DICOM PS3.5 2020a - Data Structures and Encoding

7.3 Little Endian Byte Ordering (Current)
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## 7.3 Little Endian Byte Ordering

All nonretired Transfer Syntaxes in DICOM require the use of Little Endian Byte Ordering.

Little Endian byte ordering is defined as follows:

- In a binary number consisting of multiple bytes (e.g., a 32-bit unsigned integer value, the Group Number, the Element Number, etc.), the least significant byte shall be encoded first; with the remaining bytes encoded in increasing order of significance.

- In a character string consisting of multiple 8-bit single byte codes, the characters will be encoded in the order of occurrence in the string (left to right).

Big Endian byte ordering was previously described but has been retired, See PS3.5 2016b.

### Note

The packing of bits within values of OB or OW Value Representation for Pixel Data and Overlay Data is described in Section 8. The OL and OV Value Representations are not used for Pixel Data or Overlay Data.

Byte ordering is a component of an agreed upon Transfer Syntax (see Section 10). The default DICOM Transfer Syntax, which shall be supported by all AEs, uses Little Endian encoding and is specified in Section A.1. Alternate Little Endian Transfer Syntaxes are also specified in Annex A.

### Note

The Command Set structure as specified in PS3.7 is encoded using the Little Endian Implicit VR Transfer Syntax.

In the case of Little Endian encoding, Big Endian Machines interpreting Data Sets shall do 'byte swapping' before interpreting or operating on certain Data Elements. The Data Elements affected are all those having VRs that are multiple byte Values and that are not a character string of 8-bit single byte codes. VRs constructed of a string of characters of 8-bit single byte codes are really constructed of a string of individual bytes, and are therefore not affected by byte ordering. The VRs that are not a string of characters and consist of multiple bytes are:

- 2-byte US, SS, OW and each component of AT

- 4-byte OF, OL, UL, SL, and FL

- 8 byte OD, OV, FD, SV and UV

### Note

For the above VRs, the multiple bytes are presented in increasing order of significance when in Little Endian format. For example, an 8-byte Data Element with VR of FD, might be written in hexadecimal as 68AF4B2CH, but encoded in Little Endian would be 2C4BAF68H.

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DICOM PS3.5 2020a - Data Structures and Encoding