Overview: 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 Sections 7.1.2. and 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 A two-byte character string 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 PS 3.6 of the DICOM Standard. The two character VR shall be encoded using characters 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.
or
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). 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 of the DICOM Standard).
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 PS 3.6 of the DICOM Standard, 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 of the DICOM Standard. The VMs of Standard Data Elements are specified in the Data Dictionary in PS 3.6 of the DICOM Standard.
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 of the DICOM Standard.
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