DICOM Value Representations
The DICONDE Value Representation of a Data Element describes the data type and format of that Data Element’s Value(s).
Values with VRs constructed of character strings, except in the case of the VR UI, shall be padded with SPACE characters (20H, in the Default Character Repertoire) when necessary to achieve even length. Values with a VR of UI shall be padded with a single trailing NULL (00H) character when necessary to achieve even length. Values with a VR of OB shall be padded with a single trailing NULL byte value (00H) when necessary to achieve even length.
All new VRs defined in future versions of DICOM shall be of the same Data Element Structure as defined in Section 7.1.2 with reserved bytes after the VR and a 32-bit unsigned integer VL (i.e., following the format for VRs such as OB or UT), and may or may not permit Undefined Length.
Notes
Since all new VRs will be defined as specified in Section 7.1.2, an implementation may choose to ignore VRs not recognized by applying the rules stated in Section 7.1.2.
When converting a Data Set from an Explicit VR Transfer Syntax to a different Transfer Syntax, an implementation may copy Data Elements with unrecognized VRs in the following manner:
- If the endianness of the Transfer Syntaxes is the same, the Value of the Data Element may be copied unchanged and if the target Transfer Syntax is Explicit VR, the VR bytes copied unchanged. In practice this only applies to Little Endian Transfer Syntaxes, since there was only one Big Endian Transfer Syntax defined.
- If the source Transfer Syntax is Little Endian and the target Transfer Syntax is the (retired) Big Endian Explicit VR Transfer Syntax, then the Value of the Data Element may be copied unchanged and the VR changed to UN, since being unrecognized, whether or not byte swapping is required is unknown. If the VR were copied unchanged, the byte order of the Value might or might not be incorrect.
- If the source Transfer Syntax is the (retired) Big Endian Explicit VR Transfer Syntax, then the Data Element cannot be copied, because whether or not byte swapping is required is unknown, and there is no equivalent of the UN VR to use when the Value is big endian rather than little endian.
The issues of whether or not the Data Element may be copied, and what VR to use if copying, do not arise when converting a Data Set from Implicit VR Little Endian Transfer Syntax, since the VR would not be present to be unrecognized, and if the Data Element VR is not known from a data dictionary, then UN would be used.
An individual Value, including padding, shall not exceed the Length of Value, except in the case of the last Value of a multi-valued field as specified in Section 6.4.
The lengths of Value Representations for which the Character Repertoire can be extended or replaced are expressly specified in characters rather than bytes in Table 6.2-1. This is because the mapping from a character to the number of bytes used for that character’s encoding may be dependent on the character set used.
Escape Sequences used for Code Extension shall not be included in the count of characters.
Table 6.2-1. DICOM Value Representations
| VR | Name | Definition | Character Repertoire | Length of Value |
|---|---|---|---|---|
| AE | Application Entity | A string of characters that identifies an Application Entity with leading and trailing spaces (20H) being non-significant. A Value consisting solely of spaces shall not be used. | Default Character Repertoire excluding character code 5CH (the BACKSLASH “" in ISO-IR 6), and all control characters. | 16 bytes maximum |
| AS | Age String | A string of characters with one of the following formats – nnnD, nnnW, nnnM, nnnY; where nnn shall contain the number of days for D, weeks for W, months for M, or years for Y.Example: “018M” would represent an age of 18 months. | “0”-“9”, “D”, “W”, “M”, “Y” of Default Character Repertoire | 4 bytes fixed |
| AT | Attribute Tag | Ordered pair of 16-bit unsigned integers that is the Value of a Data Element Tag.Example: A Data Element Tag of (0018,00FF) would be encoded as a series of 4 bytes in a Little-Endian Transfer Syntax as 18H,00H,FFH,00H.NoteThe encoding of an AT Value is exactly the same as the encoding of a Data Element Tag as defined in Section 7. | not applicable | 4 bytes fixed |
| CS | Code String | A string of characters identifying a controlled concept. Leading or trailing spaces (20H) are not significant.Alternatively, in the context of a Query with Empty Value Matching (see PS3.4), a string of two QUOTATION MARK characters, representing an empty key Value. | Uppercase characters, “0”-“9”, the SPACE character, and underscore “_”, of the Default Character RepertoireIn the context of a Query with Empty Value Matching (see PS3.4), the QUOTATION MARK character is allowed. | 16 bytes maximumIn the context of a Query with Empty Value Matching (see PS3.4), the length is 2 bytes fixed. |
