Материал: part05

•​yyyy identifies a specific organization as registered by an International Code Designator registration authority (see​ ISO 6523).​

3.​Example components of a <suffix> for unique identification of an image could include:​ •​product​

•​system identifier​

•​study, series and image numbers​ •​study, series and image date & times.​

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DExamplesofVariousPixelDataandOverlay​

Encoding Schemes (Informative)​

D.1 Detailed Example of Pixel Data Encoding​

As specified in PS3.3, Image Pixel Data is stored within the Value of the Pixel Data Element (7FE0,0010). The order in which Pixel​ Data for an image plane is encoded is from left to right and top to bottom, a row at a time (see Figure D-1).​

Image Plane

Pixel i

Pixel i-1

Pixel i+2

Pixel i+1

Figure D-1. An Image Pixel Plane​

An individual pixel may consist of one or more Pixel Sample Values (e.g., color or multi-planar images). Each Pixel Sample Value​ can be expressed as either a binary 2's complement integer or a binary unsigned integer, as specified by the Pixel Representation​ DataElement(0028,0103).ThenumberofbitsineachPixelSampleValueisspecifiedbyBitsStored(0028,0101).For2'scomplement​ integer Pixel Samples the sign bit is the most significant bit of the Pixel Sample Value.​

A Pixel Cell is the container for a Pixel Sample Value and optionally additional bits. These additional bits are used to place Pixels on​ certain boundaries (byte, word, etc.). A Pixel Cell exists for every individual Pixel Sample Value in the Pixel Data. The size of the​ Pixel Cells is specified by Bits Allocated (0028,0100) and is greater than or equal to the Bits Stored (0028,0101). The placement of​ the Pixel Sample Values within the Pixel Cells is specified by High Bit (0028,0102).​

Any restrictions on the characteristics of a Pixel Cell and the Pixel Sample Value contained therein are specific to the Information​ Object Definition (e.g., Image Object) containing the Pixel Data Element (see PS3.3).​

The Pixel Data Element, as specified by the DICOM Default Transfer Syntax in Section 10.1, has a Value Representation of OW​ (Other Word). The Pixel Data in DICOM, as it was in ACR-NEMA 2.0, is packed, except that Bits Allocated is always either 1, or a​ multiple of 8 (see Figure D-2). One way to visualize this packed encoding is to imagine encoding the Pixel Cells as a concatenated​ stream of bits from the least significant bit of the first Pixel Cell up through the most significant bit of the last Pixel Cell. Within this​ stream, the most significant bit of any Pixel Cell is followed by the least significant bit of the next Pixel Cell. The Pixel Data can then​ be broken up into a stream of physical 16-bit words, each of which is subject to the byte ordering constraints of the Transfer Syntax.​

Allother(non-default)DICOMTransferSyntaxesmakeuseofexplicitVRencoding.FortheseTransferSyntaxes,allPixelDatawhere​ Bits Allocated is less than or equal to 8 may be encoded with an explicit VR of OB (see Annex A). As in the OW case, Pixel Cells are​ packed together, but in this case the Pixel Data is broken up into a stream of physical 8-bit words.​

Note​

For Pixel Data encoded with an explicit VR of OB, the encoding of the Pixel Data is unaffected by byte ordering.​

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Figure D-2. Encoding (Packing) of Arbitrary Pixel Data with a VR of OW​

With the exception of single bit images, Pixel Cells begin and end on byte or word boundaries and such that the Pixel Sample Value​ contained within also fits 'neatly' within a cell.​

Figure D-3 is an example of Pixel Data encoding using the Value representation of OW for the purposes of clarification. The Example​ is a valid example for a CT Image Information Object.​

Example: CT Pixel Cell

Pixel Sample

15 14131211 10 9 8 7 6 5 4 3 2 1 0

Bits Allocated = 16

Bits Stored = 12

High Bit = 11

Figure D-3. Example Pixel Cells​

Figure D-4 shows Pixel Data constructed of these example Pixel Cells as they are packed into a stream of 16-bit words.​

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Example: CT Pixel Data Value

Figure D-4. Example Pixel Cells Packed into 16-bit Words (VR = OW)​

Byte ordering becomes a consideration when we represent the Pixel Data physically, in memory, a file, or on a network.​

In the memory of a byte-addressable Big Endian machine, the highest order byte (bits 8 - 15) in each 16-bit word has a binary address​ of x...x0. While in a byte-addressable Little Endian machine, the lowest order byte (bits 0 - 7) in each 16-bit word has a binary address​ of x...x0. Figure D-5 pictures our example Pixel Data streams as they would be addressed in the memory of both a Big Endian and​ a Little Endian machine.​

Example: CT Pixel Data Value in Memory

MSB = Most Significant Byte

LSB = Least Significant Byte

Figure D-5. Example Pixel Cells Byte Ordered in Memory (VR = OW)​

Byte ordering is also specified as part of the negotiated Transfer Syntax used in the exchange of a DICOM message. Sixteen bit​ words are transmitted across the network (a byte at a time) least significant byte first in the case of a Little Endian Transfer Syntax​ (see Figure D-6).​

Example: CT Pixel Data Value Byte Stream (VR = OW)

Little Endian Transfer Syntax

byte 5 MSb

...

...

MSb = Most Significant Bit

LSb = Least Significant Bit

Figure D-6. Sample Pixel Data Byte Streams (VR = OW)​

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As a last pair of examples, for Pixel Data having the Value Representation OW and the following attributes: 8 bits allocated, 8 bits​ stored, and a high bit of 7; the resulting byte streams pictured in Figure D-7 are as they would be transmitted across a network and/or​ stored on media. For Pixel Data having the same attributes, but having the explicit Value Representation OB; the resulting byte​ streams are unaffected by byte ordering and are pictured in Figure D-8.​

8 Bit Pixel Data Byte Stream (VR = OW; 8 bits allocated, high bit of 7)

Little Endian Transfer Syntax

Figure D-7. Sample Pixel Data Byte Streams for 8-bits Allocated and 8-bits Stored (VR = OW)​

8 Bit Pixel Data Byte Stream (VR = OB; 8 bits allocated, high bit of 7)

Little Endian Transfer Syntax

Figure D-8. Sample Pixel Data Byte Streams for 8-bits Allocated and 8-bits Stored (Explicit VR = OB)​

D.2 Various Additional Examples of Pixel and Overlay Data Cells​

The following examples further illustrate the use of the data elements for Bits Allocated (0028,0100), Bits Stored (0028,0101) and​ High Bit (0028,0102) in the encoding of Pixel and Overlay Data. All examples show sample Pixel Cells before being encoded in byte​ streams (and before being affected by a particular Transfer Syntax).​

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