7.2.2 Units of Measurement
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### 7.2.2 Units of Measurement

Context Group 82 is defined to include all units of measurement relevant to DICOM IODs. In the past it was envisaged that an extensible list of pre-coordinated codes would be included in the mapping resource.

DICOM has now adopted the Unified Codes for Units of Measurement (UCUM) standard for all units of measurement. This coding scheme allows for the "construction" of pre-coordinated codes from atomic components.

The specialization of the UCUM standard as it is used in DICOM involves the following rules:

- the Coding Scheme Designator is specified as "UCUM"

- the version of UCUM from which a code is constructed is not required, as it is not needed to resolve ambiguity in the Code Value or Code Meaning; however, there is no restriction on the version being specified in Coding Scheme Version

- the Code Value will be constructed from UCUM and make use of the "case-sensitive" form of UCUM code (e.g., "ml/s")

- the Code Meaning for other than UCUM unity may be one of the following:

  - the "print" value specified in UCUM (e.g., "mmHg" for Code Value mm[Hg])

  - the same string as sent in the Code Value (e.g., "ml/s")

  - constructed from the "names" of individual components using the Americanized form of name (e.g., "milliliters/second")

  - constructed from the "names" of individual components using the European form of name (e.g., " millilitres /second")

- In the case of UCUM unity ("1", or curly braces expression) it is forbidden to use "1" as a Code Meaning. Annex G provides Code Meanings for a Code Value (0008,0100) of 1. A Template or Context Group may constrain the Code Meaning according to the following rules:

  - UCUM default unit 1 shall use one of the Code Meaning synonyms specified in Annex G

  - ratios of identically dimensioned values may use ({ratio}, UCUM, "ratio")

  - unitless numeric scores may use ({M:N}, UCUM, "range: M:N") to specify the minimum and maximum value, for example, ({0:10}, UCUM, "range: 0:10")

  - counts using UCUM annotation shall always use the text within the curly braces as the Code Meaning, for example, ({masses}, UCUM, "masses")

  - compositions of a curly braces expression with other UCUM values may use a conventional clinical representation, for example, ({H.B.}/min, UCUM, "BPM")

The UCUM standard states that the preferred display values for codes deg (degrees of plane angle) and Cel (degrees Celsius) are "°" and "°C". However, the character ° does not have a representation in the DICOM default character set (ASCII, ISO-IR 6). The Code Meaning specified in this Part therefore uses "deg" and "C". SOP Instances that specify a Specific Character Set that allows the character ° may use Code Meanings "°" and "°C".

### Note

1. Code Meaning "C" formally conflicts with the Code Meaning for Coulomb. In the context of DICOM use, the possibility of confusion to a user based on the display of the Code Meaning is considered remote, as there is little use of Coulomb in imaging, and the context of the displayed item Concept Name would resolve between temperature and electric charge. Automated processing based on the Code Values should not face an issue as the Code Values differ.

2. The character ° has Unicode code point U+00B0, and is represented as 0xB0 in ISO-IR 100 (Latin-1), ISO-IR 101 (Latin-2), ISO-IR 109 (Latin-3), ISO-IR 110 (Latin-4), ISO-IR 126 (Greek), ISO-IR 138 (Hebrew), and ISO-IR 148 (Latin-5). It is not encodable in ISO-IR 13 (Katakana), ISO-IR 144 (Cyrillic), ISO-IR 127 (Arabic), or ISO-IR 166 (Thai).

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