Extended Offset Table clarification Status Assigned Date of Last Update 2024/02/29 Person Assigned David Clunie Submitter Name David Clunie Submission Date 2024/02/23 Correction Number CP-2388 Log Summary: Extended Offset Table clarification Name of Standard PS3.3, PS3.5 Rationale for Correction: The Extended Offset Table introduced by CP 1818 contains offsets and lengths and requires each frame be encoded in a single fragment. Yet the description of the length attribute implies that the offsets may be present when frames are fragmented. Clarify that this is not the case (since the "and every frame is encoded as a single fragment" conditional clause is always true). Also, note that lengths may be odd if the compressed bitstream is an odd length (i.e., padding is not included). Also, clarify the description of when the Basic Offset Table is not present (i.e., the Item Tag is always present, but when it is zero value length, the table is absent). Also, explain why offset table is only present for Encapsulated Format. [Open Issue: Is there value in relaxing the requirement tbat the BOT and EOT are mutually exclusive? Is there a use case for both? E.g., legacy readers might use the BOT, new readers might take advantage of the EOT being in the metadata, or having the additional length information.] [Open Issue: Is there value in adding another optional Attribute that is the byte offset from the start of the PS3.10 file to the beginning of the Pixel Data 7FE0 group? This would allow readers to skip to the extended offset table and then the pixel data without fully parsng the header before that point. If assigned a low group,element tag number, it would be encoded early in the stream, e.g., after the retired (0008,0001) Length to End (ACR-NEMA message length); alternatively, it could be in the PS3.10 meta information (group 0x0002).] [Ed.Note. Account for changes in CP 2362, which adds text to the Extended Offset Table description note.] Correction Wording:

Image Pixel Module Attributes

Attribute Name	Tag	Type	Attribute Description
Extended Offset Table	(7FE0,0001)	3	Byte offsets of the Item Tags of the Frames in the Sequence of Items in Encapsulated Pixel Data encoded in Pixel Data (7FE0,0010). See . May only be present when: Pixel Data (7FE0,0010) is present, and the Transfer Syntax uses Encapsulated Format for the Pixel Data (7FE0,0010), and the Transfer Syntax encodes Frames in separate Fragments, and the Basic Offset Table is not present (i.e., in the first Item of Pixel Data (7FE0,0010) is not present (its Item has zero length), and each Frame is entirely contained within one Fragment. Unlike a Basic Offset Table, an Extended Offset Table is not permitted to be empty.
Extended Offset Table Lengths	(7FE0,0002)	1C	Byte lengths of the Frames in the Sequence of Items in Encapsulated Pixel Data encoded in Pixel Data (7FE0,0010). See . Required if Extended Offset Table (7FE0,0001) is present and every frame is encoded as a single fragment. The length information is not sent when there are multiple fragments per frame, since it is necessary to assemble the frame from fragments with delimiters rather than assuming a single contiguous span of bytes.This Attribute is only sent when each Frame is entirely contained within one Fragment as a single contiguous span of bytes so that it is not necessary to assemble the frame from fragments with delimiters. The length encoded in this Attribute may be an odd number if the compressed bitstream for the frame is an odd length; i.e., it does not include any trailing padding required to make the Item an even length.

The Extended Offset Table (7FE0,0001) Value shall contain byte offsets to the first byte of the Item Tag of the first fragment for every frame in the Pixel Data (7FE0,0010) Sequence. The byte offsets are measured from the first byte of the first Item Tag following the empty (zero length) Basic Offset Table item, i.e., the Item Tag of the first fragment of the first frame. If present, the first entry will always be 00000000H. The offset table is only present when the Pixel Data (7FE0,0010) is encoded in Encapsulated Format, since the position of uncompressed frames encoded in Native Format can be computed from their known fixed length (i.e., derived from Rows (0028,0010), Columns (0028,0011), Samples per Pixel (0028,0002) and Bits Allocated (0028,0100)).

Amend DICOM PS3.5 as follows (changes to existing text are bold and underlined for additions and struckthrough for removals):

The encoding of the Data Elements of the Data Set shall be as follows according to their Value Representations: ... For the Value Representations OB, OL, OV and OW, the encoding shall meet the following specification depending on the Data Element Tag: Pixel Data (7FE0,0010) may be encapsulated or native. It shall be encapsulated if present in the top-level Data Set (i.e., not nested within a Sequence Data Element). ... If encapsulated, it has the Value Representation OB and is an octet-stream resulting from one of the encoding processes. It contains the encoded pixel data stream fragmented into one or more Item(s). This Pixel Data Stream may represent a Single or Multi-frame Image. See and . ... The first Item in the Sequence of Items before the encoded Pixel Data Stream shall be a Basic Offset Table item. The Basic Offset Table Item Value, however, is not required to be present: When the Item Value is not present, the Item Length shall be zero (00000000H) (see ). When the Item Value is present, the Basic Offset Table Item Value shall contain concatenated 32-bit unsigned integer values that are byte offsets to the first byte of the Item Tag of the first fragment for each frame in the Sequence of Items. These offsets are measured from the first byte of the first Item Tag following the Basic Offset Table item (see ). For a Multi-Frame Image containing only one frame or a Single Frame Image, the Basic Offset Table Item Value may be present or not. If present it will contain a single 00000000H value. Decoders of encapsulated pixel data, whether Single Frame or Multi-Frame, need to accept both an empty Basic Offset Table (zero length) and a Basic Offset Table filled with 32 bit offset values. A Basic Offset Table Item Value is not permitted (i.e., the Item Length of the first Item will be zero) if Extended Offset Table (7FE0,0001) is present. If the Basic Offset Table Item Value is not zero length, the first entry will always be 00000000H. ...

Examples of Elements for an Encoded Two-Frame Image Defined as a Sequence of Three Fragments with Basic Table Item Values

Pixel Data Element Tag	Value Representation		Data Element Length	Data Element
(7FE0, 0010) with VR of OB	OB	0000H Reserved	FFFF FFFFH Undefined Length	Basic Offset Table with Item Value			First Fragment (Frame 1) of Pixel Data
				Item Tag	Item Length	Item Value	Item Tag	Item Length	Item Value
				(FFFE, E000)	0000 0008H	0000 0000H 0000 0646H	(FFFE, E000)	0000 02C8H	Compressed Fragment
4 bytes	2 bytes	2 bytes	4 bytes	4 bytes	4 bytes	0008H bytes	4 bytes	4 bytes	02C8H bytes

Examples of Elements for an Encoded Two-Frame Image Defined as a Sequence of Three Fragments with Basic Table Item Values (continued)

Data Element Continued
Second Fragment (Frame 1) of Pixel Data			Third Fragment (Frame 2) of Pixel Data			Sequence Delimiter Item
Item Tag	Item Length	Item Value	Item Tag	Item Length	Item Value	Sequence Delimiter Tag	Item Length
(FFFE, E000)	0000 036EH	Compressed Fragment	(FFFE, E000)	0000 0BC8H	Compressed Fragment	(FFFE, E0DD)	0000 0000H
4 bytes	4 bytes	036EH bytes	4 bytes	4 bytes	0BC8H bytes	4 bytes	4 bytes