8.2.6 MPEG2 MP@HL IMAGE COMPRESSION

MPEG2 Main Profile at High Level (MP@HL) corresponds to what is commonly known as HDTV (‘High Definition Television’). DICOM provides a mechanism for supporting the use of MPEG2 MP@HL Image Compression through the Encapsulated Format (see PS 3.3). Annex A defines a Transfer Syntax that references the MPEG2 MP@HL Standard.

Note: MPEG2 compression is inherently lossy. 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 MPEG2 MP@HL are also beyond the scope of this standard.

The use of the DICOM Encapsulated Format to support MPEG2 MP@HL 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 that are consistent with the characteristics of the compressed data stream, with some specific exceptions noted here. The Pixel Data characteristics included in the MPEG2 MP@HL bit stream shall be used to decode the compressed data stream.

Note: These requirements are specified in terms of consistency with what is encapsulated, rather than in terms of the uncompressed pixel data from which the compressed data stream may have been derived.

When decompressing, should the characteristics explicitly specified in the compressed data stream 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.

The requirements are:

Note: The value of Planar Configuration (0028,0006) is irrelevant since the manner of encoding components is specified in the MPEG2 standard, hence it is set to 0.

Table 8-2MPEG2 MP@HL IMAGE TRANSFER SYNTAX FRAME RATE ATTRIBUTES

Video Type Spatial resolution layer Frame Rate (see Note 2) Frame Time (see Note 3)
30 Hz HD Single level, Enhancement 30 33.33 ms
25 Hz HD Single level, Enhancement 25 40.0 ms
60 Hz HD Single level, Enhancement 60 16.17 ms
50 Hz HD Single level, Enhancement 50 20.00 ms

Notes: 1. The requirements on rows and columns are to maximize interoperability between software environments and commonly available hardware MPEG2 encoder/decoder implementations. Should the source picture have a lower value, it should be re-formatted accordingly by scaling and/or pixel padding prior to MPEG2 encoding.

2. The frame rate of the acquiring camera for ’30 Hz HD’ MPEG2 may be either 30 or 30/1.001 (approximately 29.97) frames/sec. Similarly, the frame rate in the case of 60 Hz may be either 60 or 60/1.001 (approximately 59.94) frames/sec This may lead to small inconsistencies between the video timebase and real time.

3. The Frame Time (0018,1063) may be calculated from the frame rate of the acquiring camera. A frame time of 33.367 ms corresponds to 29.97 frames per second.

4. The value of chroma_format for this profile and level is defined by MPEG as 4:2:0.

5. Examples of screen resolutions supported by MPEG2 MP@HL are shown in Table 8-y. Frame rates of 50 Hz and 60 Hz (progressive) at the maximum resolution of 1080 by 1920 are not supported by MP@HL. Interlace at the maximum resolution is supported at a field rate of 50 Hz or 60 Hz, which corresponds to a frame rate of 25 Hz or 30 Hz respectively as described in Table 8-y.

6. An MPEG2 MP@HL decoder is able to decode bitstreams conforming to lower levels. These include the 1080 by 1440 bitstreams of MP@H-14, and the Main Level bitstreams used in the existing MPEG2 MP@ML transfer syntax in the Visible Light IOD.

7. MP@H-14 is not supported by this transfer syntax.

8. The restriction of DAR to 16:9 is required to ensure interoperability because of limitations in commonly available hardware chipset implementations for MPEG2 MP@HL.

Table 8-3

Examples of MPEG2 MP@HL screen resolution

Rows Columns Frame rate Video Type Progressive or Interlace
1080 1920 25 25 Hz HD P
1080 1920 29.97, 30 30 Hz HD P
1080 1920 25 25 Hz HD I
1080 1920 29.97, 30 30 Hz HD I
720 1280 25 25 Hz HD P
720 1280 29.97, 30, 30 Hz HD P
720 1280 50 50 Hz HD P
720 1280 59.94, 60 60 Hz HD P

One fragment shall contain the whole MPEG2 bit stream.

Notes: 1. If a video stream exceeds the maximum length of one fragment (approximately 4 GB), it may be sent as multiple SOP Instances.

2. This constraint limits the length of the compressed bit stream to no longer than 2 32 -2 bytes.

The Basic Offset Table in the Pixel Data (7FE0,0010) shall be empty (present but zero length).

Note: The Basic Offset Table is not used because MPEG2 contains its own mechanism for describing navigation of frames. To enable decoding of only a part of the sequence, MPEG2 manages a header in any group of pictures (GOP) containing a time_code – a 25-bit integer containing the following: drop_frame_flag, time_code_hours, time_code_minutes, marker_bit, time_code_seconds and time_code_pictures.

Any audio components present within the MPEG2 MP@HL bit stream shall comply with the restrictions as for MPEG2 MP@ML as stated in Section 8.2.5.