Delimitation of the Sequence of Items

Delimitation of the last Item of a Sequence of Items, encapsulated in a Data Element of Value Representation SQ, shall be in one of the two following ways:

Explicit Length: The number of bytes (even) contained in the Data Element Value (following but not including the Data Element Length Field) is encoded as a 32-bit unsigned integer value (see Section 7.1 in PS 3.5 of the DICOM Standard). This length shall include the total length resulting from the sequence of zero or more items conveyed by this Data Element. This Data Element Length shall be equal to 00000000H if the sequence of Items contains zero Items.
Undefined Length: The Data Element Length Field shall contain a Value FFFFFFFFH to indicate an Undefined Sequence length. It shall be used in conjunction with a Sequence Delimitation Item. A Sequence Delimitation Item shall be included after the last Item in the sequence. Its Item Tag shall be (FFFE,E0DD) with an Item Length of 00000000H. No Value shall be present.

The encoder of a Sequence of Items may choose either one of the two ways of encoding. Both ways of encoding shall be supported by decoders of the Sequence of Items.

Note: The Sequence Delimitation Item Tag (FFFE,E0DD) is different from the Item Delimitation Tag (FFFE,E00D) in that it indicates the end of a Sequence of Items whose Length was left undefined. If an undefined length Item is the last Item of a Sequence of Items of undefined length, then an Item Delimitation Tag will be followed by a Sequence Delimitation Tag.

Note: An encapsulated Data Set does not include the Specific Character Set data element unless the Specific Character Set Attribute is defined as part of the IOD for that sequence.

If an encapsulated Data Set includes the Specific Character Set Attribute, it shall apply only to the encapsulated Data Set. If the Attribute Specific Character Set is not explicitly included in an encapsulated Data Set, then the Specific Character Set value of the encapsulating Data Set applies.

The following is an illustrative example.

DE	Data Element
DE-SQ	Sequence of Items
DE-SQD	Sequence Delimitation Item
DE-I	Item
DE-ID	Item Delimitation Data Element

Please note the DE-SQD and DE-ID elements are added internally. You do not have to explicitly add these elements.