DICOM PS3.5 2018b - Data Structures and Encoding

A.2 DICOM Little Endian Transfer Syntax (Explicit VR)
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## A.2 DICOM Little Endian Transfer Syntax (Explicit VR)

This Transfer Syntax applies to the encoding of the entire DICOM Data Set. This implies that when a DICOM Data Set is being encoded with the DICOM Little Endian Transfer Syntax the following requirements shall be met:

1. The Data Elements contained in the Data Set structure shall be encoded with Explicit VR (with a VR Field) as specified in Section 7.1.2.

2. The encoding of the overall Data Set structure (Data Element Tags, Value Length, and Value) shall be in Little Endian as specified in Section 7.3.

3. The encoding of the Data Elements of the Data Set shall be as follows according to their Value Representations:

   - For all Value Representations defined in this part, except for the Value Representations OB and OW, the encoding shall be in Little Endian as specified in Section 7.3.

   - For the Value Representations OB, OL and OW, the encoding shall meet the following specification depending on the Data Element Tag:

     - Pixel Data (7FE0,0010)

       - where Bits Allocated (0028,0100) has a value greater than 8 shall have Value Representation OW and shall be encoded in Little Endian;

       - where Bits Allocated (0028,0100) has a value less than or equal to 8 shall have the Value Representation OB or OW and shall be encoded in Little Endian.

       ### Note

       The OL Value Representation is not used for Pixel Data, even if it has a Bits Allocated (0028,0100) of 32, since OL was added to the standard after the encoding of Pixel Data had been established

     - Overlay Data (60xx,3000)

       - shall have the Value Representation OB or OW and shall be encoded in Little Endian.

         ### Note

         Previous versions of the Standard specified that the choice of OB or OW VR was based on whether or not Overlay Bits Allocated (60xx,0100) was greater than, or less than or equal to, 8. However, since only one bit plane can be encoded in each Overlay Data (60xx,3000) Element, no value of Overlay Bits Allocated other than 1 makes sense. Such a restriction is now present in PS3.3.

     - Waveform Data (5400,1010) has the Value Representation specified in its Explicit VR Field. The component points shall be encoded in Little Endian.

     - Red Palette Color Lookup Table Data (0028,1201), Green Palette Color Lookup Table Data (0028,1202), Blue Color Palette Lookup Table Data (0028,1203) and Alpha Palette Color Lookup Table Data (0028,1204) have the Value Representation OW and shall be encoded in Little Endian.

       ### Note

       Previous versions of the Standard either did not specify the encoding of Red Palette Color Lookup Table Data (0028,1201), Green Palette Color Lookup Table Data (0028,1202) and Blue Color Palette Lookup Table Data (0028,1203) in this Part, but specified a VR of US or SS in PS3.6-1993, or specified OW in this Part but a VR of US, SS or OW in PS3.6-1996. The actual encoding of the values and their byte order would be identical in each case, though the explicitly encoded VR field would be different. However, an explicit VR of US or SS cannot be used to encode a table of 216 elements, since the Value Length is restricted to 16 bits.

     - Red Palette Color Lookup Table Descriptor (0028,1101), Green Palette Color Lookup Table Descriptor (0028,1102) and Blue Palette Color Lookup Table Descriptor (0028,1103) have the Value Representation SS or US (depending on rules specified in the IOD in PS3.3), and shall be encoded in Little Endian. The first and third values are always interpreted as unsigned, regardless of the Value Representation.

     - Segmented Red Palette Color Lookup Table Data (0028,1221), Segmented Green Palette Color Lookup Table Data (0028,1222) and Segmented Blue Palette Color Lookup Table Data (0028,1223) have the Value Representation OW and shall be encoded in Little Endian.

     - LUT Data (0028,3006) has the Value Representation US or OW and shall be encoded in Little Endian.

       ### Note

       Previous versions of the Standard did not specify the encoding of these Data Elements in this Part, but specified a VR of US or SS in PS3.6-1998. However, an explicit VR of US or SS cannot be used to encode a table of 216 elements, since the Value Length is restricted to 16 bits. Hence a VR of OW has been added. Moreover this element is always unsigned, therefore the VR of SS has been removed. The actual encoding of the values and their byte order would be identical in each case, though the explicitly encoded VR field would be different.

     - LUT Descriptor (0028,3002) has the Value Representation SS or US (depending on rules specified in the IOD in PS3.3), and shall be encoded in Little Endian. The first and third values are always interpreted as unsigned, regardless of the Value Representation.

     - Blending Lookup Table Data (0028,1408) has the Value Representation OW and shall be encoded in Little Endian.

     - Track Point Index List (0066,0129) has the Value Representation OL and shall be encoded in Little Endian and is always interpreted as unsigned.

### Note

1. For Data encoded with the Value Representation OB, the Data encoding is unaffected by byte ordering.

2. Encoding of Curve Data (5000,3000) and Audio Sample Data (5000,200C) was previously defined but has been retired. See PS3.5-2004.

3. Vertex Point Index List (0066,0025), Edge Point Index List (0066,0024), Triangle Point Index List (0066,0023) and Primitive Point Index List (0066,0029) were previously defined with a value representation of OW and always interpreted as unsigned, but have been retired. These have been replaced by corresponding OL data elements, which allow values larger than 65535 to index the full range of points that can be encoded in Point Coordinates Data (0066,0016). See PS3.5-2015c.

This DICOM Explicit VR Little Endian Transfer Syntax shall be identified by a UID of Value "1.2.840.10008.1.2.1".

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DICOM PS3.5 2018b - Data Structures and Encoding