| DA | Date | A string of characters of the format YYYYMMDD; where YYYY shall contain year, MM shall contain the month, and DD shall contain the day, interpreted as a date of the Gregorian calendar system.Example:“19930822” would represent August 22, 1993.NoteThe ACR-NEMA Standard 300 (predecessor to DICOM) supported a string of characters of the format YYYY.MM.DD for this VR. Use of this format is not compliant.See also DT VR in this table.Dates before year 1582, e.g., used for dating historical or archeological items, are interpreted as proleptic Gregorian calendar dates, unless otherwise specified.Alternatively, in the context of a Query with Empty Value Matching (see PS3.4), a string of two QUOTATION MARK characters, representing an empty key Value. | “0”-“9” of Default Character RepertoireIn the context of a Query with Range Matching (see PS3.4), the character “-” is allowed, and a trailing SPACE character is allowed for padding.In the context of a Query with Empty Value Matching (see PS3.4), the QUOTATION MARK character is allowed. | 8 bytes fixedIn the context of a Query with Range Matching (see PS3.4), the length is 18 bytes maximum.In the context of a Query with Empty Value Matching (see PS3.4), the length is 2 bytes fixed. |
| DS | Decimal String | A string of characters representing either a fixed point number or a floating point number. A fixed point number shall contain only the characters 0-9 with an optional leading “+” or “-” and an optional “.” to mark the decimal point. A floating point number shall be conveyed as defined in ANSI X3.9, with an “E” or “e” to indicate the start of the exponent. Decimal Strings may be padded with leading or trailing spaces. Embedded spaces are not allowed.NoteData Elements with multiple Values using this VR may not be properly encoded if Explicit-VR Transfer Syntax is used and the VL of this Data Element exceeds 65534 bytes. | “0”-“9”, “+”, “-”, “E”, “e”, “.” and the SPACE character of Default Character Repertoire | 16 bytes maximum |
| DT | Date Time | A concatenated date-time character string in the format:YYYYMMDDHHMMSS.FFFFFF&ZZXXThe components of this string, from left to right, are YYYY = Year, MM = Month, DD = Day, HH = Hour (range “00” - “23”), MM = Minute (range “00” - “59”), SS = Second (range “00” - “60”).FFFFFF = Fractional Second contains a fractional part of a second as small as 1 millionth of a second (range “000000” - “999999”).&ZZXX is an optional suffix for offset from Coordinated Universal Time (UTC), where & = “+” or “-”, and ZZ = Hours and XX = Minutes of offset.The year, month, and day shall be interpreted as a date of the Gregorian calendar system.A 24-hour clock is used. Midnight shall be represented by only “0000” since “2400” would violate the hour range.The Fractional Second component, if present, shall contain 1 to 6 digits. If Fractional Second is unspecified the preceding “.” shall not be included. The offset suffix, if present, shall contain 4 digits. The string may be padded with trailing SPACE characters. Leading and embedded spaces are not allowed.A component that is omitted from the string is termed a null component. Trailing null components of Date Time indicate that the Value is not precise to the precision of those components. The YYYY component shall not be null. Non-trailing null components are prohibited. The optional suffix is not considered as a component.A Date Time Value without the optional suffix is interpreted to be in the local time zone of the application creating the Data Element, unless explicitly specified by the Timezone Offset From UTC (0008,0201).UTC offsets are calculated as “local time minus UTC”. The offset for a Date Time Value in UTC shall be +0000.Alternatively, in the context of a Query with Empty Value Matching (see PS3.4), a string of two QUOTATION MARK characters, representing an empty key Value.NoteThe range of the offset is -1200 to +1400. The offset for United States Eastern Standard Time is -0500. The offset for Japan Standard Time is +0900.The RFC 2822 use of -0000 as an offset to indicate local time is not allowed.A Date Time Value of 195308 means August 1953, not specific to particular day. A Date Time Value of 19530827111300.0 means August 27, 1953, 11;13 a.m. accurate to 1/10th second.The Second component may have a Value of 60 only for a leap second.The offset may be included regardless of null components; e.g., 2007-0500 is a legal Value. | “0”-“9”, “+”, “-”, “.” and the SPACE character of Default Character RepertoireIn the context of a Query with Empty Value Matching (see PS3.4), the QUOTATION MARK character is allowed. | 26 bytes maximumIn the context of a Query with Range Matching (see PS3.4), the length is 54 bytes maximum.In the context of a Query with Empty Value Matching (see PS3.4), the length is 2 bytes fixed. |
