DICOM provides a mechanism for supporting the use of JPEG Image Compression through the Encapsulated Format (see PS 3.3). Annex A defines a number of Transfer Syntaxes that 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:
Note: The DICOM conformance statement shall differentiate whether or not the implementation is capable of simply receiving or receiving and processing JPEG encoded images (see PS 3.2).
The use of the DICOM Encapsulated Format to support JPEG Compressed Pixel Data requires that the Data Elements which are related to the Pixel Data encoding (e.g. Photometric Interpretation, Samples per Pixel, Planar Configuration, Bits Allocated, Bits Stored, High Bit, Pixel Representation, Rows, Columns, etc.) shall contain values which are consistent with the characteristics of the compressed data stream . The Pixel Data characteristics included in the JPEG Interchange Format shall be used to decode the compressed data stream.
Note: These requirements were formerly specified in terms of the "uncompressed pixel data from which the compressed data stream was derived". However, since the form of the "original" uncompressed data stream could vary between different implementations, this requirement is now specified in terms of consistency with what is encapsulated. When decompressing, should the characteristics explicitly specified in the compressed data stream (e.g. spatial subsampling or number of components or planar configuration) be inconsistent with those specified in the DICOM Data Elements, those explicitly specified in the compressed data stream should be used to control the decompression. The DICOM data elements, if inconsistent, can be regarded as suggestions as to the form in which an uncompressed data set might be encoded.
Note: Those characteristics not explicitly specified in the compressed data stream (e.g. color space which is not specified in the JPEG Interchange Format), or implied by the definition of the compression scheme (e.g. always unsigned in JPEG), can therefore be determined from the DICOM Data Element in the enclosing data set. For example a Photometric Interpretation of "YBR_FULL_422" would describe the color space that is commonly used to lossy compress images using JPEG. It is unusual to use an RGB color space for lossy compression, since no advantage is taken of correlation between the red, green and blue components (e.g. of luminance), and poor compression is achieved.
Note: Should the compression process be incapable of encoding a particular form of pixel data representation (e.g. JPEG cannot encode signed integers, only unsigned integers), then ideally only the appropriate form should be "fed" into the compression process. However, for certain characteristics described in DICOM Data Elements but not explicitly described in the compressed data stream (such as Pixel Representation), then the DICOM Data Element should be considered to describe what has been compressed (e.g. the pixel data really is to be interpreted as signed if Pixel Representation so specifies).
Note: DICOM Data Elements should not describe characteristics that are beyond the capability of the compression scheme used. For example, JPEG lossy processes are limited to 12 bits, hence the value of Bits Stored should be 12 or less. Bits Allocated is irrelevant, and is likely to be constrained by the Information Object Definition in PS 3.3 to values of 8 or 16. Also, JPEG compressed data streams are always color-by-pixel and should be specified as such (a decoder can essentially ignore this element however as the value for JPEG compressed data is already known).