DICOM PS3.5 2021b - Data Structures and Encoding

7 The Data Set (Current)
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# 7 The Data Set

A Data Set represents an instance of a real world Information Object. A Data Set is constructed of Data Elements. Data Elements contain the encoded Values of Attributes of that object. The specific content and semantics of these Attributes are specified in Information Object Definitions (see PS3.3).

The construction, characteristics, and encoding of a Data Set and its Data Elements are discussed in this section. Pixel Data, Overlays, and Curves are Data Elements whose interpretation depends on other related elements.

## 7.1 Data Elements

A Data Element is uniquely identified by a Data Element Tag. The Data Elements in a Data Set shall be ordered by increasing Data Element Tag Number and shall occur at most once in a Data Set.

### Note

A Data Element Tag may occur again within Nested Data Sets (see Section 7.5).

Two types of Data Elements are defined:

- Standard Data Elements have an even Group Number that is not (0000,eeee), (0002,eeee), (0004,eeee), or (0006,eeee).

  ### Note

  Usage of these groups is reserved for DIMSE Commands (see PS3.7) and DICOM File Formats.

- Private Data Elements have an odd Group Number that is not (0001,eeee), (0003,eeee), (0005,eeee), (0007,eeee), or (FFFF,eeee). Private Data Elements are discussed further in Section 7.8.

### Note

Although similar or related Data Elements often have the same Group Number; a Data Group does not convey any semantic meaning.

A Data Element shall have one of three structures. Two of these structures contain the VR of the Data Element (Explicit VR) but differ in the way their lengths are expressed, while the other structure does not contain the VR (Implicit VR). All three structures contain the Data Element Tag, Value Length and Value for the Data Element. See Figure 7.1-1.

Implicit and Explicit VR Data Elements shall not coexist in a Data Set and Data Sets nested within it (see Section 7.5). Whether a Data Set uses Explicit or Implicit VR, among other characteristics, is determined by the negotiated Transfer Syntax (see Section 10 and Annex A).

### Note

VRs are not contained in Data Elements when using DICOM Default Transfer Syntax (DICOM Implicit VR Little Endian Transfer Syntax).

Figure 7.1-1. DICOM Data Set and Data Element Structures

### 7.1.1 Data Element Fields

A Data Element is made up of fields. Three fields are common to all three Data Element structures; these are the Data Element Tag, Value Length, and Value Field. A fourth field, Value Representation, is only present in the two Explicit VR Data Element structures. The Data Element structures are defined in Section 7.1.2 and Section 7.1.3. The definitions of the fields are:

Data Element Tag

  An ordered pair of 16-bit unsigned integers representing the Group Number followed by Element Number.

Value Representation

  Two single byte characters containing the VR of the Data Element. The VR for a given Data Element Tag shall be as defined by the Data Dictionary as specified in PS3.6. The two byte VR shall be encoded using only upper case letters from the DICOM default character set.

Value Length

  Either:

  - a 16 or 32-bit (dependent on VR and whether VR is explicit or implicit) unsigned integer containing the Explicit Length of the Value Field as the number of bytes (even) that make up the Value. It does not include the length of the Data Element Tag, Value Representation, and Value Length Fields.

  - a 32-bit Length Field set to Undefined Length (FFFFFFFFH). Undefined Lengths may be used for Data Elements having the Value Representation (VR) Sequence of Items (SQ) and Unknown (UN). For Data Elements with Value Representation OW or OB Undefined Length may be used depending on the negotiated Transfer Syntax (see Section 10 and Annex A).

  ### Note

  1. The decoder of a Data Set should support both Explicit and Undefined Lengths for VRs of SQ and UN and, when applicable, for VRs of OW and OB.

  2. The 32-bit Value Length Field limits the maximum size of large data values such as Pixel Data sent in a Native Format (encoded in Transfer Syntaxes that use only the unencapsulated form).

Value Field

  An even number of bytes containing the Value(s) of the Data Element.

  The data type of Value(s) stored in this field is specified by the Data Element's VR. The VR for a given Data Element Tag can be determined using the Data Dictionary in PS3.6, or using the VR Field if it is contained explicitly within the Data Element. The VR of Standard Data Elements shall agree with those specified in the Data Dictionary.

