DICOM-具有定义长度的SequenceDelimitationItem

Question

Is it ok to use SequenceDelimitationItem along with Defined Length in DICOM?

Is it one way: if Undefined length then SequenceDelimitationItem must be present?

Or is it two ways: if and only if IDI present if and only if Undefined Length is set?

I think the first way of looking at this simplifies reading algorithms a bit. Plus the unknown sequences lengths could be calculated once and saved without having to parse whole unknown substructure?

Answer 1

DICOM standard allows mixing of explicit length and undefined length elements in a single dataset. As for example, a SQ Data Element of Undefined Length encapsulating Items of Explicit Length (see PS 3.5 Table 7.5-2.). In this case you will need the Sequence Delimitation Item (FFFE, E0DD) without the Item delimitation tag (FFFE, E00D).

Another example provided by the standard shows a SQ Data Element of Undefined Length encapsulating Items of both Explicit and Undefined Length (see Table 7.5-3). This will require Item Delimitation Tag (FFFE, E00D) for the undefined length item plus the Sequence Delimitation Item (FFFE, E0DD).

Also, note that explicit length sequence is not very common.

Answer 2

see Dicom part 5, 7.5.2.

If you have undefined length you definitely have to use Sequence Delimitation Items (Tags) and and Item Delimitation Tags

If you have defined length (n) you can put everything in that n bytes (including any tag and item you want). But it will not be under the scope of Dicom definition. It will be just data. and what is in there depends on the meaning of that data.