Encoded Pixel Data of various bit depths shall be accommodated. The following three Data Elements shall define the Pixel structure:
- Bits Allocated (0028,0100)
- Bits Stored (0028,0101)
- High Bit (0028,0102)
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 PS 3.3 of the DICOM Standard.
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 in PS 3.5 of the DICOM Standard 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 PS 3.3. 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.
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).