Pixel data conveyed in the Pixel Data Element (7FE0,0010) may be sent either in a Native (uncompressed) Format or in an Encapsulated Format (e.g., compressed) defined outside the DICOM standard.
If Pixel Data is sent in a Native Format, the Value Representation OW is most often required. The Value Representation OB may also be used for Pixel Data in cases where Bits Allocated has a value less than or equal to 8, but only with Transfer Syntaxes where the Value Representation is explicitly conveyed (see Annex A in PS 3.5 of the DICOM Standard).
Note: The DICOM default Transfer Syntax (Implicit VR Little Endian) does not explicitly convey Value Representation and therefore the VR of OB may not be used for Pixel Data when using the default Transfer Syntax.
Native format Pixel Cells are encoded as the direct concatenation of the bits of each Pixel Cell, where the most significant bit of a Pixel Cell is immediately followed by the least significant bit of the next Pixel Cell. The number of bits of each Pixel Cell is defined by the Bits Allocated (0028,0100) Data Element Value. When a Pixel Cell crosses a word boundary in the OW case, or a byte boundary in the OB case, it shall continue to be encoded, least significant bit to most significant bit, in the next word, or byte, respectively (see Annex D in PS 3.5 of the DICOM Standard). For Pixel Data encoded with the Value Representation OW, the byte ordering of the resulting 2-byte words is defined by the Little Endian or Big Endian Transfer Syntaxes negotiated at the Association Establishment (see Annex A in PS 3.5 of the DICOM Standard).
Note:
a) For Pixel Data encoded with the Value Representation OB, the Pixel Data encoding is unaffected by Little Endian or Big Endian byte ordering.
b) If encoding Pixel Data with a Value for Bits Allocated (0028,0100) not equal to 16 be sure to read and understand Annex D in PS 3.5 of the DICOM Standard.
If sent in an Encapsulated Format (i.e., other than the Native Format) the Value Representation OB is used. The Pixel Cells are encoded according to the encoding process defined by one of the negotiated Transfer Syntaxes (see Annex A in PS 3.5 of the DICOM Standard). The encapsulated pixel stream of encoded pixel data is segmented in one or more Fragments which convey their explicit length. The sequence of Fragments of the encapsulated pixel stream is terminated by a delimiter, thus allowing the support of encoding processes where the resulting length of the entire pixel stream is not known until it is entirely encoded. This Encapsulated Format supports both Single-Frame and Multi-Frame images (as defined in PS 3.3 of the DICOM Standard).