The Pixel Data Element (7FE0,0010) and Overlay Data Element (60xx,3000) shall be used for the exchange of encoded graphical image data. These elements along with additional Data Elements, specified as Attributes of the Image Information Entities defined in PS 3.3, shall be used to describe the way in which the Pixel Data and Overlay Data are encoded and shall be interpreted. Finally, depending on the negotiated Transfer Syntax (see Section 10 and Annex A), Pixel Data may be compressed.
The Pixel Data Element (7FE0,0010) and Overlay Data Element (60xx,3000) have a VR of OW or OB, depending on the negotiated Transfer Syntax (see Annex A). The only difference between OW and OB being that OB, a string of bytes, shall be unaffected by Byte Ordering (see Section 7.3).
Encoded Pixel Data of various bit depths shall be accommodated. The following three Data Elements shall define the Pixel structure:
Each Pixel Cell shall contain a single Pixel Sample Value. The size of the Pixel Cell shall be specified by Bits Allocated (0028,0100). Bits Stored (0028,0101) defines the total number of these allocated bits that will be used to represent a Pixel Sample Value. Bits Stored (0028,0101) shall never be larger than Bits Allocated (0028,0100). High Bit (0028,0102) specifies where the high order bit of the Bits Stored (0028,0101) is to be placed with respect to the Bits Allocated (0028,0100) specification. Bits not used for Pixel Sample Values can be used for overlay planes described further in PS3.3.
Note: For example, in Pixel Data with 16 bits (2 bytes) allocated, 12 bits stored, and bit 15 specified as the high bit, one pixel sample is encoded in each 16-bit word, with the 4 least significant bits of each word not containing Pixel Data. See Annex D for other examples of the basic encoding schemes.
Restrictions are placed on acceptable Values for Bits Allocated (0028,0100), Bits Stored (0028,0101), and High Bit (0028,0102) and are specified in the Information Object Definitions in PS3.3. Also, the Value Field containing Pixel Data, like all other Value Fields in DICOM, shall be an even number of bytes in length. This means that the Value Field may need to be padded with data that is not part of the image and shall not be considered significant. If needed, the padding bits shall be appended to the end of the Value Field, and shall be used only to extend the data to the next even byte increment of length.
In a multi-frame object that is transmitted in Native Format, the individual frames are not padded. The individual frames shall be concatenated and padding bits (if necessary) apply to the complete Value Field.
Note: Receiving applications should be aware that some older applications may send Pixel Data with excess padding, which was not explicitly prohibited in earlier versions of the Standard. Applications should be prepared to accept such Pixel Data elements, but may delete the excess padding. In no case should a sending application place private data in the padding data.
The field of bits representing the value of a Pixel Sample shall be a binary 2's complement integer or an unsigned integer, as specified by the Data Element Pixel Representation (0028,0103). The sign bit shall be the High Bit in a Pixel Sample Value that is a 2's complement integer. The minimum actual Pixel Sample Value encountered in the Pixel Data is specified by Smallest Image Pixel Value (0028,0106) while the maximum value is specified by Largest Image Pixel Value (0028,0107).
Encoded Overlay Planes always have a bit depth of 1, and are encoded separately from the Pixel Data in Overlay Data (60xx,3000). The following two Data Elements shall define the Overlay Plane structure:
Note: There is no Data Element analogous to Bits Stored (0028,0101) since Overlay Planes always have a bit depth of 1.
Note: Restrictions on the allowed values for these Data Elements are defined in PS 3.3. Formerly overlay data stored in unused bits of Pixel Data (7FE0,0010) was described, and these attributes had meaningful values but this usage has been retired. See PS 3.5 2004. For overlays encoded in Overlay Data Element (60xx,3000), Overlay Bits Allocated (60xx,0100) is always 1 and Overlay Bit Position (60xx,0102) is always 0.
For Overlay Data Element (60xx,3000), the Value Representation OW is most often required. The Value Representation OB may also be used for Overlay Data in cases where the Value Representation is explicitly conveyed (see Annex A).
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.
Overlay Data is encoded as the direct concatenation of the bits of a single Overlay Plane, where the first bit of an Overlay Plane is encoded in the least significant bit, immediately followed by the next bit of the Overlay Plane in the next most significant bit. When the Overlay Data 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). For Overlay 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).
Note: For Overlay Data encoded with the Value Representation OB, the Overlay Data encoding is unaffected by Little Endian or Big Endian byte ordering.