Overview: Compression

JPEG Compression

DICOM provides a mechanism for supporting the use of JPEG Image Compression through the Encapsulated Format (see PS 3.3 of the DICOM Standard). Annex A defines a number of Transfer Syntaxes which reference the JPEG Standard and provide a number of lossless (bit preserving) and lossy compression schemes.

Note: The context where the usage of lossy compression of medical images is clinically acceptable is beyond the scope of the DICOM Standard. The policies associated with the selection of appropriate compression parameters (e.g. compression ratio) for JPEG lossy compression is also beyond the scope of this standard.

In order to facilitate interoperability of implementations conforming to the DICOM Standard which elect to use one or more of the Transfer Syntaxes for JPEG Image Compression, the following policy is specified:

• Any implementation which conforms to the DICOM Standard and has elected to support any one of the Transfer Syntaxes for lossless JPEG Image Compression, shall support the following lossless compression: The subset (first-order horizontal prediction [Selection Value 1) of JPEG Process 14 (DPCM, non-hierarchical with Huffman coding) (see Annex F of the DICOM Standard).

• Any implementation which conforms to the DICOM Standard and has elected to support any one of the Transfer Syntaxes for 8-bit lossy JPEG Image Compression, shall support the JPEG Baseline Compression (coding Process 1).

• Any implementation which conforms to the DICOM Standard and has elected to support any one of the Transfer Syntaxes for 12-bit lossy JPEG Image Compression, shall support the JPEG Compression Process 4.

Note: The DICOM conformance statement shall differentiate between implementations that can simply receive JPEG encoded images and those that can receive and process JPEG encoded images (see PS 3.2 of the DICOM Standard).

The use of the DICOM Encapsulated Format to support JPEG Compressed Pixel Data implies that the Data Elements which are related to the Native Format Pixel Data encoding (e.g. Bits Allocated, Bits Stored, High Bit, Pixel Representation, Rows, Columns, etc.) shall contain values which are consistent with the characteristics of the uncompressed pixel data from which the compressed Data Stream was derived. The Pixel Data characteristics included in the JPEG Interchange Format shall be used to decode the compressed data stream.

Run Length Encoding Compression

DICOM provides a mechanism for supporting the use of Run Length Encoding (RLE) Compression which is a byte oriented lossless compression scheme through the encapsulated Format (see PS 3.3 of this Standard). Annex G of the DICOM Standard defines RLE Compression and its Transfer Syntax.

Note: The RLE Compression algorithm described in Annex G of the DICOM Standard is the compression used in the TIFF 6.0 specification known as the "PackBits" scheme.

The use of the DICOM Encapsulated Format to support RLE Compressed Pixel Data implies that the Data Elements which are related to the Native Format Pixel Data encoding (e.g. Bits Allocated, Bits Stored, High Bit, Pixel Representation, Rows, Columns, etc.) shall contain values which are consistent with the characteristics of the uncompressed pixel data from which the compressed data was derived.