| FL | Floating Point Single | Single precision binary floating point value represented in [IEEE 754] binary32 format. All [IEEE 754] values are permitted, including NaN (Not a Number) and infinity values. | not applicable | 4 bytes fixed |
| FD | Floating Point Double | Double precision binary floating point value represented in [IEEE 754] binary64 format. All [IEEE 754] values are permitted, including NaN (Not a Number) and infinity values. | not applicable | 8 bytes fixed |
| IS | Integer String | A string of characters representing an Integer in base-10 (decimal), shall contain only the characters 0 - 9, with an optional leading “+” or “-”. It may be padded with leading and/or trailing spaces. Embedded spaces are not allowed.The integer, n, represented shall be in the range:-231<= n <= (231-1). | “0”-“9”, “+”, “-” and the SPACE character of Default Character Repertoire | 12 bytes maximum |
| LO | Long String | A character string that may be padded with leading and/or trailing spaces. The character code 5CH (the BACKSLASH “" in ISO-IR 6) shall not be present, as it is used as the delimiter between Values in multi-valued Data Elements. The string shall not have Control Characters except for ESC. | Default Character Repertoire and/or as defined by (0008,0005) excluding character code 5CH (the BACKSLASH “" in ISO-IR 6), and all Control Characters except ESC when used for [ISO/IEC 2022] escape sequences. | 64 chars maximum (see Note in Section 6.2) |
| LT | Long Text | A character string that may contain one or more paragraphs. It may contain the Graphic Character set and the Control Characters, CR, LF, FF, and ESC. It may be padded with trailing spaces, which may be ignored, but leading spaces are considered to be significant. Data Elements with this VR shall not be multi-valued and therefore character code 5CH (the BACKSLASH “" in ISO-IR 6) may be used. | Default Character Repertoire and/or as defined by (0008,0005) excluding Control Characters except TAB, LF, FF, CR (and ESC when used for [ISO/IEC 2022] escape sequences). | 10240 chars maximum (see Note in Section 6.2) |
| OB | Other Byte | An octet-stream where the encoding of the contents is specified by the negotiated Transfer Syntax. OB is a VR that is insensitive to byte ordering (see Section 7.3). The octet-stream shall be padded with a single trailing NULL byte value (00H) when necessary to achieve even length. | not applicable | see Transfer Syntax definition |
| OD | Other Double | A stream of [IEEE 754] binary64 values. All [IEEE 754] values are permitted, including NaN (Not a Number) and infinity values. OD is a VR that requires byte swapping within each 64-bit word when changing byte ordering (see Section 7.3). | not applicable | 232-8 bytes maximum |
| OF | Other Float | A stream of [IEEE 754] binary32 values. All [IEEE 754] values are permitted, including NaN (Not a Number) and infinity values. OF is a VR that requires byte swapping within each 32-bit word when changing byte ordering (see Section 7.3). | not applicable | 232-4 bytes maximum |
| OL | Other Long | A stream of 32-bit words where the encoding of the contents is specified by the negotiated Transfer Syntax. OL is a VR that requires byte swapping within each word when changing byte ordering (see Section 7.3). | not applicable | see Transfer Syntax definition |
| OV | Other 64-bit Very Long | A stream of 64-bit words where the encoding of the contents is specified by the negotiated Transfer Syntax. OV is a VR that requires byte swapping within each word when changing byte ordering (see Section 7.3). | not applicable | see Transfer Syntax definition |
| OW | Other Word | A stream of 16-bit words where the encoding of the contents is specified by the negotiated Transfer Syntax. OW is a VR that requires byte swapping within each word when changing byte ordering (see Section 7.3). | not applicable | see Transfer Syntax definition |