  The Value Multiplicity specifies how many Values with this VR can be placed in the Value Field. If the VM is greater than one, multiple values shall be delimited within the Value Field as defined previously in Section 6.4. The VMs of Standard Data Elements are specified in the Data Dictionary in PS3.6.

  Value Fields with Undefined Length are delimited through the use of Sequence Delimitation Items and Item Delimitation Data Elements, which are described further in Section 7.5.

### 7.1.2 Data Element Structure with Explicit VR

When using the Explicit VR structures, the Data Element shall be constructed of four consecutive fields: Data Element Tag, VR, Value Length, and Value. Depending on the VR of the Data Element, the Data Element will be structured in one of two ways:

- for VRs of AE, AS, AT, CS, DA, DS, DT, FL, FD, IS, LO, LT, PN, SH, SL, SS, ST, TM, UI, UL and US the Value Length Field is the 16-bit unsigned integer following the two byte VR Field (Table 7.1-2). The value of the Value Length Field shall equal the length of the Value Field.

- for all other VRs the 16 bits following the two byte VR Field are reserved for use by later versions of the DICOM Standard. These reserved bytes shall be set to 0000H and shall not be used or decoded (Table 7.1-1). The Value Length Field is a 32-bit unsigned integer.

  - for VRs of OB, OD, OF, OL, OV, OW, SQ and UN, if the Value Field has an Explicit Length, then the Value Length Field shall contain a value equal to the length (in bytes) of the Value Field, otherwise, the Value Field has an Undefined Length and a Sequence Delimitation Item marks the end of the Value Field.

  - for all other VRs with a 32-bit Value Length Field, the Value Length Field shall contain a value equal to the length (in bytes) of the Value Field.

  ### Note

  VRs of SV, UC, UR, UV and UT may not have an Undefined Length, i.e.,a Value Length of FFFFFFFFH.

Table 7.1-1. Data Element with Explicit VR other than as shown in Table 7.1-2

Tag | VR | Value Length | Value
--- | --- | --- | ---
Group Number (16-bit unsigned integer) | Element Number (16-bit unsigned integer) | VR (2 single byte characters) | Reserved (2 bytes) set to a value of 0000H | 32-bit unsigned integer | Even number of bytes containing the Data Element Value(s) encoded according to the VR and negotiated Transfer Syntax. Delimited with Sequence Delimitation Item if of Undefined Length.
2 bytes | 2 bytes | 2 bytes | 2 bytes | 4 bytes | 'Value Length' bytes if of Explicit Length

Table 7.1-2. Data Element with Explicit VR of AE, AS, AT, CS, DA, DS, DT, FL, FD, IS, LO, LT, PN, SH, SL, SS, ST, TM, UI, UL and US

Tag | VR | Value Length | Value
--- | --- | --- | ---
Group Number (16-bit unsigned integer) | Element Number (16-bit unsigned integer) | VR (2 single byte characters) | (16-bit unsigned integer) | Even number of bytes containing the Data Element Value(s) encoded according to the VR and negotiated Transfer Syntax.
2 bytes | 2 bytes | 2 bytes | 2 bytes | 'Value Length' bytes

### 7.1.3 Data Element Structure with Implicit VR

When using the Implicit VR structure the Data Element shall be constructed of three consecutive fields: Data Element Tag, Value Length, and Value (see Table 7.1-3). If the Value Field has an Explicit Length then the Value Length Field shall contain a value equal to the length (in bytes) of the Value Field. Otherwise, the Value Field has an Undefined Length and a Sequence Delimitation Item marks the end of the Value Field.

Table 7.1-3. Data Element with Implicit VR

Tag | Value Length | Value
--- | --- | ---
Group Number (16-bit unsigned integer) | Element Number (16-bit unsigned integer) | 32-bit unsigned integer | Even number of bytes containing the Data Elements Value encoded according to the VR specified in PS3.6 and the negotiated Transfer Syntax. Delimited with Sequence Delimitation Item if of Undefined Length.
2 bytes | 2 bytes | 4 bytes | 'Value Length' bytes or Undefined Length

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DICOM PS3.5 2021b - Data Structures and Encoding