixels are laid out.
Each data element carries four things: the tag, a value representation (VR) saying what kind of value it is, the length of the value, and the value itself.
Value representations
The VR is a two-letter code. A few you will see constantly:
| VR | Meaning | Example |
|---|---|---|
PN | Person name, with family and given names separated by ^ | DOE^JANE |
LO | Long string, up to 64 characters | a patient ID |
CS | Code string, a short defined term | CT, MR |
DA | Date, as YYYYMMDD | 20261023 |
UI | Unique identifier (UID), dot-separated numbers | 1.2.840.10008.5.1.4.1.1.2 |
OB, OW | Other byte or word: raw binary data | pixel data |
A file or transfer can use explicit VR, where each element states its VR, or implicit VR, where the receiver looks the VR up in the data dictionary. The transfer syntax says which.
Tags you will meet most
| Tag | Name | VR |
|---|---|---|
(0010,0010) | Patient's Name | PN |
(0010,0020) | Patient ID | LO |
(0010,0030) | Patient's Birth Date | DA |
(0008,0020) | Study Date | DA |
(0008,0050) | Accession Number | SH |
(0008,0060) | Modality | CS |
(0008,0016) | SOP Class UID | UI |
(0008,0018) | SOP Instance UID | UI |
(0020,000D) | Study Instance UID | UI |
(0020,000E) | Series Instance UID | UI |
(7FE0,0010) | Pixel Data | OB or OW |
The three instance UIDs build the hierarchy every PACS relies on: a study contains series, and a series contains instances. If two studies share a Study Instance UID, or one study arrives with two, the archive will file images in the wrong place. That is why an importer keeps the original UIDs when it brings in outside images.
Private tags
Elements in an even-numbered group are defined by the standard. Elements in an odd-numbered group are private: a vendor uses them for its own data, such as scanner settings, and reserves a block for them with a private creator element. Other systems may not understand private tags, and they can hold patient information, so check them before you share images for research or support.
The file meta information
A DICOM file starts with a 128-byte preamble, the letters DICM, and then group 0002: the file meta information. It records the transfer syntax the rest of the file uses, which is how a reader knows whether to expect explicit or implicit VR and whether the pixel data is compressed.
Where the definitions live
Every standard tag, its name, VR, and value multiplicity is listed in the DICOM data dictionary, PS3.6. The encoding rules are in PS3.5, and the file format in PS3.10. For how objects move between systems, see DICOM vs PACS.
Sources: PS3.5 Data Structures and Encoding; PS3.6 Data Dictionary; PS3.10 Media Storage and File Format.