| PN | Person Name | A character string encoded using a 5 component convention. The character code 5CH (the BACKSLASH “" in ISO-IR 6) shall not be present, as it is used as the delimiter between Values in multi-valued Data Elements. The string may be padded with trailing spaces. For human use, the five components in their order of occurrence are: family name complex, given name complex, middle name, name prefix, name suffix.NoteHL7 prohibits leading spaces within a component; DICOM allows leading and trailing spaces and considers them insignificant.Any of the five components may be an empty string. The component delimiter shall be the caret “^” character (5EH). There shall be no more than four component delimiters, i.e., none after the last component if all components are present. Delimiters are required for interior null components. Trailing null components and their delimiters may be omitted. Multiple entries are permitted in each component and are encoded as natural text strings, in the format preferred by the named person.For veterinary use, the first two of the five components in their order of occurrence are: responsible party family name or responsible organization name, patient name. The remaining components are not used and shall not be present.This group of five components is referred to as a Person Name component group.For the purpose of writing names in ideographic characters and in phonetic characters, up to 3 groups of components (see Annex H, Annex I and Annex J) may be used. The delimiter for component groups shall be the equals character “=” (3DH). There shall be no more than two component group delimiters, i.e., none after the last component group if all component groups are present. The three component groups of components in their order of occurrence are: an alphabetic representation, an ideographic representation, and a phonetic representation.Any component group may be absent, including the first component group. In this case, the person name may start with one or more “=” delimiters. Delimiters are required for interior null component groups. Trailing null component groups and their delimiters may be omitted.Precise semantics are defined for each component group. See Section 6.2.1.2.For examples and notes, see Section 6.2.1.1. | Default Character Repertoire and/or as defined by (0008,0005) excluding character code 5CH (the BACKSLASH “" in ISO-IR 6) and all Control Characters except ESC when used for [ISO/IEC 2022] escape sequences. | 64 chars maximum per component group(see Note in Section 6.2) |
| SH | Short String | A character string that may be padded with leading and/or trailing spaces. The character code 05CH (the BACKSLASH “" in ISO-IR 6) shall not be present, as it is used as the delimiter between Values for multi-valued Data Elements. The string shall not have Control Characters except ESC. | Default Character Repertoire and/or as defined by (0008,0005) excluding character code 5CH (the BACKSLASH “" in ISO-IR 6) and all Control Characters except ESC when used for [ISO/IEC 2022] escape sequences. | 16 chars maximum (see Note in Section 6.2) |
| SL | Signed Long | Signed binary integer 32 bits long in 2’s complement form. Represents an integer, n, in the range:- 231<= n <= 231-1. | not applicable | 4 bytes fixed |
| SQ | Sequence of Items | Value is a Sequence of zero or more Items, as defined in Section 7.5. | not applicable (see Section 7.5) | not applicable (see Section 7.5) |
| SS | Signed Short | Signed binary integer 16 bits long in 2’s complement form. Represents an integer n in the range:-215<= n <= 215-1. | not applicable | 2 bytes fixed |
| ST | Short Text | A character string that may contain one or more paragraphs. It may contain the Graphic Character set and the Control Characters, CR, LF, FF, and ESC. It may be padded with trailing spaces, which may be ignored, but leading spaces are considered to be significant. Data Elements with this VR shall not be multi-valued and therefore character code 5CH (the BACKSLASH “" in ISO-IR 6) may be used. | Default Character Repertoire and/or as defined by (0008,0005) excluding Control Characters except TAB, LF, FF, CR (and ESC when used for [ISO/IEC 2022] escape sequences). | 1024 chars maximum (see Note in Section 6.2) |
| SV | Signed 64-bit Very Long | Signed binary integer 64 bits long. Represents an integer n in the range:- 263<= n <= 263-1. | not applicable | 8 bytes fixed |
| TM | Time | A string of characters of the format HHMMSS.FFFFFF; where HH contains hours (range “00” - “23”), MM contains minutes (range “00” - “59”), SS contains seconds (range “00” - “60”), and FFFFFF contains a fractional part of a second as small as 1 millionth of a second (range “000000” - “999999”). A 24-hour clock is used. Midnight shall be represented by only “0000” since “2400” would violate the hour range. The string may be padded with trailing spaces. Leading and embedded spaces are not allowed.One or more of the components MM, SS, or FFFFFF may be unspecified as long as every component to the right of an unspecified component is also unspecified, which indicates that the Value is not precise to the precision of those unspecified components.The FFFFFF component, if present, shall contain 1 to 6 digits. If FFFFFF is unspecified the preceding “.” shall not be included.Examples:“070907.0705 " represents a time of 7 hours, 9 minutes and 7.0705 seconds.