6 Value Encoding

Preferred repertoires as defined in ENV 41 503 and ENV 41 508 for the use in Western and Eastern Europe, respectively, are: ISO-IR 100, ISO-IR 101, ISO-IR 144, ISO-IR 126. See Section 6.1.2.3.

Information Object Definitions using different character sets cannot rely per se on lexical ordering or string comparison of Data Elements represented as character strings. These operations can only be carried out within a given character repertoire and not across repertoire boundaries.

The code table is split into the GL area, which supports a 94 character set only (bit combinations 02/01 to 07/14) plus SPACE in 02/00, and the GR area, which supports either a 94 or 96 character set (bit combinations 10/01 to 15/14 or 10/00 to 15/15). The default character set (ISO-IR 6) is always invoked in the GL area.

All character sets specified in [ISO/IEC 8859] include ISO-IR 6. This set will always be invoked in the GL area of the code table and is the equivalent of ASCII [ANSI X3.4]), whereas the various extension repertoires are mapped onto the GR area of the code table.

The 8-bit code table of [JIS X 0201] includes ISO-IR 14 (romaji alphanumeric characters) as the G0 code element and ISO-IR 13 (katakana phonetic characters) as the G1 code element. ISO-IR 14 is identical to ISO-IR 6, except that bit combination 05/12 represents a "¥" (YEN SIGN) and bit combination 07/14 represents an over-line.

When the Value of Specific Character Set (0008,0005) is either "ISO_IR 13" or "ISO 2022 IR 13", the graphic character represented by the bit combination 05/12 is a "¥" (YEN SIGN) in the character set of ISO-IR 14.

The expected behavior on conversion during store-and-forward operations needs to be equivalent to the action of separating a multi-valued character stream for multi-valued VRs into individual values between 05/12 byte delimiters and to recombine them separated by 05/12 byte delimiters, regardless of which Graphic Character 05/12 represents in the respective Character Set.

Graphic Characters that match the delimiter specified for the Character Set for multi-valued VRs cannot be represented as Values in that Character Set. I.e., a BACKSLASH encoded as 05/12 cannot be present within a Value (as opposed to between Values) in the Default Character Set and a YEN SIGN encoded as 05/12 cannot be present within a Value in [JIS X 0201].

Considerations on the Handling of Unsupported Character Sets:

In DICOM, character sets are not negotiated between Application Entities but are indicated by a conditional Attribute of the SOP Common Module. Therefore, implementations may be confronted with character sets that are unknown to them.

The Unicode Standard includes a substantial discussion of the recommended means for display and print for characters that lack font support. These same recommendations may apply to the mechanisms for unsupported character sets.

The machine should print or display such characters by replacing all unknown characters with the four characters "\nnn", where "nnn" is the three digit octal representation of each byte.

An example of this for an ASCII based machine would be as follows:

Character String: Günther

Encoded representation: 04/07 15/12 06/14 07/04 06/08 06/05 07/02

ASCII based machine: G\374nther

Implementations may also encounter Control Characters that they have no means to print or display. The machine may print or display such Control Characters by replacing the Control Character with the four characters "\nnn", where "nnn" is the three digit octal representation of each byte.

Considerations for missing fonts

The Unicode standard and the [GB 18030] standard define mechanisms for print and display of characters that are missing from the available fonts. If GBK is specified in Specific Character Set (0008,0005), the [GB 18030] rules of print and display of characters shall apply. The DICOM Standard does not specify user interface behavior since it does not affect network or media data exchange.

The Unicode and [GB 18030] standards have distinct YEN SIGN, BACKSLASH, and several forms of reverse solidus. The separator for multi-valued Data Elements in DICOM is the character valued 05/12 regardless of what glyph is used to enter or display this character. The other reverse solidus characters that have a very similar appearance are not separators. The choice of font can affect the appearance of 05/12 significantly. Multi-byte encoding systems, such as [GB 18030], [GBK] and [ISO/IEC 2022], may generate encodings that contain a byte valued 05/12. Only the character that encodes as a single byte valued 05/12 is a delimiter.

For multi-valued Data Elements, existing implementations that are expecting only single-byte replacement character sets may misinterpret the Value Multiplicity of the Data Element as a consequence of interpreting 05/12 bytes in multi-byte characters or [ISO/IEC 2022] escape sequences as delimiters, and this may affect the integrity of store-and-forward operations. Applications that do not explicitly state support for [GB 18030], [GBK] or [ISO/IEC 2022] in their conformance statement, might exhibit such behavior.

Defined Terms for Specific Character Set (0008,0005) are defined in PS3.3.

Support for Japanese kanji (ideographic), hiragana (phonetic), katakana (phonetic), Korean (Hangul phonetic and Hanja ideographic) and Chinese characters is defined in PS3.3.

The Chinese Character Set (GB18030) and Unicode [ISO/IEC 10646] do not allow the use of Code Extension Techniques. If either of these character sets is used, no other character set may be specified in the Specific Character Set (0008,0005) Attribute, that is, it may have only one Value.