“1010” represents a time of 10 hours, and 10 minutes.“021 " is an invalid Value.NoteThe ACR-NEMA Standard 300 (predecessor to DICOM) supported a string of characters of the format HH:MM:SS.frac for this VR. Use of this format is not compliant.See also DT VR in this table.The SS component may have a Value of 60 only for a leap second.Alternatively, in the context of a Query with Empty Value Matching (see PS3.4), a string of two QUOTATION MARK characters, representing an empty key Value. | “0”-“9”, “.” and the SPACE character of Default Character RepertoireIn the context of a Query with Range Matching (see PS3.4), the character “-” is allowed.In the context of a Query with Empty Value Matching (see PS3.4), the QUOTATION MARK character is allowed. | 14 bytes maximumIn the context of a Query with Range Matching (see PS3.4), the length is 28 bytes maximum.In the context of a Query with Empty Value Matching (see PS3.4), the length is 2 bytes fixed. |
| UC | Unlimited Characters | A character string that may be of unlimited length that may be padded with trailing spaces. The character code 5CH (the BACKSLASH “" in ISO-IR 6) shall not be present, as it is used as the delimiter between Values in multi-valued Data Elements. The string shall not have Control Characters except for ESC. | Default Character Repertoire and/or as defined by (0008,0005) excluding character code 5CH (the BACKSLASH “" in ISO-IR 6), and all Control Characters except ESC when used for [ISO/IEC 2022] escape sequences. | 232-2 bytes maximumSee Note 2 |
| UI | Unique Identifier (UID) | A character string containing a UID that is used to uniquely identify a wide variety of items. The UID is a series of numeric components separated by the period “.” character. If a Value Field containing one or more UIDs is an odd number of bytes in length, the Value Field shall be padded with a single trailing NULL (00H) character to ensure that the Value Field is an even number of bytes in length. See Section 9 and Annex B for a complete specification and examples. | “0”-“9”, “.” of Default Character Repertoire | 64 bytes maximum |
| UL | Unsigned Long | Unsigned binary integer 32 bits long. Represents an integer n in the range: 0 <= n < 232. | not applicable | 4 bytes fixed |
| UN | Unknown | An octet-stream where the encoding of the contents is unknown (see Section 6.2.2). | not applicable | Any length valid for any of the other DICOM Value Representations |
| UR | Universal Resource Identifier or Universal Resource Locator (URI/URL) | A string of characters that identifies a URI or a URL as defined in [RFC3986]. Leading spaces are not allowed. Trailing spaces shall be ignored. Data Elements with this VR shall not be multi-valued.Alternatively, in the context of a Query with Empty Value Matching (see PS3.4), a string of two QUOTATION MARK characters, representing an empty key Value.NoteBoth absolute and relative URIs are permitted. If the URI is relative, then it is relative to the base URI of the object within which it is contained, or to a base URI in another Attribute as specified by the Information Object Definition. | The subset of the Default Character Repertoire required for the URI as defined in IETF RFC3986 Section 2, plus the space (20H) character permitted only as trailing padding.Characters outside the permitted character set must be “percent encoded”.NoteThe BACKSLASH (5CH) character is among those disallowed in URIs.In the context of a Query with Empty Value Matching (see PS3.4), the QUOTATION MARK character is allowed. | 232-2 bytes maximum.See Note 2.In the context of a Query with Empty Value Matching (see PS3.4), the length is 2 bytes fixed. |
| US | Unsigned Short | Unsigned binary integer 16 bits long. Represents integer n in the range: 0 <= n < 216. | not applicable | 2 bytes fixed |
| UT | Unlimited Text | A character string that may contain one or more paragraphs. It may contain the Graphic Character set and the Control Characters, CR, LF, FF, and ESC. It may be padded with trailing spaces, which may be ignored, but leading spaces are considered to be significant. Data Elements with this VR shall not be multi-valued and therefore character code 5CH (the BACKSLASH “" in ISO-IR 6) may be used. | Default Character Repertoire and/or as defined by (0008,0005) excluding Control Characters except TAB, LF, FF, CR (and ESC when used for [ISO/IEC 2022] escape sequences). | 232-2 bytes maximumSee Note 2 |
| UV | Unsigned 64-bit Very Long | Unsigned binary integer 64 bits long. Represents an integer n in the range: 0 <= n < 264. | not applicable | 8 bytes fixed |
Source: https://dicom.nema.org/medical/dicom/current/output/html/part05.html#sect_6.2